The evaluation of culture media for the isolation of salmonellae from faeces.

Abstract

The sensitivity of different culture media for isolating salmonellae from the faeces of man, dog, horse, cow, sheep, pig, chicken, duck and turkey has been investigated. Suitable dilutions of tissue fluids of animals that had died from salmonella infection, and in which the numbers of viable salmonellae could be accurately estimated, had previously been added to the faecal specimens. Tissue fluids containing Salmonella dublin, typhi-murium, thompson, cholerae-suis, gallinarum and pullorum were used.Selenite and tetrathionate media were greatly superior to liquid desoxycholatecitrate medium, liquid Wilson and Blair medium, cacotheline broth and brilliant green peptone water. By the use of either selenite or tetrathionate media it was usually possible to recover salmonellae from faecal specimens to which less than ten salmonellae had been added. They could nearly always be recovered from specimens containing 100 salmonellae.Salmonellae were more easily recovered from the faeces of some species of animals than others. For selenite, the order of ease of recovery was horse, followed by sheep, human, cow, chicken, pig, turkey, dog and duck faeces, and for tetrathionate, horse, sheep, human, dog, pig, cow, turkey, chicken and duck faeces.Selenite medium was preferable to tetrathionate for examining cow and chicken faeces but the reverse was true in the case of dog faeces; slight differences only were noted in the other species. Taken as a whole, selenite was slightly superior to tetrathionate, butbest results were obtained by the use of both media.Some types of salmonellae were easier to recover from faeces than others.Of six salmonella types, Salm. thompson was easiest to recover followed by Salm. typhi-murium, dublin, gallinarum, pullorum and cholerae-suis, in that order.Erroneous results were obtained when laboratory cultures were used instead of infected tissue fluids.It was necessary to add several thousand salmonellae to faeces before they could be recovered by direct culture on desoxycholate-citrate-agar or Wilson and Blair solid medium.Salm. cholerae-suis and Salm. abortus-ovis were exceptional in that direct plating was superior to the use of enrichment media.With lightly infected specimens an incubation period of 24–30 hr. was optimum for selenite and tetrathionate media. A longer period was detrimental with tetrathionate but not with selenite medium.The combination of selenite and solid Wilson and Blair medium was sometimes too inhibitory to permit the growth of salmonellae.There was little advantage in preparing selenite medium with mannitol instead of lactose.