Prevalence, enterotoxin-gene profiles, antimicrobial resistance, and genetic diversity of Bacillus cereus group in aquatic environments and shellfish

Bacterial strains of the Bacillus cereus group produce various toxins that cause diarrheal and emetic food poisoning. In this study, five main oyster farming areas and 15 fishing ports in Taiwan were examined for the status of B. cereus group bacteria inhabiting seawater and shellfish. On average, bacteria of the B. cereus group were detected in 32.6% of the seawater samples (n = 89) and 2.5% of the oysters (n = 81) in the oyster farming areas and in 7.9% of the seawater samples (n = 202) and 0.68% of the shellfish products (n = 292) in fishing ports. To trace the potential source of B. cereus group bacteria in intertidal oyster farming areas, we simultaneously explored their terrestrial river basins. In total, 44 B. cereus group strains were purified and cultured from water and shellfish for the analysis of virulence genes, panC gene typing, antibiotic susceptibility testing, and enterobacterial repetitive intergenic consensus PCR (ERIC-PCR) fingerprinting. The enterotoxin-coding genes nheABC, hblCDA, entFM, cytK-1, and cytK-2 were detected in 70.4%, 84.1%, 97.7%, 72.7%, and 75% of the total isolates, among which 40.9% carried all these genes. According to panC gene analysis, the dominant isolates belonged to the panC group IV. In antibiotic susceptibility tests, most B. cereus group isolates were resistant to ampicillin (97.7%) and sulfamethoxazole/trimethoprim (100%). The percentage of multidrug resistant B. cereus group isolates was 34.6%. Finally, the 44 B. cereus group isolates were classified into 43 types and categorized into five clusters using ERIC-PCR fingerprinting. The B. cereus group isolates from different oyster farming areas were concentrated within the two main clusters; however, those from river basins displayed a wide genetic diversity, indicating the presence of multiple sources of B. cereus group bacteria in river basins.