Influence of dairy products environment on the growth of Bacillus cereus

Bacillus cereus is a spore forming foodborne pathogen associated with two types of diseases, the emetic and the diarrheal syndromes [1]. Raw milk could be contaminated at farm level but also post-pasteurization recontamination of milk and dairy products by vegetative cells or endospores could occur [2, 3]. It is well known that pH is one of the most important parameters used to control bacterial replication in foodstuffs and nowadays low-pH foods are widely produced and consumed as they guarantee bacterial stability. Dairy products are characterized by different substrate acidity levels that can significantly influence bacterial growth. In the present study the ability of two Bacillus cereus strains, one clinical human isolate (GPe2) and one isolated from a dairy product (D43), were investigated to evaluate the in vitro growth at different pH values (from 3.5 to 7.5) at two different temperatures (15 and 37\ub0C). Moreover, the spores germination and growth of these two strains in different typologies of dairy products (unflavoured yogurt, taleggio cheese, mascarpone cheese, raw and pasteurized milk) was investigated at 15\ub0C. The spores were inoculated (inoculum concentration around 2-3 Log CFU/g) and microbiological analyses were performed at settled times during the storage of the products at 15\ub0C, minimum temperature of growth of the two microroganisms. B. cereus GPe2 and D43 showed their ability to grow at 37\ub0C from 5.5 to 7.5 and from 5.0 to 7.5, respectively. The growth ability of the two strains was moderately affected by the temperature of 15\ub0C, as growth was detected in a pH range between 5.5 and 7.5 and between 6.0 and 7.5 for GPe2 and D43 strains, respectively. Considering the behaviour in dairy substrates, no growth was observed in yogurt, likely due to the combined effect of low pH (7 Log CFU/g). An inhibitory effect of the natural microflora on B. cereus growth may also be assumed for taleggio cheese and raw milk, as in these matrices high natural Lactic Acid Bacteria (LAB) population and permissive pH values (>5.8 in taleggio cheese, >7 in raw milk) were detected. Finally, in pasteurized milk and mascarpone cheese, where pH was not preventive for the growth of B. cereus and where LAB loads were not significant, growth occurred quickly up to loads nearly of 7 Log CFU/g. Especially for the last two products, where pH and natural microflora do not exert an inhibition, correct hygienic procedures should be regularly applied during the production with the goal to reduce the risk of contamination by pathogenic B. cereus strains