High Genetic Diversity and Virulence Potential in Bacillus cereus sensu lato Isolated from Milk and Cheeses in Apulia Region, Southern Italy

Abstract

The Bacillus cereus group includes species that act as food-borne pathogens causing diarrheal and emetic symptoms. They are widely distributed and can be found in various foods. In this study, out of 550 samples of milk and cheeses, 139 (25.3%) were found to be contaminated by B. cereus sensu lato (s.l.). One isolate per positive sample was characterized by Multilocus Sequence Typing (MLST) and for the presence of ten virulence genes. Based on MLST, all isolates were classified into 73 different sequence types (STs), of which 12 isolates were assigned to new STs. Virulence genes detection revealed that 90% and 61% of the isolates harboured the nheABC and the hblCDA gene cluster, respectively. Ninety-four percent of the isolates harboured the enterotoxin genes entS and entFM; 8% of the isolates possessed the ces gene. Thirty-eight different genetic profiles were identified, suggesting a high genetic diversity. Our study clearly shows the widespread diffusion of potentially toxigenic isolates of B. cereus s.l. in milk and cheeses in the Apulia region highlighting the need to adopt GMP and HACCP procedures along every step of the milk and cheese production chain in order to reduce the public health risk linked to the consumption of foods contaminated by B. cereus s.l.

Keywords: B. cereus; MLST; dairy products; food safety.

Figure 1

Minimum spanning tree analysis, using BioNumerics version 7.1, of the 139 B. cereus isolates based on allelic profiles of seven housekeeping genes. Each circle corresponds to a sequence type (ST) and the size of the circle is related to the number of isolates found with that profile. Each colour inside the circles represents the source origin of the strains (green, Caciocavallo; red, Mozzarella; indigo, Ricotta; yellow, milk).

Figure 2

Bionumerics phylogenetic tree built on 139 B. cereus isolates. The tree is based on 73 ST profiles. The presence of virulence factors are marked in blue colour. The source and STs are listed next the tree.