The prevalence and characterization of Bacillus cereus isolated from raw and pasteurized buffalo milk in southwestern China

Bacillus cereus is an important food-borne pathogenic bacteria and a putrid microorganism in the dairy industry. Raw and pasteurized buffalo milk play important roles in the dairy market in southwestern China. However, the reports on the prevalence and characterization of B. cereus strains isolated from the above sources are lacking. In this study, 150 raw buffalo milk samples and 300 pasteurized buffalo milk samples were collected from 3 provinces in southwestern China. The genotype, virulence gene distribution, antibiotic resistance, and biofilm-forming ability of isolates were analyzed. Ninety-six B. cereus strains were isolated and identified: 50 isolates (33.3%) from buffalo raw milk and 46 isolates (15.3%) from pasteurized buffalo milk. These strains were classified into 41 sequence types (ST) and 5 groups, of which ST857 was the predominant ST. The detection rates of virulence genes nheABC cluster, hblACD cluster, cytK, bceT, entFM, hlyII, and cesB were 89.6%, 13.5%, 64.6%, 71.9%, 84.4%, 62.5%, and 6.25%, respectively. The antimicrobial susceptibility testing showed that more than 90% of the isolates were susceptible to gentamicin, chloramphenicol, ciprofloxacin, erythromycin, vancomycin, and tetracycline, as well as resistant to ampicillin, cefepime, oxacillin, and rifampin. The results of biomass biofilm evaluation of the isolates on the stainless-steel tube showed that the optical density values at a wavelength of 595 nm of all strains in group I were greater than 1, with the strongest overall biofilm-forming ability among 5 groups, and the overall biofilm-forming ability of group III was the weakest. There was a relationship between the biofilm-forming ability and phylogenetic relationship of B. cereus strains. Taken together, our findings are the first to report the contamination situation and characterization of B. cereus isolated from raw and pasteurized buffalo milk in southwestern China as well as indicate the potential risk posed by this pathogen to dairy industry and public health.