Temperature characterization of psychrotrophic and mesophilic Bacillus species from milk

Abstract

A total of 50 isolates of Bacillus spp. and one reference strain were investigated for their growth at 6·5°C for 10 d, 30°C for 3 d and 40°C for 2 d. The results obtained differentiated three physiological groups: one clearly psychrotrophic (able to grow at 6·5°C in 10 d, but not at 40°C in 2 d), one intermediate in psychrotrophy (it grew at both 40 and 6·5°C) and one mesophilic (capable of growth at 30 and 40°C, but not at 6·5°C). The proportion of strains in the second group was higher among isolates of B. cereus than for other Bacillus spp. However, the proportion of real mesophilic strains was lower for B. cereus isolates. Psychrotrophic B. cereus grew better at both 6·5 and 30°C than other psychrotrophic Bacillus spp. Using eight strains, a correlation between differential growth at mesophilic temperatures (count at 30°C minus count at 40°C) and a standard psychrotrophic count at 6·5°C for 10 d (r=0·95) was obtained in mixed cultures when the psychrotrophic flora count was [les ]1 log (cfu/ml) lower than the mesophilic count.