GROWTH AND COMPETITIVE NATURE OF Yersinia enterocolitica IN WHOLE MILK

A combination of four Y. enterocolitica strains was inoculated with and without five competing psychrotrophic bacteria, into milk that had been pasteurized using ultra‐high‐temperature procedures. Milk inoculated at two levels with Y. enterocolitica was held at 3°C and 25°C, without competing bacteria and monitored for growth. At 3°C, cells per ml of milk increased from 250 to more than 4.6 × 10 7 in less than 3 wk. Growth and identification were determined for Y. enterocolitica and competing organisms in milk held for 10 days at 3°C. After 10 days, the total count was 1.7 × 10 7 cells per ml of the inoculated milk. Pseudomonas fluorescens accounted for 43% of the 60 randomly picked colonies that were representative of the microbial population in the milk. Competing organisms were isolated in the following order of decreasing frequency: Micrococcus varians, Alcaligenes faecalis, Achromobacter pestifer , and Bacillus cereus. Y. enterocolitica was not present among the colonies randomly picked, but was detected in low numbers in Bismuth Sulfite Agar spread plates. This work indicated that Y. enterocolitica has the capacity for growth in milk at refrigeration temperatures, but is a poor competitor with common spoilage organisms.