THE SUITABILITY OF CERTAIN OBLIGATELY ANAEROBIC NONSPOREFORMING ENTERIC BACTERIA, AS PART OF A MORE EXTENDED BACTERIAL ASSOCIATION, AS INDICATORS OF 'FAECAL CONTAMINATION'OF FOODS

SUMMARY: The suitability, as indicators of the faecal contamination of foods, of two types of obligately anaerobic nonsporeforming bacteria which are very numerous in faeces, viz. members of the genera Bacteroides and Bifidibacterium , was tested. For the counting of Bacteroides , Beerens'bile-azide agar (1957) enriched with 10% (v/v) of blood, and incubated in oval cross section tubes, was found useful. Bifidibacterium could be counted very well in Frisell's tomato-azide agar (1951), incubated in the same way . Both media being not too selective, confirmation of representative numbers of colonies was necessary. After microscopical examination, the biochemical study of properties such as catalase reaction, oxygen tolerance, carbohydrate fermentation and nitrate reduction may be required.
The 3 Bacteroides strains tested died off rather rapidly when present in sterile ice cream under conditions where Enterobacteriaceae and faecal streptococci increased in numbers. Accordingly they were never found in considerable numbers in some 25 samples of food which appeared to contain very high numbers of other bacteria of faecal origin. The 8 Bifidibacterium strains investigated appeared more resistant. However, under conditions permitting growth, they increased less than the Enterobacteriaceae and faecal streptococci; and where the latter did not increase but did survive, the Bifidibacterium decreased in numbers. In line with these findings Bifidibacterium was not even found in numbers of the order of 10/g in the 25 samples of food mentioned above. Neither Bacteroides nor Bifidibacterium can therefore be considered suitable as an indicator of the contamination of foods with bacteria of faecal origin and of the subsequent growth of these bacteria therein.