Prevalence, Virulence Genes, Antimicrobial Susceptibility, and Genetic Diversity of Bacillus cereus Isolated From Pasteurized Milk in China

Abstract

<i>Bacillus cereus</i> is a common and important food-borne pathogen that can be found in various food products. Due to low-temperature sterilization for a short period of time, pasteurization is not sufficient for complete elimination of <i>B. cereus</i> in milk, thereby cause severe economic loss and food safety problems. It is therefore of paramount importance to perform risk assessment of <i>B. cereus</i> in pasteurized milk. In this study, we isolated <i>B. cereus</i> from pasteurized milk samples in different regions of China, and evaluated the contamination situation, existence of virulence genes, antibiotic resistance profile and genetic polymorphism of <i>B. cereus</i> isolates. Intriguingly, 70 samples (27%) were found to be contaminated by <i>B. cereus</i> and the average contamination level was 111 MPN/g. The distribution of virulence genes was assessed toward 10 enterotoxigenic genes (<i>hblA, hblC, hblD, nheA, nheB, nheC, cytK, entFM, bceT</i>, and <i>hlyII</i>) and one emetic gene (<i>cesB</i>). Forty five percent strains harbored enterotoxigenic genes <i>hblACD</i> and 93% isolates contained <i>nheABC</i> gene cluster. The positive rate of <i>cytK, entFM, bceT, hlyII</i>, and <i>cesB</i> genes were 73, 96, 75, 54, and 5%, respectively. Antibiotic susceptibility assessment showed that most of the isolates were resistant to β-lactam antibiotics and rifampicin, but susceptible to other antibiotics such as ciprofloxacin, gentamicin and chloramphenicol. Total multidrug-resistant population was about 34%. In addition, <i>B. cereus</i> isolates in pasteurized milk showed a high genetic diversity. In conclusion, our findings provide the first reference on the prevalence, contamination level and characteristics of <i>B. cereus</i> isolated from pasteurized milk in China, suggesting a potential high risk of <i>B. cereus</i> to public health and dairy industry.