A SYSTEM FOR THE BACTERIOLOGICAL EXAMINATION OF WATER

333. The coliform flora of milk and dairy products

1. The sources of error and general significance of the presumptive coliform test have been studied, with special reference to manufactured dairy products.

2. Eosine methylene blue agar has been found reasonably satisfactory for isolation of cultures but unsatisfactory as a direct plating medium.

3. A wide survey of the coliform flora of raw milk and milk products has been made over a period of 11 years. The data were collected from 24,952 samples and include the incidence of presumptive positive tests at each stage of manufacturing processes, the occurrence of ‘false-positive’ tests, the detailed distribution of coliform types in individual dairy products, and some preliminary observations on ‘heat-resistant’ coliform strains.

4. Material examined included raw milk, pasteurized milk, ice cream, milk powder, condensed milk, pasteurized cream, clotted cream, butter, soft cheese, Cheddar cheese, processed cheese, swabs of dairy plant, churn rinses, and the atmosphere of dairy factories.

5. Detailed confirmatory tests were performed on 2490 presumptive positives, from which 2508 coliform cultures were isolated and classified.

6. A general ecological survey of the frequency of individual coliform types in dairying operations as a whole has been attempted.

7. Particular factors which may affect the distribution of coliform types in specific products or situations have been shown to include heat resistance, resistance to drying, chemical composition of the substrate, the effect of storage, and conditions prevailing on plant surfaces. It is suggested that the coliform flora may be partly the result of adaptation to conditions associated with each individual product.

8. The value of the presumptive coliform test has been discussed in respect of public health requirements, as a plant-control method, and in relation to economic aspects. The test is believed to be of great utility in the plant-control laboratory but of little value for public health purposes.

The Bacteriological Examination of Mussels

1. This paper reports the results obtained by the cultivation of the bacteria of ten batches each of ten mussels, 100 mussels in all, in lactose bile broth.

2. From these mussels 371 cultures of bacteria were obtained. 30·7 per cent, of these were not Gram-negative bacilli, 10·0 per cent. were Gram-negative bacilli which did not ferment lactose and glucose, 59·3 per cent. were coliform bacilli.

3. By reading the results in lactose bile broth after 24 hours' incubation, and by regarding as positive those tubes which show strong acid and good gas production the most reliable results were obtained. 93·0 per cent. of all typical, and 83·8 per cent. of all typical and atypicalB. colifound gave this reaction. 37·6 per cent. of the organisms giving this reaction were typicalB. coli, 61·1 per cent. of them were typical or atypicalB. coli, and 97·2 per cent. of them were coliform bacilli.

4. The writer's method is described in detail. Its essentials are the rejection of shell fluid, the standardisation of the volume of each mussel at 25 c.c. by the addition of saline, the inoculation of three volumes of mussel mince into lactose bile broth—0·5, 0·1 and 0·02 cc., each in duplicate, and the regarding as positive each of these amounts only when acid and gas are produced in 24 hours in both tubes.

5. It is suggested that a batch of mussels showing more than seven positives in the tubes inoculated with 0·5 cc.. of mince, or more than three in the tubes inoculated with 0·1 c.c. or more than one in the tubes inoculated with 0·02 c.c. should be considered as contaminated to an undesirable extent.

6. Three types of solid media—Salle's, MacConkey's and Burke-Gaffney's— were tested. None were considered suitable for the bacteriological examination of mussels.

7. Reasons are given for considering any solid medium inferior to fluid medium for the bacteriological examination of mussels.