CONYERSION OF SOMATIC `ANTIGENS IN SALMONELLA     69

VIROLOGY 6, 68-91 (1958)

Conversion of Somatic Antigens in salmonella by Phage
   Infection Leading to Lysis or Lysogeny'

HISAO UETAKE,~ S. E. LURIA, AND J. W. BUREOUS

Department of Bacteriology, University of Illinois, Urbana, Illinois

           Accepted October 9, 15457
The conversion of Salmonella anatultr (group Et, somatic antigens 3, 10)
to group I& (antigens 3, 15) by infection with phage cl5 and its virulent mu-
tant P vir has been analyzed. Antigen 15 appears in the cells within a few
minutes after infection with either phage. The receptors for another phage,
ear, which can be adsorbed only by cells with antigen 15, appear with the new
antigen. The receptors for phage CM, which is adsorbed only by cells with
antigen 10, disappear following infection. In the early stages the cells have
both antigens 10 and 15 and both sets of phage receptors. The cells that SUF-
vive infection segregate lysogenic and nonlysogenic cells in their progeny.
In the nonlysogenic segregants the changes in antigenic structure and in
phage receptors initiated by phage infection persist for several generations;
later, the cells revert to the typical Ei condition. The results indicate that the
converted characters are controlled by vegetative phage as well as by pro-
phage. Their bearing on the mechanism of conversion is discussed.

INTRODUCTION

Organisms of the genus Salmonella undergo changes in somatic anti-
gens upon lysogeniaation with certain phages (Iseki and Sakai, 1953b;
Uetakeetal., 1955; Terada et al., 1956;Uetake, 1956,1957; Harada, 1956;
Nakagawa, 1957). Among these changes is the conversion of strains be-
longing to the group El, with 0 antigens 3,10, to group Ez, with 0 antigens 3,
15 when they become lysogenic for phages carried by organisms of groups
E, and ES. Reversion from Ez to E1, with loss of lysogeny, is observed by
cultivation of Ez cells in media containing anti-15 serum. In fact, most
of the naturally occurring Ez organisms are the exact lysogenic counter-
1 Supported by grants from the American Cancer Society, Inc., and from the
National Institute of Allergy and Infectious Diseases, Public Health Service (No.
E-1867).

* Fellow of the Rockefeller Foundation; on leave of absence from Sapporo
Medical College, Sapporo, Japan.

                        68

parts of known E1 organisms; and the inference seems justified that the
same conversions observed under laboratory conditions also occur in
the natural habitat (Uetake et al., 1955).

The appearance of new cell properties, such as antigens, in lysogenized
strains has been named "lysogenic conversion" (Lederberg, 1955). It is
generally assumed that it expresses the function of the prophage inte-
grated in the genetic material of the host. We describe in this paper a
system which permits us to study the early manifestation of new proper-
ties following phage infection and the correlation between the presence
Df such properties and of phage elements in the bacterial cell.
Specifically, using a phage that converts group E, organisms to group
Ez, we have studied not only the antigenic factors 10 and 15, which are
shifted in the conversion, but also the sensitivity of the infected cells to
two unrelated phages, which are adsorbed only by cells with antigen 10
or antigen 15 respectively. The results show that the phage-induced
changes appear within minutes after infection, can persist phenotypically
in cells that have segregated away the genetic determinants of phage
production, and are produced equally well in cells that give a productive,
lytic response to infection as in cells that give a reductive, lysogenic
response. Thus, the changes in surface properties of the cells are the ex-
pression of the phage material acting as a genetic determinant in all its
forms.

MATERIALS AND METHODS

The experimental system. The basic experimental system to be described
consists of Salmonella anatum 293 (= strain A) and of phage Gs active
upon it. Upon infection with t Ifi the surviving cells of A give rise to
                          ,
lysogenic progeny A(P). Strains A(P) differ from A in somatic antigens.
Strain A is a typical Er group Salmonella, with somatic antigens 3,lO.
Strain A(c'~) is a typical Ez group Salmonella, with somatic antigens
3,15.
In addition, strain A(e15) is unable to adsorb certain phages, including
P itself and an unrelated phage C a~, which can infect strain A; instead,
strain A(P) is sensitive to another unrelated phage t34, which is not
adsorbed by A. These properties are listed in Table 1. The experiments
ta be described investigate the appearance and persistence of the above
differential characters in cells of strain A infected with phage P and in
their progeny.
Bacterial strains. All Salmonella strains used are international st,andarcl


70           UETAKE, LURIA AND BURROUS                    CONVER~~I~N OF SOMATIC ANTIGENS IN a~ucoiv~zm     71

                    TABLE 1
PROPERTIES OF Salmonella anatum STRAIN A AND ITS LYSWENIC DERIVATIVES

Strain

:: (e'6)
A (@, 8')

o-Antigens             Adsorption of phages

          &b or dS Vi?     gr4, or CUl      e" or 2' Dir

3, 10        +         +          -
3, 15          -          -        +
3, 15, 34       -          -

strains obtained from the Enterobacteriaceae Committee of Japan.
Strain A is 8. analum 293 (group Er, antigens 3,lO: e ,h-1,6 . . .). It gives
an appreciable proportion of rough variants, which fail to adsorb phage
and may complicate the serological tests. For each experiment, a single
smooth colony was picked into broth and grown to the desired concen-
tration. Under these conditions only about 0.1 to 0.2 % rough cells were
present, a proportion that does not interfere with the experiments. The
experimental cultures were adjusted to the desired titer by calorimetric
test and assayed by colony count. The generation time of strains A and
A(e16) growing in a water bath at 37" is 30 minutes. A streptomycin-
resistant derivative AS' was obtained for use in the assay of free phage
(Bertani, 1951).

Phage strains. Phage en is a temperate phage isolated from S. newing-
ton CZ (group Ez, antigenic structure 3,15: e , h-l ,6,. . .) and propagated
on S. anatum A (Uetake et al., 1955). The latent period of this phage on A
is 55 minutes at 37"; the burst size is about 400. At a multiplicity of in-
fection of 5, 20 to 40% of the infected cells survive and give rise to
lysogenic progeny A(e15), with the antigenic structure: 3,15: e,h-1,6. . . .
The frequency of liberation of phage E 15 from A(&) is of the order of lop4
per cell per generation under the standard conditions of these experi-
ments. Strain A(@) is induced to lysis and phage liberation by ultra-
violet light.

Phage $6 a& is a virulent mutant of e16, which lyses all susceptible
cells it infects. Latent period, burst size, and serological properties are
the same for en and till, vir.

Phage Cw is a virulent mutant of phage ga41, isolated from S. thomas-
uille 341 (Uchida et al., 1956; Uetake, 1957). These phages are adsorbed
by most Salmonella strains of group E1, but not by strains of group Ez.
The adsorption rate constant on strain A is 6 X 10Mg min-' per cell.

Over 150 phage particles per cell can be adsorbed by growing cells of
strain A. The latent period is 30 minutes; burst size about 100. Phage
Can is serologically unrelated to cl6 by cross-neutralization tests.
Phage c34 vir is a virulent mutant of temperate phage e34 c, which was
isolated from S. thomasville 341. These phages can infect cells with anti-
gens 3,15; the temperate phage, upon lysogenization, converts the
somatic antigens to 3,15,34 (Uetake, 1956; Harada, 1956; Nakagawa,
1957). The adsorption rate constant is 3 X lWg min-1 per cell. The
maximum adsorption capacity is about 25 phages per cell. The latent
period is 35 minutes; burst size about 100. Phages c34c and c31 vir do not
adsorb on strain A (nor on any other strains without antigen 15). They
are serologically unrelated to phages e16 and C3d1. Phage ea4 tir for our
work was propagated on S. anatum A do, a defective derivative of strain
A(m) that has antigen 15 but liberates no phage en (Uetake, to be pub-
lished). This avoids the presence of any cl6 phage in lysates of G4 vir.
Phage stocks were prepared in aerated nutrient broth cultures or by the
agar layer method.
Media. Difco nutrient broth (8 g/liter in demineralized water, plus

5 g/liter NaCI) was used throughout. Platings were made on nutrient
agar (1.1% and 0.65 % agar in nutrient broth for bottom and top layers,
respectively) except for viable cell counts, which employed a tryptone-
yeast extract-glucose agar.
Serological tests. Antibacterial antisera were prepared by standard
methods (Kauffmann, 1954). Anti-10 serum was obtained by absorbing
OTantiseprm against strain A withcells of A(e16) ; anti-15 serum by absorb-
ing antiserum against A(e15) with cells of A. Test for antigens in colonies
was done by slide agglutination, using anti-10 and anti-15 sera.

Tube agglutination tests were done using 0.2 ml of serum dilution in
saline plus one drop of cell suspension in 7-n-m tubes; readings were
done after incubation for 2 hours in a water bath at 50" and overnight
at room temperature. Cells from experimental cultures were heated for
4 minutes in boiling water, centrifuged and resuspended in saline for use
as antigens.
Antiphage sera were prepared in rabbim by subcutaneous inoculations
followed by intravenous injections. The sera used had the following
titers: anti-&--80 min+; anti-C&--750 min-I; anti+ tir-70 min-`.
The sera were used without previous absorption of antibacterial anti-
bodies. In the experiments, possible agglutinating action of antibacterial


72          UETAKE, LURIA AND BURROUS

CONVERSION OF SOMATIC ANTIGENS IN SALb_ONELLA    73

antibodies in antiphage sera was prevented by using low bacterial con-
centrations, and interference by these antibodies with phage adsorption
was tested in control experiments. There is no cross reaction between the
antigens of strain A and of the phage e ls detectable by phage neutraliza-
tion tests.

Tests for phage-carrying colonies were done by replica plating (Leder-
berg and Lederberg, 1952) using velveteen pads. The colonies to be
tested were replicated onto plates with a 4-ml overlay of 1.1% nutrient
agar containing indicator .cells. The indicator plates (which could be
stored in the refrigerator) were dried open for 30 minutes at 37" before
replication.

Tests for killing of cells by phages CM and e34 vir. The cells to be tested
were exposed at a density of about lo6 cells per milliliter to phage with
a titer of 10D particles per milliliter. During exposure to phage the mix-
tures were kept at 20" to reduce bacterial multiplication. At intervals,
samples were diluted 1:5 in a mixture of anti-C&n serum (1: 100) and
anti-e" serum (1:50), kept at 37" for 5 minutes, and plated for viable
counts, adding to each plate one drop of the same mixture of sera, to
minimize reinfection of surviving cells on the plates. A uniform serum
treatment was used in tests for killing by phage C341 alone, by 634 vir
alone, or by a mixture of the two. Suitable controls indicated that the
killing rates were not affected by the presence of anti& serum, which
was also present in the experimental mixtures.

Tests for killing of cells by phage t16 uir were done in a similar manner,
except that serum anti@ was either omitted from the experimental
mixtures or, if present, was rendered ineffective by suitable dilution
before testing. Anti-G6 serum was used on the plates.
Tests for adsorption of phages by cells of strain A infected with g16 were
done by chilling, centrifuging, and resuspending the cells to the desired
density. The details are described in the appropriate section.

RESULTS

The basic experiment. In a typical experiment, a culture of strain A
from a single smooth colony was grown with aeration to a density of
about 2 X lO* cells per milliliter. Equal volumes of the culture and of
phage ~16 at a titer of 1 X log particles per milliliter were mixed, and the
mixture was kept at 37' without aeration. After 10 minutes, when over
95% of the phage was adsorbed, the mixture received an equal volume
of anti-El6 serum diluted 1: 50. After another 10 minutes, a 1: 50 dilution

was made in broth containing anti@ serum diluted 1:400.3 At 90 min-
utes, a further dilution 1: 8 was made in the same inedium, and a further
dilution 1: 120 was made at 180 minutes. Thus the cells were kept at
low density and reinfection with phage G6 was prevented by the anti-
serum. Parallel dilutions without serum were used for determination of
unadsorbed phage, latent period, and burst size. Samples were taken at
intervals and tested in the following ways:
1. Viable cells, by colony count.
2. Cells carrying phage e 15, by replication of colonies onto indicator
plates.
3. Antigens 10 or 15 in colonies, by slide agglutination.
4. Presence of antigens 10 and 15, by tube agglutination after heat-
killing the cells and resuspending them in saline.
5. Killing of the cells by phage C&a.
6. Killing of the cells by phage c34 vir.
7. Killing of the cells by a mixture of phages Call, and c34 wir.
8. Killing of the cells by phage t16 vir.
9. Ability of the cells to adsorb phages C3d1 and g3( vir, and rates of
phage adsorption.'
Not all these properties were tested in each experiment at all time in-
tervals; but enough duplications were made and the results were repro-
ducible enough to permit us to draw a composite picture. A typical ex-
periment is detailed in Fig. 1.
The most variable feature is the proportion of infected cells that sur-
vive infection, which varies between 20 and 40 % in different experiments
a8 measured at 20 minutes after infection. This variability only gives a
factor 2 of uncertainty in bacterial titers, which does not interfere with
the planning of dilutions. The infected survivors resume multiplication
within 30 minutes after infection and multiply at the same rate as un-
infected bacteria.

We shall discuss individually the evolution of the various properties
of these surviving bacteria (see Table 2) and then give a general picture

a The anti-P serum used in these experiments could not be absorbed with A
or A(P) cells to remove antibacterial antibodies because such absorption left
in the serum some materials that interfered with adsorption of other phages by the
cells. Clumping of the cells was prevented by keeping them at low concentration
and by careful resuspension before using them.
4 This effect of antiserum on the adsorption rate of P uir is probably due to the
presence of antibacterial antibodies in the antiphage serum.


74          UETARE, LURIA AND BURROUS                CONVERSION OF SOMATIC ANTIGENS IN SAL~KWE&LA     75

Id1    I     I     I     I     t     I
     80   I20  180 240 300 360      J

        MINUTES

FIG. 1. A typical experiment of infection of strain A with phage ~16. The curve
of the colony counts of uninfected cells was obtained from a portion of the culture
that received no phage. The phage plaque counts were measured on a sample
diluted in broth without anti-@ serum after the first 10 minutes of serum treat-
ment. The phage-carrier cells were determined by replica plating. The values for
cells resistant to phages P vir and to C S*I, respectively, correspond to measure.
ments after 20 or 30 minutes of exposure to the test phages. Abscissa: time after
infection. Ordinate: values referred to the initial phage-cell mixture.

of this evolution and an interpretation of the genetic functions of
phage e16.

tI2wrier state, lyeogeny, and segregation of nonlysogenic progeny. *When
the colonies formed by the surviving bacteria are tested by replication
on an elk-sensitive indicator, at first practically all colonies are phage
carriers and, if picked and restreaked, contain lysogenic bacteria. As the
bacteria multiply, the number of phage-carrier colonies does not in-
crease as rapidly; indeed, after an initial rise the number remains ap-
proximately constant for 2 or 3 generations, so that their proportion de-
creases from nearly 100% to about 10%. Thereafter, the number of
phage-carrier colonies begins to increase and rises in parallel with the
total cell count (see Table 2 and Fig. 1). This indicates that, following in-

fection, the surviving cells segregate out for several generations a mix-
ture of sensitive cells and phage-carrier cells. Such segregation has been
observed with Escherichiu coli infected with phage X (Lieb, 1953) and
with S. typhi murium infected with phage P22 (Levine, 1957).
The pattern of segregation was confirmed by single clone experiments,
in which at 10 minutes an infection mixture, prepared as usual, was
diluted in broth to a concentration of about 2 surviving cells per milli-
liter and 0.2-ml samples were distributed into individual tubes. After
incubation for about 3 hours, the contents of each tube were tested for
free phage cl6 by spot test on streptomycin agar seeded with strain ASr;
then, after addition of anti-@ serum the samples were plated for colonies
on separate plates. All plates with colonies were replicated for phage-
carrier test. The results are shown in Table 3.
These experiments show that the presence of lytic responses (free
phage) and of surviving cell clones arise by independent events, since
their joint frequency is similar to the product of the frequencies of the
two individual events (50 % plates with phage; 39 % plates with cells;
19% plates with both). There is an excess of plates with nonlysogenic
cells only over the number expected from the fact that in the experi-
ments of Table 3 early platings contain ahnost only phage-earner col-
onies. This suggests that under the conditions of the single clone experi-

TABLE 2

PROPERTIES OF THE PROGENY CELLS FROM BACTERIA INFECTED WITH PHAGE ~16

Time after
infection
(rain)

                      Cells killed by
Phage-carrier Cells killed by
colonies
  (%I     pha e cw  Cells killed by
                phage r" air   B mixture of
      8%)       1%)     Ciu and r't t+  Cells killed by
                  (%)    phage tk6 SiP
                                 (%I

    30       98       996      746      96b
    60       85       99*      926      71b        0
    90       75       78       95      97
   120       61       69
   150       30
   180        12        0       90      90        46
   300        12
   360        11       85        7       96
* Tested on separate samples without anti-c" serum.
* These values are for an exposure of 20 minutes to the test phage at 20". All
other values are for a 30-minute exposure.
NOTE: All values in the table are from a single experiment, except the one in
italics.


76           UETAKE, LURIA AND BURROW                     CONVERSION OF SOMATIC ANTIGENS IN BALMONELLA     77

TABLE 3

SEGREGATION OF NONLYSOGENIC CELLS IN THE PROGENY OF CELLS INFECTED
                  WITH PWAGE 6'S

    Plates with              Number of colonies
       Colonies      Number        Average muubeepef colonies
               of plates
Free   Phage-   NOW          Carrier   Noncarrier   Carrier   NOD
                                             carrier   TOtJll
phw   calmer   carrier

     -           32
-    + - .o
     -    -I-      8            245         30.6   30.6
    + f     10      78    765 7.8    76.5 84.3
+ - -.29
+ + -      0
+ - +     11           282          25.5   25.5
+ + +     10     80    549 8.0 54.9   62.9
                         -I -

100     153    1341

Total : 1999

Plates with free phage (bursts) = 50
Plates with colonies = 39

Plates with phage-carrier colonies only = 0
Plates with noncarrier colonies only = 19
Plates with both types of colonies = 20
Average number of bacterial cells per tube (lysed or survivors) = 1.14
Average number of bacterial clones per plate = 0.49
Average number of clones with carrier cells per plate =I 0.22
Average number of clones without carrier cells per plate = 0.27

Approximate proportion of fk x  % ?4e %2   364
phage-carrier colonies in
plates with colonies of both
types

Number of plates        2 3 7 5 2 1   Total : a0
NOTE: A mixture of A cells (1.2 X 108/ml) and phage er6 (5.0 X lO*/ml) ~88'

kept at 37" for 10 minutes, diluted 1:2 X 107 in broth, and distributed in 0.2-ml
samples in 100 tubes. The tubes were incubated for 170 minutes at 37", then placed
in a water bath at 10'. Each sample was tested for free phage tr5 by spot test
(about 0.01 ml) on a streptomycin-agar plate seeded with cells A&. One drop of
a I:50 dilution of anti-@ serum was added to each tube, and the tubes were re-
turned to 37" for 10 minutes, then again to 10'. The contents of each tube wen
plated by spreading on individual agar plates. All plates with colonies werf
counted and replicated for phage-carrier test.

ment some infected cells give rise to few lysogenic progeny cells or none
at all. This is supported by findings in some preliminary experiments
with single ceil isolation pedigrees (carried out Dr. D. K. Fraser), in
which some clones at the early cell divisions gave repeated cell deaths
without phage liberation. The possibility of a frequent early death of
potentially lysogenic progeny cells is also suggested by the significantly
lower colony counts in those plates that have only nonlysogenic colonies,
compared with the counts in plates ,with lysogenic colonies (see Table 3).

The most relevant finding shown in Table 3 is that all clones contain-
ing lysogenic colonies also contain nonlysogenic, sensitive cells. This
conhrms the existence of segregation. The average number of cells per
clone is 37 (about 5.2 generations). The proportion of lysogenic colonies
in plates with lysogenics was 158/1313 = 13%; the proportion of lyso-
genies among all colonies was 158/1841 = 8.5%. These values are in
agreement with the final proportion of about 10 % lysogenics found in
the mass growth experiments. The frequency distribution of the propor-
tion of lysogenics in the plates, ranging from $$j to g4, is an index of the
variability of the pattern of segregation, as observed also with phage
P22 of S. @phi murium (Levine, 1957).
As expected, all phage-carrier colonies gave a positive slide agglutina-
tion reaction with anti-15 serum, since they contained lysogenic cells
A(S). Conversely, all colonies without phage gave a positive reaction
with anti-10 serum and a negative one with anti-15 serum.
Evolution of antigens 10 and 15 in the infected cells. Table 4 shows that
the infected cells of strain A were already agglutinable by anti-15 serum
15 minutes after infection; they were agglutinated at the full serum titer
by 30 to 45 minutes. Additional tests showed some detectable agglu-
tination after 3 to 5 minutes and definite agglutination at 8 minutes. Mean-
while, the agglutinability by serum anti-10 persisted, apparently un-
changed. Judging from the completeness of agglutination by either
serum, it seems clear that antigens 10 and 15 coexist on the same cells.
Note that a mixture of 10 % A(e15) cells and 90 % A cells did not produce
visible agglutination with anti-15 serum. Hence, over 10% of the cells
must have developed some antigen 15 within 3 to 5 minutes.
A remarkable finding is that the same result is obtained when phage
t16 is replaced by tJ6 sir, a virulent mutant, which does not lysogenize
and which causes 99 % of the infected cells to lyse between 60 and 120
minutes after infection (see Table 4). Here too, antigen 15 is detected
within less than 10 minutes. This finding indicates that the formation of


78         UETAKE, LtJBJA AND BURROWS
antigen 15 can be controlled by the phage in its vegetative state and does
not require the presence of an established prophage. Note also that the
development of antigen 15 in infected cells is scarcely affected by the
multiplicity of infection (between 1.8 and 14).

Table 4 also shows tests with anti-10 and anti-15 sera on cells taken
180 and 360 minutes after infection with P, at which times the propor-
tion of phage-carrying cells has decreased to about 10% (stable lyso-
genies). The cell suspensions are agglutinated by both sera, but the
degree of agglutination by anti-15 serum is stronger at 180 minutes,
while agglutination by anti-10 serum is stronger at 320 minutes. These
data are not quantitative enough to decide whether the extent of the
antigenic reaction with anti-15 corresponds to the proportion of cells
that carry phage E 15; but, qualitatively, the evolution-of antigens parallels
the evolution of lysogeny and segregation.

Appemxmce and development of susceptibility to phage 9 vir. Since
phage 84 vim is not adsorbed by cells of strain A but is adsorbed by A(P),

                    TABLE 4
AGGLUTINATION REACTIONS OF CELLS OF STRAIN A INFECTED WITH PHAGE tu

1 Time 1

Serum anti-10

            after
   Antigen     infec-
            tion
       (min) 1:zoo 1:400
___----
Cells A      15 ++++-l-i-
infected     20 ++++++
with t16     25 ++++++
         30 +-t+++t
         45 +++++t
         60 -t++++t
         180 ++ +
         360 +++++t
           ---
Cells A   15 +++++-t
infected     20 +++++-t
with do U~T'   25 +++++-t
     30 +++I+++
     45 +++I+++
     60 t+t'++t
    -- I-
Cells A        +++ ++-I
uninfected     I
Cells A@`)

1:800 1:1600  1

--
t++ +-t
t++ +++
t++ +++
t++ ++
-I-+ ++
++ ++
+ +
t++ -I-+
--
t++ ++
+++ ++-t
+++ ++-t
+++ ++
++-t ++
+++ ++
--
++-I- ++

--
- -

_-

--

--

-

i-

: 3200

++
++
++
++
+
+
+
+

    serum ad-15

1:lOO   1:200   1:400
-__-
+ + f
+ + -I-
++ ++ +
+++t++ ++
-l--t++++ ++
++++++ ++
,t++ +++ ++
I +i- ++ i-f

-___--
++ + + f
++ +-I- ++ +
++ ++++++ ++
++ ++++++ ++
++ +-I-++++ S-t
+-I- +++ ++ +
p-F-
++ - - -

----
- +++ +++ ++-

-

1
-

-.

--

;:
-

-

:a00
-
-

f
i-
+
f
tt
+
f
-

+
+
1
t
-

-
tt
-

    CONVERSION OF SOMATIC ANTIGENS IN SALbiONELLA     79
killing every infected cell, the receptors for this phage must appear dur-
ing lysogenization. Their formation can be followed by measuring the
killing by 8 vir of cells that survive infection by t15. The results are
shown in Table 2 and Fig. 2. The development of susceptibility to e34 vir
is very rapid: 74 % of the cells are susceptible after 30&m&s and 90 %
after 60 minutes. This is consistent with the rapid appearance of anti-
gen 15.

When the rate of killing of cells by @ vir is studied, as shown in Fig. 2
it becomes clear that not only the proportion of cells susceptible td
phage G4 vir increases with time up to a maximum, -but also the rate at
which the cells are killed increases from 0 to a maximum. This rate
never quite reaches, for the whole population, the rate of killing for the
established lysogenic strain A(e15). This is as expected'if the receptors
were formed slowly and if the rate of killing paralleled the number of
effective receptors per cell.

-I

F'IG. 2. The survival of bacteria exposed to phage P vir at various times after
infection with phage t Is. Mixtures contained 10D phage particles and about 10'
cells. The figure near each curve indicates the time between infection wi,h P
and challenge with es4 uir. On the abscissa is the duration of exposure to phage ~34
tir. Curve A: uninfected cells of strain A. Curve A(@): established lysogenic
cells.


80         UETAKE, LURIA AND BURROUS
As time proceeds, the proportion of cells susceptible to phage ta4 vir
rises first to a maximum of about 95 %, then diminishes, until after 360
minutes only about 10 % of the cells are sensitive to ca-' vir. It is interest-
ing to compare the evolution of the sensitivity to tM with that of the
phage-carrier state (see above). The comparison can be made by inspec-
tion of Table 2. It is clear that the proportion of cells sensitive to c34 tir
continues to increase while the proportion of phage-carrier cells is
decreasing. The former is still 90% at 180 minutes, when the surviving
cells have already undergone 5-6 division cycles and the phage-carrier
cells have been reduced to the level of the stable lysogenics for e15 (about
10%). That the acquisition of sensitivity to 6% wir in noncarrier cells is
not permanent is shown by the fact that at 360 minutes, after 6 addi-.
tional generations, the proportions of ti6 lysogenics and ez4 sensitives are
the same. Hence, we conclude that susceptibility to ca4 tir, arising under
the control of phage t15, can persist for several cell generations after
the $6 phage-elements have been removed by segregation. The meaning
of this persistence will be discussed in a later section.
Evolution of susceptibility to phage Ga. Since phage CM adsorbs only
to cells with antigen 10, such as strain A, and not to cells of strain A(elF),
it is expected that the receptors for CM, and hence the sensitivity of the
cells to this phage, will be affected during lysogenization by phage c16.
The findings are shown in Table 2 and Fig. 3.
At 30 and 60 minutes after infection, 99% of the cells were killed by
exposure to phage C1hl for 20 minutes, although more slowly at the later
time. By 90 minutes, however, only 78 % of the cells were killed, and at
180 minutes practically all cells were resistant to phage CM. By this
time the cells with phage cl5 are only about 10 %. At 360 minutes 80 % of
the cells are again killed by C&41, but the rate of killing is still definitely
lower than for uninfected cells of strain A.
These results indicate that susceptibility to phage Carl is greatly re-
duced folIoWing infection with phage e15, until after about 5-6 genera-
tions most of the progeny of the E 15-infected cells is fully resistant to
phage C341. The Carl receptors later reappear in the nonlysogenic progeny;
their amounts, however, are not yet normal after 11-12 generations. It
is important to note that reappearance of sensitivity to phage CM lags
far behind the segregation of e 16-free cells, in the same way as the disap-
pearance of sensitivity to ea4 vir does.
Adsurptim of phages CM and t 34 LINT, and evohtion of the cell Teceptofs.
Three interpretations of the above findings are possible: either, follow-

CONVERSION OF SOMATIC! ANTIGENS IN SALHOivE~LA     81

I         I         I
IO        0
MINUL     30

FIG. 3. The survival of bacteria exposed to phage GUI at various times after
infection with phage e". Notations as in Fig. 2.

ing infection with phage c Is, the receptors for Cad1 present in uninfected
A cells are blocked, destroyed, or inactivated; or they are preserved but
no further synthesized, being diluted among the progeny cells; or they
are synthesized at a rate insufficient to keep pace with cell division. The
same hypotheses apply to the loss of sensitivity to phage 6% vir in the
later evolution of the progeny cells. We have attempted to decide among
these possibilities by quantitative tests of adsorption of phages Cau and
tX v-i~ on progeny cells at various times after infection with phage tls.
For these experiments, cells infected as usual with e15 were diluted in
broth with anti+ serum to such an extent that at the desired times the
bacterial titer would be approximately 1 X lO'/ml. At these times, 20-
ml samples were taken, assayed for viable count, chilled and centrifuged.
The cells were collected in 1.2 ml of cold broth, carefully resuspended
and again assayed. These cells, with titers of 14 X 108/ml, were used for
adsorption tests. A mixture of phages C&l and @ vim, each at 1 X lO'/ml,
was added and samples were taken after 2, 5, and 10 minutes. These
were diluted 1: 100 in broth with chloroform (to kill the bacterial cells)
and plated 39-40 minutes later; thus, only the free phage was assayed.
Phage ca4 tir was assayed on strain A do and phage Call was assayed on


82           UETAKE, LURIA AND BURROUS

strain A, adding 2 drops of serum anti-@ (1: 50) to each plate. Most
of the phage P produced by bacteria lysing during the adsorption test
is eliminated by the serum; even in controls with no serum added, how-
ever, the phage cl5 present does not interfere with the assay of phage
Carl. The adsorption rate constants for strain A and A(P) were measured
in the same experiments.

The results of such an experiment are shown in Table 5. The adsorp-
tion rate constants are given as the means of the values derived from the
platings at 2, 5, and 10 minutes. This procedure is justified for eU uir,
where the adsorption is generally a simple exponential function of time;
less so for phage Ca41, which is adsorbed more rapidly in the first few
minutes. Table 5 also `gives for comparison the proportion of phage-
carrier cells found in the same experiments. It is clear that the adsorption
capacity per cell for phage Ca,r decreases rapidly after infection, while
the adsorption capacity for phage ca4 tir appears and increases to a maxi-
mum. Later, the adsorption capacity for C&i reappears and increases
again, while that for ea4 vir rapidly decreases. As expected, the timing of
these changes in adsorption rates parallels closely that of the changes in
susceptibility to the phages, as can be seen by comparison with Table2
and Fig. 1.

More interesting are the quantitative data. The adsorption rate for
ear vim reaches the maximum value of 1.8 X 1O-g at 150 minutes, that
is, when the phage-carrier cells are already reduced to 24%. This maxi-
mum rate is the same as the rate for the established lysogenic strain
A(P) tested under the same conditions, that is, grown in broth contain-
ing anti-P serum (1:400) for 2 hours. The decrease in adsorption rate
for @ wir thereafter is such that at 300 minutes the rate is only about
X0 the maximum rate per cell. In the interval between 150 and 390
minutes, the adsorption rates observed correspond closely (see Table 5)
to the rates calculated by assuming that the population consists of two
fractions: one, lysogenic, comprising 8% of the population, with maxi-
mum adsorption rate of 1.8 X 10eg per cell; the other, with adsorption
rate per cell corresponding to the maximum rate divided by the number
of cells derived from each cell at 150 minutes. We interpret this as show-
ing that in the cells without phage $6 the receptors for e34 wir that have
been formed are diluted and distributed among the progeny cells but are
not destroyed or otherwise inactivated. Note that in this period the
proportion of cells susceptible to phage P tir decreases from 90 to 8 %.

The situation is different with regard to phage Can. Here the adsorp-

      CONVERSION OF SOMATIC ANTIGENS IN SALhiONELLA     83

                    TABLE 5
ADSORPTION RATE OF PHAQES Cal AND es4 vir ON PROQENY CELLS FROM BACTERIA

Time
after
infection

-

_

C&/mP

30   4.8 x 107    -
60   1.0 x 108   90
120   5.2 X 108   62.5
1.50   8.9 x 108   24.5
180   2.1 x 100    8.1
240   1.2 X 10'0   6.8
300   8.2 x 10'0   7.8

_

Observed

st.

Calculated*

2.6 X 10-O 2.9 x 10-9
3.6 X IO-`* 5.6 X 10-r"
<l x lo-`0 3.3 x 10-10
3.6 X 10-I"
1.2 X 10-9
1.9 x 10-g

Strain A

6 X 1OP in
broth or
in anti-
&erum

Strain A(G)

-

-.

,
,

INFECTED WITH cl6

Adsorption rate constant
per cell per minute for
  phwe GUI = kc

Adsorption rate constant per
cell per minute for &age
    8' vir = La,

Observed  I Calculated*

-  I

1.1 x 10-9
1.7 x IO-9
1.8 x 10-g
7.7 x 10-o  8.6 x 10-10
4.0 X lo-10  2.6 X lo-`0
1.9 X lo-10  1.6 X lo-l0

3 X 10-O in
broth;
1.8 X 10-9
in anti-cl5
serum

o Values refer to the initial mixture of phage and bacteria.
b Calculated on the assumption of dilution of receptors by a factor of 2 at each
cell generation (see text).

tion rate constant per cell decreases between 0 and 150 minutes from
6 X UP9 to less than 1 X lo-lo. This decrease is more rapid than cal-
culated from simple dilution of existing receptors (see Table 5). It is
possible that the results may be affected by presence of the anti-El5 serum.
Although this serum does not interfere with adsorption of phage Carl by
strain A, it cannot be excluded that it might affect the adsorption by
bacteria with rapidly changing surface structure. There is additional
evidence, however, that an actual loss of effective receptors for C3(i takes
place. The growing cells of strain A can adsorb at least 150 particles of
phage C341 per cell, as shown by additional experiments. If simple dilu-
tion took place, after 3 generations each cell should have, on the average,


84           UETAKE, LURIA AND BURROUS

about 20 receptors and, after 5 generations, about 5 receptors. Yet, more
and more cells become totally resistant to phage C&al in the killing tests,
in which the phage is used in large excess. Here, too, it is difficult to
exclude a possible role of antibody in anti-@ serum in preventing adsorp-
tion of and killing by phage C 3u. This question will have to be solved
experimentally before we can conclude with certainty that the receptors
for phage CL are actually destroyed or otherwise inactivated during the
first 2 hours after infection.

Coexistence and interactions of antigens 10 and i6 and of receptors for
phages ~84 and &I. The tube agglutination tests have indicated that early
after infection most cells carry both antigens 10 and 15. The results of
phage sensitivity and bhage adsorption tests likewise show that the
receptors for phages ear vim and Ca4i, whose presence is closely associated
with that of antigen 15 and antigen 10, respectively, are also present to-
gether in cells descended from e 15-infected bacteria. In fact, the data in
Table 3 show that the sum of the proportions of cells killed by either P
wir or Cm at 30, 60, and 90 minutes were 173, 191, and 173%, respec-
tively. Thus, at least 73 to 91% of the cells must have receptors for both
phages.                                                   c
This does not mean that both antigens or both receptors can be syn-
thesized in the same cell. All our results are compatible with the hy-
pothesis that a cell can only form, either antigen 10 and receptor for
Cal ("complex lo"), or antigen 15 and receptor for e3* vim ("complex
15"), as though the two synthesizing mechanisms were mutually ex-
clusive. Formation of complex 15 is initiated rapidly by infection with
G5. Formation of complex 10 is initiated by the removal of phage P
during segregation, but is apparently delayed for several generations, at
least as tested by the reappearance of receptors for phage C&. The
possible interpretations of the competition and of the delay will be dis-
cussed later.

An unexpected observation was made when the e15-infected cells were
exposed at various times to a mixture of tU v-ir and CM phages at the
same titers as used singly. The results, shown on Fig. 4, should be com-
pared with those of Figs. 2 and 3. It was expected that all cells would at
all times be killed by the mixture and that the rate of killing would be at
least as fast as that by either phage alone. Instead, it is seen that at
least at 60 minutes, the mixture of phages kills fewer cells and more
slowly than phage C341 alone. This puzzling result may reflect an inhibi-
tion by eM vir of the adsorption of phage CM by 60minute cells. Experi-

    CONVERSION OF SOMATIC ANTIGENS IN SALMONELLA     85

ments to test such an inhibition directly, however, gave erratic results
and allowed no definite conclusion to be reached.

Immunity to phage t15 vim. Immunity of bacteria stably lysogenic for
prophage en to lysis by superinfection with cl5 vir cannot be tested be-
cause cl5 and G5 vir are not adsorbed by cells lysogenic for e15. A test can
be made, however, during the period of segregation. The results of such
tests are included in Table 2. It is seen that at 60 minutes all cells sur-
vive exposure to cl5 wir, while at 180 minutes only about half survive.
The failure to be killed by elfi vim may reflect either immunity or absence
of receptors. Since at 60 minutes all cells are still sensitive to C341, the
resistance to cl5 vim is probably a refl,ection of immunity. The results at
180 minutes show that receptors for C311 and to t15, although they are
both present in cells of strain A and absent from cells of strain A(@),
are presumably not identical. This is in agreement with the observation
that strain A(g,,,) can adsorb e
Uetake, 1957).        l5 but not Carl (Uchida et al., 1956;
Tests for antigens mprotoplasts. Zinder (personal communication) has

detected antigen 1 in protoplasts of S. typhi murium. It seemed worth
while testing whether antigen 10, which is not detectable in whole cells

I

360'
SO'

rt `A (c")

FIQ. 4. The survival of bacteria exposed to a mixture of Carl and P vir. Nota-
tions as in Fig. 2.


86          UETAKEi, LURIA AND BURROUS

CONVERSION OF SOMATIC ANTIQENS IN SAI,bfONELLA     87

of strain A(G), may still be present on the protoplasts, that is, on cells
without cell walls.

Protoplasts were prepared from cells A and A(E~~) by the method of
Fraser and Mahler (1957) and Fraser et al. (1957). Over 90 % of the
cells were transformed into protoplasts, that is, made round, nonviable,
and sensitive to disruption by osmotic shock. They were washed and
resuspended in a stabilizing medium (0.2% NaCl with 5% crystalline
bovine serum albumin and. 1O-2 M MgSO4) and tested for agglutinability
by anti-10 and anti-15 sera. The mixtures were incubated at 37" for 2 hours
and at 5" overnight. The results were clear-cut. Protoplasts from strain
A cells are agglutinated by anti-10 serum only; those from strain A(~ll)
are agglutinated by anti-15 serum only. The agglutination end point for
the protoplasts is about the same as for intact cells in the same medium.
The failure to reveal antigen 10 in the protoplasts of A(@) does not
exclude, of course, that this antigen may be present in some masked
form in the cells of this strain.

DISCUSSION

Infection of S. anatum strain A with phage G results iu a rapid change
in properties of the bacterial surface, recognizable as formation of somatic
antigen 15, disappearance of antigen 10, acquisition of receptors for
phage ea4, and loss of receptors for phage Carl. Although the various
properties are recognized by different tests and may reflect different
details of specificity in the surface structures of the bacteria, their pat-
tern of formation and disappearance following infection with phage Ebb is
consistent enough to justify for purposes of discussion the assumption of
two alternative sets of characters, complex 10 and complex 15, each re-
sponsible for the antigenic and the phage receptor properties. Complex,
15 is genetically determined by the activity of phage e15; complex 10 by
its absence.

It should be realized that complex 10 and complex 15 are defined opera-
tionally only by the changes that the component properties undergo
during lysogenization by phage c Is. Although the receptor for phage ex6 is
present in strain A and absent in strain A(e16), we do not include it as
part of the definition of complex 10, because its changes during lyso-
genization have not been investigated in detail. The fact that receptors
for phage Cl341 can be absent from cells with antigen 10 (Uchida et al.,
1956) does not invalidate the inclusion of these receptors in defining
complex 10 with regard to the function of phage G5 as genetic
determinant.

The most remarkable finding is the extremely rapid appearance of
complex 15 in the infected bacteria and its relation to the activity of
phage ~1~ in any one of its forms. By the agglutination test, complex 15
can be detected within 5 minutes after infection. After 30 minutes, when
agglutination by anti-15 serum is almost as extensive as with cells of
A(el6); about 75 % of the cells can already be killed by phage G4 vir, and
by 60 minutes the average amount of ea4 receptors, measured by the rate
of phage adsorption per cell, is over half the maximum.

The rapidity of these events makes it possible to detect them by the
agglutination test in cells destined to lysis, using as determining phage
the mutant cl6 uir instead of G6. This has allowed us to prove that com-
plex 15 is formed equally well in the ly&-awaiting cells. Hence, the de-
termination of complex I5 does not require the function of a stable
prophage, which may be part of the bacterial chromosome (see Bertani,
1957). The assumption of such a requirement has been present in the
expression "lysogenic conversion," by which the occurrence of novel
properties in lysogenized bacteria has been named. At least for the anti-
genie change produced by phage e15 in Salmonella, our results show that,
provided suitable tests are available, the phage-determined properties
can be found even in bacteria that carry the determining phage in the
vegetative state. Additional evidence supporting this conclusion is being
obtained with other phage-controlled antigenic conversions of Salmonella
(Zinder, personal communicatOion; Fraser, Luria and Burrous, unpub-
lished) .

It has been pointed out before (Luria, 1954) that all alterations in the
patterns of biosynthesis in phage-infected bacteria may be considered as
an expression of the genetic function of phage, even though the phe-
nomena related to production of mature, infectious phage are not ob-
viously recognizable as bact.erial heredity (unless this is extended to
include virus infection as infectious heredity). An important example is
the recently discovered enzyme that syr$hesizes hydroxymethyl-
deoxycytidylic acid in E. coli cells infected with phage T2 (Flaks and
Cohen, 1957). This enzyme is present only in phage-infected cells. A
difference between the functions of vegetative phage and of prophage,
which might be considered as a form of "position effect," is observed in
the failure of the prophage as such to initiate phage maturation in the
host cell, as well as in differences in immunity and phage resistance
patterns between lysogenic cells and cells with vegetative phage (S.
Lederberg, 1957). It may be recalled that abortive transduction (Stocker
et al., 1953; Stocker, 1956; Lederberg, 1956; Ozeki, 1956) has aheady


88       UETAKE, LUBIA AND BURROUS
indicated that cell functions can be controlled by newly introduced,
unestablished fragments of genetic material.
The fact that complex 15 is formed in infected cells during the vege-
tative growth cycle of phage E l6 removes one of the criteria used to dis-
tinguish between conversion and transduction. In conversion, a phage
has been supposed to confer the new character to the lysogenized cells
even when previously grown on cells that did not have the character in
question. Since the character "complex 15" presumably appears in all
cells where ~16 grows, the distinction between conversion and obligatory
transduction becomes a matter of definition. In fact, high frequency
transduction by phage X is apparently mediated by defective phage
particles that have incorporated into their own genome a fragment of
bacterial genome (Arber et al., 1957). A converting phage may have a
similar origin. Alternatively, a converting phage may be a fragment of
bacterial genome that has acquired by mutation a viral function, that is,
the ability to control its own maturation into an infectious particle.
The loss of receptors for phage e l6 upon acquisition of complex 15, de-
termined by that phage, exemplifies a situation in which a lysogenic
bacterium fails to adsorb the phage related to its prophage. This situa-
tion is apparently common, although by no means general, in Salmonella
(see Boyd, 1954; Uchida el al., 1956). The frequent association between
phage activity and genetic control of surface constituents in a fragment
of genetic material may not be fortuitous. We may speculate as to its
relation to the ability of the phage genome to control the synthesis of the
specific apparatus by which the phage particles will combine with the
surface receptors of the host cell.

The change from complex 10 to complex 15 is not simply a change
from the expression of one genetic potentiality of the host cell to another
potentiality, triggered by the infecting phage. The eventual loss of
complex 15 in nonlysogenic segregants from infected cells is proof that
the phage genome is required to maintain the new character. In pre-
liminary tests, cells exposed to phage t I6 inactivated by a heavy dose of
ultraviolet light showed no antigenic changes. With phage that had re-
ceived a lower dose of ultraviolet light some antigen 15 appeared. It
should be possible to decide by quantitative studies whether the antigen-
controlling function can be dissociated from the reproductive function
of the phage.

The cells converted from complex 10 to complex 15 must retain the
genetic factors for complex 10 since the strains lysogenic for G6 revert

CONVERSION OF SOMATIC ANTIGENS IN SALMONELLA     89

specifically to the antigenic form 3,lO when they lose the prophage
(Uetake et al., 1955). These nonlysogenic derivatives can be isolated by
cultivation in presence of anti-15 serum. Phage cl6 appears to introduce
an additional dominant genetic element rather than to replace it by a
different one.

Bruner and Edwards (1948) reported the isolation of variants belong-
ing to group Ea (antigens 3,15) by cultivation of El cells in media with
anti-10 serum. Their anti-10 serum had been produced by absorption
with heavy suspensions of bacteria belonging to group E,. These may
have contained converting phage. When the experiment was repeated
using anti-10 serum prepared by absorption with washed, killed cells of
group Ez, the antigenic change was not observed (Iseki and Sakai,
1953a; Uetake, unpublished experiments).
Our experiments have shown that complex 15, although determined
by phage e15, can be present in bacteria that by segregation have lost
the phage genetic material, at least as detectable by production of phage
or of specific immunity in their progeny. In the transition period the
cells have both complex 10 and complex 15, as shown by agglutination
and phage-sensitivity tests. Such a coexistence has already been re-
ported, presumably as a permanent condition, in S. lexington and S.
maeallen infected with phage cl5 (Uetake et al., 1955; Nakagawa, 1955).
In S. anatum, however, the coexistence is a transient one, observed both
during t,he disappearance of complex 10 in the recently infected cells
and during its reappearance in cells that have segregated away the
phage @. What is the mechanism of these replacement phenomena?
The results on the receptors for the test phages, C&r and @ tir, are
compatible with `e assumption that, following infection with phage
.2
tlK, the existin complex 10 disappears faster than by simple dilutions
among the progeny cells, whereas in cells that lose phage cl5 the newly
formed complex 15 is simply diluted away. The results are not con-
clusive, because the experiments may be complicated by unrecognized
effects of the experimental conditions on adsorption of the test phages,
as explained in the experimental section. If the complex 10 initiallypres-
ent actually disappears, it may either decay by intrinsic instability, or
be sloughed off by the host cells, or masked by complex 15, or actually
transformed into complex 15. These possibilities should be amenable to
test by chemical isolation and isotope transfer experiments on the
somatic antigens. Additional information should be obtained by analysis
of other conversion phenomena observed in the E group of Salmonella


90           UETAKE, LURIA AND BURROUS

(Uetake et al., 1955) as well as in the B group (Iseki and Kashiwag,
1955; Terada et al., 1956). The observed transitions between alternative
cell properties in the phase of segregation of phage elements should prp-
vide a useful material for the study of gene action. It may, in fact, be
ultimately possible to detect various gene products-the antigen itself
and its component parts, as well as enzymatic or otherwise functional
constituents of the antigen-forming system-and to relate their presence,
amount, and function to the actual presence of the determining genetic
element in a cell or in its direct ancestors.

ACKNOWLEDGMENT

The senior author wishes to express his appreciation to Dr. T. Uchida, who
collaborated in the early phase of this work.

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