Temperature characterization of psychrotrophic and mesophilic Bacillus species from milk

A total of 50 isolates of Bacillus spp. and one reference strain were investigated for their growth at 6.5 degrees C for 10 d, 30 degrees C for 3 d and 40 degrees C for 2 d. The results obtained differentiated three physiological groups : one clearly psychrotrophic (able to grow at 6.5 degrees C in 10 d, but not at 40 degrees C in 2 d), one intermediate in psychrotrophy (it grew at both 40 and 6.5 degrees C) and one mesophilic (capable of growth at 30 and 40 degrees C; but not at 6.5 degrees 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 degrees C than other psychrotrophic Bacillus spp. Using eight strains, a correlation between differential growth at mesophilic temperatures (count at 30 degrees C minus count at 40 degrees C) and a standard psychrotrophic count at 6.5 degrees C for 10 d (r = 0.95) was obtained in mixed cultures when the psychrotrophic flora count was less than or equal to 1 log (cfu/ml) lower than the mesophilic count.