270
Effect of Penicillin Injections on Staphylococci
[XVII
Minett 7 found that hemolysis on ox-blood or sheep-blood agar and coagulation of human or
rabbit-blood plasma were characteristic of staphylococci isolated from milk samples, from udder
quarters affected with chronic mastitis, and from the affected quarters of cows showing symptoms of
acute, septic parenchymatous mastitis, and that it is improbable that cocci recovered from samples
of milk from normal quarters, which do not possess these properties, are pathogenic. Plastridge
and his associates 8 examined staphylococci isolated from milk samples from individual quarters of
milking cows for their ability to hemolyze ox-blood and coagulate human-blood plasma, for pigment
formation, leucocyte count, and other properties, using the leucocyte count as an index of patho-
genicity.
TABLE I
Schedule followed in taking milk samples and making injections of penicillin
Time
First day . . . . .
3 p.m . . . .
9 p.m
.
.
.
.
Second day . . . .
3
a.m.
.
.
.
9 a.m. . . .
3
p.m.
.
.
.
9
p.m.
.
.
.
Third day . .
.
.
3 a.m
.
.
.
.
9 a.m
.
.
.
.
.
3 p.m
.
.
.
.
Quarters received 8 injections
at 6 hour intervals
Took Sample 1 . . . . .
Milked out and injected quarter . .
Milked out and injected quarter . .
Took Sample 2 . . . . .
Milked out and injected quarter . .
Milked out and injected quarter . .
Took Sample 3 . . . . .
Milked out and injected quarter . .
Milked out and injected quarter . .
Took Sample 4
.
.
.
.
.
Milked out and injected quarter . .
Milked out and injected quarter . .
Took Sample 5 . . . . .
Quarters received 4 injections
at 12 hour intervals
Took Sample 1
Milked out and injected quarter
. . . .
Took Sample 2
Milked out and injected quarter
. . . . .
Took Sample 3
Milked out and injected quarter
. . . .
Took Sample 4
Milked out and injected quarter
. . . .
Took Sample 5
Took Samples 6, 7, 8, and 9 on the third, sixth, tenth, and thirteenth days following, immediately before the evening
milking.
From the results obtained, they concluded that ability to coagulate rabbit or human-blood plasma
is more closely related to pathogenicity than any other property. The greater number of the coaguse
positive strains were hemolytic but some were not. They also concluded that staphylococci from
the udder which are hemolytic on ox-blood agar are usually pathogenic whether they have coagulative
ability or not, as are also staphylococci associated with a leucocyte count of 500,000 or over provided
the cow is not in the early or late stage of lactation. They found no definite correlation between
pigment formation and the leucocyte count. Nearly as many strains of staphylococci associated
with a low leucocyte count produced pigmented colonies as those associated with a high count.
Chapman, Berens, Peters, and Curcio, 9 who studied staphylococci of human origin and determined
pathogenicity by injecting cultures into rabbits, found that while strains which coagulated human-
blood plasma were usually pathogenic regardless of the color of the colonies, those which produced
hemolytic yellow colonies were usually pathogenic regardless of coagulative ability and hemolytic
noncoagulating strains that produced white colonies were usually nonpathogenic.
Plan of experiment
In selecting the cows for this experiment, ability to hemolyze ox-blood, coagulate human-blood
plasma, and produce pigment were used, together with the leucocyte count, to differentiate patho-
genic staphylococci from the nonpathogenic cocci (micrococci) which are commonly present in cow's
milk. The methods by which these determinations were made are described below.
Samples of milk were taken from each quarter of the udder at the time of the evening milking
but before the cow was milked. After the udder had been prepared in the usual way to prevent
external contamination, three streams were expressed from the quarter and discarded, and 15 to
20 c.c. of milk were then drawn into a sterile container, which was placed in a refrigerator until
removed to the laboratory.