The San Francisco call. [volume] (San Francisco [Calif.]) 1895-1913, 1904-08-01
Counting Bacteria.


In a great many cities where the
quality Of the milk supply is given
some attention the bacterial test has
been adopted among the others for the
determination of the quality of the
milk. This has just been done in Bos-
ton. In that city a standard which
would amount to about 100000 bsc-
terra. to the quantity of milk which an
ordinary. household tumbler would
hold has been adopted
1t may be interesting to learn how
the psychologists ever counted all
these germs. or how they manage to
count the germs in samples from Is
10 80 different places every day. DR..
Slack, says the Boston Transcript, who
attends TO it all personally, goes out
every morning to collect samples. He
visits the dairy stations along the rail-
roads, and eventually will visit stores
and restaurants also, and collects
about 80 samples. He draws the milk
from the can by means of a small ster-
ilized and marked glass tube. The large
tube is then placed in the box, and the
small tube is put out of commission
for the present, or until it has been
sterilized again. Another tube is filled
in the same way from another can,
perhaps at another station, and when
30 samples have been taken the bacter-
iologist returns to the laboratory on
Sudbury street to begin his research
work. The. temperature Of the milk
ia the big glass tubes is kept just above
freezing, by ice in both ends of the
copper box, so that the germs in the
milk have no opportunity to multiply
from the time the sample is taken until
its purity has been tested.
At the laboratory the bacteriologist
draws about 10 drops from the supply
in the big tube-the exact amount be-
ing one c. c.-and deposit it in q jar.
containing 100 C. C: of sterilized water
He shakes the jar well and then draws
one c. c. from the diluted contents and
drops it into another bottle containing
100 c. c. of pure steriIized water. Only
one c.. c. (about Is drops( from the
contents Of this second bottle is ex-
amined and it. is only matter of
arithmetic. to. determine that the milk
subjected to. the actual bacterial test
has been diluted 10000 times, so that
.the number of germs found in the Is
drops from the second. bottle must be
multiplied by 10000 in order to find
how much bacteria there is. in Is drops
Of undiluted milk Sometimes there
are so many in the diluted. drops that
it is impracticable to count them all.
But the germs in milk are so infin-
itesimal that it is impossible to isolate
them and find even the Comparatively
few that exist in Is drops. Of so highly
diluted milk. The 10 drops, therefore,
are emptied into "a shallow glass dish
that. can be closed alright, and spread
out over coat Of agar-agar jelly, on
which the germs Will feed and multi
ply. In "a hours each germ multiplies
so enormously that it becomes sur
rounded by a cluster as large in cir
conference as a pea, and under read
/ ing glass magnifying about three times
each cluster may be distinguished with
out the slightest difficulty. Each clus-
| ter represents one original germ, and
| the total number of clusters found in
the glass dish is multiplied by 10000
/ to find how many germs there were
! in one C. C. of undiluted milk, and the
second total may be multiplied by 240
to find how many germs there are in
a tumbler full of such milk, since a
tumbler holds about 240 C. g