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bacteria in milk which is being condensed. Streptococci, which are the most abundant
organisms in ordinary milk, are the most easily destroyed, and although I have found
them present in certain samples, they are an inconspicuous feature of condensed milk.
Bacillus coli and its allies are also largely destroyed. Some observers have failed to
detect bacilli of this group in samples examined, but I have noticed them in seven or
eight specimens, and if I had employed differential culture media, I should probably
have found them more frequently present. Bacillus proteus is also occasionally found.
Staphylococci, on the other hand, are commonly very abundant, in many cases only
staphylococcus albus, but often typical staphylococcus aureus, rapidly liquefying gelatin
and fermenting mannite. Certain samples yield pure cultures of staphylococcus aureus
and albus. Bacterial spores escape destruction, and colonies of Bacillus mesentericus and
its allies are often seen in cultures, but they are rarely numerous, probably because they
are sparse and accidental air contaminations of the original milk, and cannot multiply
in condensed milk. I have not devoted special attention to the spores of anaerobic bacilli
in condensed milks, but I know that they may be present.
So far as my present observations go, the only microbes which demand more
detailed consideration are the staphylococci in condensed milk. No one who is acquainted
with the methods of preparation of this product, which is not sold as sterile, and with
the bacteriology of fresh milk, could condemn it because it contained the bacteria of
fresh milk in greatly diminished numbers, or such harmless air contaminations as
Bacillus mesentericus. But when a condensed milk yields pure and abundant cultures of
Staphylococcus pyogenes albus and aureus the question of its wholesomeness demands
very careful consideration.
When a film from the sediment of condensed milk is crowded with polynuclear
leucocytes and swarming with staphylococci, and when a cultivation yields a pure growth
of staphylococcus pyogenes aureus and albus, the inference that pus is present in the milk is
at first sight irresistible. From the examination of this series of machine skimmed milks
a general impression is left on my mind that an apparent abundance of polynuclear cells
as often associated with large numbers of staphylococci. But I must admit that this
rule is not invariable. In some cases I have found an abundance of polynuclears with
few staphylococci, and in others crowds of staphylococci with relatively few polynuclears.
Moreover, in none of these cases have I found the percentage of polynuclears increased
above that in good preserved milks.
I have therefore considered very carefully whether any alternative explanation of
the phenomenon is possible, and the following one seems to me at least conceivable.
The polynuclear cells do not, so far as I can determine, exceed in absolute numbers
what might be expected from the content in normal milk of such cells. No special
explanation is needed for their presence. As regards the staphylococci, it is known that
these organisms, indistinguishable from the ordinary pyogenic staphylococci aureus and
albus, occur in the skin of cows, and their presence in milk is well known. Amongst
non-sporing bacteria staphylococci are remarkably resistant to heat, and would be likely
to survive better than most other bacteria the processes involved in condensing a milk.
If, now, they were able to grow in the condensed milk, an explanation of the
phenomenon I am considering would be forthcoming, for we should find a pure culture
of staphylococci intermingled with the cells characteristic of pus. From the grouping
of the staphylococci in large clumps in the milk sediments, and from their active and
well stained appearance, I think it probable that they do grow in condensed milk,
though I have not at present carried out experiments to prove this.