A standard bacteriological test for molluscan shellfish should fulfil certain requirements stated: briefly, the test should show the degree of pollution, be accurate and rapid, and self-sufficient, not requiring subsequent confirmation. Review of the subject leads to recommendations, first, to overcome the technical difficulties of preparing samples for testing; secondly, on the nature of the test. In preparation: external shell sterilization can usually be omitted, shell water should be discarded and replaced by sterile water to make a total volume three times that of the body tissues. Pooling of individual shellfish into one sample is acceptable in routine examinations. In the test: a solid medium is preferable to a liquid medium, giving more accurate results, and review of existing tests leads to the conclusion that the use of roll tubes of MacConkey agar incubated at 44° should meet the requirements of a standard test.
A modification was found necessary in the MacConkey agar: a mixture of 2 % gelatin and 5 % agar is used instead of the normal 2 % agar. Mechanical rolling devices for tubes are described and figured.
Among other critical experiments, 1000 roll-tube colonies grown at 44° from shellfish included 969 with + + − − ‘IMViC’ reactions and 979 acid and gas producers at 44°. The coefficient of variation among replicate tests of samples of shellfish and water in roll tubes was not seriously greater than that for colony counts in Petri dishes at 37° with ordinary MacConkey agar.
Colonies in roll tubes incubated at 44° can be counted as conveniently and accurately as those on Petri dishes, and, in general it is concluded that the new method is more satisfactory for estimation of faecal coli than other methods at present in use.
Directions are given for the preparation of shellfish and inoculation into roll cultures, both for individual and for pooled examination, and the method of determining results is described.
The interpretation of results is discussed, and it is suggested that shellfish which in four out of five samples from the same source are free from faecal coli in 1 ml. quantities of body tissue should be regarded as satisfactory for food. The presence of more than two or three faecal coli per ml. of body tissue in any one sample calls for appropriate action according to the number present.