Influence of controlled lactic fermentation on growth and sporulation of Bacillus cereus in milk

Abstract

The growth and sporulation of Bacillus cereus NVH 45 in a fermentor with controlled pH or simulated pH conditions were investigated. The study was carried out in a fermentor to measure the influence of a rapid and a slow lactic acid production on the inhibition of B. cereus in a controlled environment during the initial part of fermentation and to observe if other factors than lactic acid influenced the inhibition. In the controlled pH experiments the pH was allowed to decrease to an end pH 5.0, 5.5 or 6.0 either by Lactobacillus casei 2756 (a fast acid producer) or Lactobacillus acidophilus NCFB 1748 (a slow acid producer). In co-cultures of Lb. casei 2756 and B. cereus NVH 45, low numbers (10–70 cfu/ml) of B. cereus NVH 45 were observed at end pH 5.5 (72 h) while at pH 5.0 no viable cells (<10 cfu/ml) were detected (48–72 h). B. cereus NVH 45 did not sporulate in co-culture with Lb. casei 2756. In co-culture with Lb. acidophilus NCFB 1748, B. cereus NVH 45 sporulated and survived as spores. In these co-cultures B. cereus NVH 45 grew to higher maximum counts (>10 7 cfu/ml) than with Lb. casei 2756 (<10 7 cfu/ml). Significantly different amounts of lactic acid were observed between the two co-cultures after 7 and 12 h. A rapid decrease of pH appears to prevent B. cereus from sporulating and it seems that it is enough to just reach pH 5.0 rapidly and keep that pH to achieve the desirable inhibition of B. cereus. In the simulated pH experiments B. cereus NVH 45 was inoculated in the fermentor and the different pH developments from different LAB strains were monitored by addition of lactic acid. These experiments showed the same tendency: a fast pH reduction during the initial hours of fermentation, simulating lactococci, resulted in complete inhibition of B. cereus NVH 45 (<10 cfu/ml). However, when simulating the pH development of the two different Lactobacillus strains, complete inhibition of B. cereus NVH 45 was not seen. In co-cultures competition for nutrients with consequences for cell density appears to be important. Based on these results it seems that B. cereus must reach a certain density to induce sporulation.