235. The enrichment ofaerogenes-cloacaetypes in milk held at low temperatures: with observations on the relative rates of growth ofaerogenes-cloacaeandb. colitypes in milk at different temperatures

Abstract

1. Seventy-five pure cultures of various types of coliform bacteria were grown in sterilized or partially sterilized milk for 24 hr. at 17° C. The cultures ofB. aerogenes, B. oxytocusandB. cloacaetypes as a rule multiplied much more rapidly than those of theB. colitypes. Similar results were obtained when twenty-nine cultures of various coliform types were grown in raw milk for 24 hr. at 17° C. There was generally little or no tendency to clumping of the organisms in the milk, even after 3 days' incubation at 17° C.2. Forty mixed cultures, each consisting of anaerogenes-cloacaetype and aB. coli, were grown in milk for 24 hr. at 17° C. In thirty-four instances theaerogenes-cloacaetype multiplied more rapidly than theB. coli.3. Twenty-one of these mixed cultures were also grown in milk for 24 hr. at 22, 30 and 37° C. At 22, as at 17° C, theaerogenes-cloacaetype multiplied as a rule more rapidly than theB. coli, while at 30 and 37°C. the reverse was the case.4. Twelve specimens of bovine faeces were inoculated into raw milk and the cultures kept at 17° C. for 36 hr.Aerogenes-cloacaetypes occurring in the faeces became enriched in the milk, the coliform flora of the milk at the end of the incubation period frequently consisting chiefly of these types.5. The greater incidence ofaerogenes-cloacaetypes in summer milk in Scotland as compared with winter milk, found in a previous investigation, may be explained by the fact that when milk is contaminated directly or indirectly with faeces, these types become enriched at the temperatures of holding commonly employed in summer. Such enrichment does not occur in winter, the holding temperature of the milk being as a rule so low that there is little or no proliferation of any coliform types. Consequently, the relative proportions of the various coliform types in winter milk tend to remain similar to those in the faeces.