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count of staphylococci without trouble. The success of volunteer
experiments will depend on the number of volunteers, the amount
of cheese eaten, the susceptibility of each volunteer and, of course,
the number and types of coagulase-positive staphylococci in the
cheese which in turn will govern the amount of enterotoxin in the
cheese. It was considered that tasting was not worth while unless
at least 15 to 20 persons per test took part.
It was reported by Dr. B. C. Hobbs, Central Public Health
Laboratory, and D. W. H. H. Merivale, Guy's Hospital, that strains
of coagulase-positive staphylococci isolated from various samples
of cheese were penicillin resistant. Professor R. Knox, Guy's Hospital,
was unable to find traces of penicillin by microbiological assay
in samples of the cheese of high and low staphylococcal counts; this
was expected since penicillinase produced by the resistant staphylococci
would have destroyed free penicillin.
Phosphatase tests on the cheese were carried out but, they could
not be relied upon to show that pasteurisation of the milk was effective
or not. The verbal statements of some of the exporters indicated
that only sub-pasteurisation temperatures of milk had been reached
It has been widely, but mistakenly, held that good cheese cannot be
made from pasteurised milk. 5
Normally, the acidity of the cheese prevents the development
of staphylococci which may be present in variable numbers in milk.
When insufficient acid is formed by a slow starter culture the staphylococci
are able to grow in the curd and persist in the cheese. This
may be due to bacteriophage action in the starter culture, but the
presence of penicillin resistant staphylococci in such large numbers in
cheese, nine to twelve months following production, strongly suggested
that penicillin was the inhibitory factor in the milk, from the
widespread use of penicillin by the New Zealand farmers in an attempt
to control mastitis in their herds. The method used was to express
a tube of penicillin ointment into the affected udder via the teat.
Some penicillin would come down with the milk eventually together
with varying numbers of penicillin resistant staphylococci. Coagulasepositive
staphylococci are commonly found in raw milk.
Quantities of bulked milk destined for cheese making, therefore,
may have had variable amounts of penicillin and staphylococci
present. Starter cultures, when added to this milk, would be killed
by the penicillin and hence acidity would not develop, and the resistant
staphylococci would multiply profusely in the curd and during
maturation.
On 12th May 1961 the first meeting of the London riparian
medical officers was held with the trade representatives to discuss
these new problems with cheese. It was considered that further
information was required on the extent of the contamination and it
was suggested that intensive sampling by batches should be carried
out. Although desirable, it was not practicable since the bacteriologists
could not have examined the thousands of cheese samples
which this would have entailed. It was, therefore, decided to carry
5 Davis J. G. Dictionary of Dairying.