Growth Characteristics of Yersinia enterocolitica in Pasteurized Skim Milk

Experiments were undertaken to determine the growth characteristics of five strains of Yersinia enterocolitica in pasteurized milk at 4°C. Pasteurized milk was inoculated with approximately 10 or 1000 cells/ml of Y. enterocolitica , and was incubated at 4°C for 0, 3, 7, 14 and 21 d. Each sample was spread-plated in duplicate on Tryptone Soya Agar, MacConkey Agar and Cefsulodin-Irgasan-Novobiocin (CIN) agar. Plates were incubated at 25°C for 48 h or at 32°C for 24 h and enumerated for total and Yersinia plate count. All five strains of Y. enterocolitica competed very well with background microflora of pasteurized milk and reached levels of log 5.0 to 7.0/ml after 7 d at 4°C. Level of inoculation had little or no effect on the total number of Y. enterocolitica after 14 or 21 d in pasteurized milk at 4°C. Generation times at 4°C were highly strain-dependent.