Prevalence, toxigenic profiles, multidrug resistance, and biofilm formation of Bacillus cereus isolated from ready-to eat cooked rice in Penang, Malaysia

Abstract

Bacillus cereus is a foodborne pathogen which causes intoxication and toxico-infections in human. Spore- and biofilm-forming abilities of B. cereus enable its persistence in various ecological niches and food products, resulting in its presence even in processed foods such as cooked rice. Violation of time-temperature control creates favourable conditions for the growth of B. cereus which then produce emetic toxin and enterotoxins. Moreover, emerging antibiotic resistance in B. cereus promotes concern such as resistant strains of B. cereus facilitate in enrichment and emergence of more antibiotic resistant strains. Hence, this study was carried out to determine the prevalence of B. cereus in 100 ready-to-eat (RTE) cooked rice samples randomly collected from food outlets in Penang Island and to study the different B. cereus strains' characteristics which are relevant to foodborne illnesses, antibiotic resistance and potentiality to cause recurrent contamination. Aerobic plate count method was used for isolation and enumeration of B. cereus and the identities of the isolates were confirmed through 16S rRNA gene PCR and sequencing. PCR analysis was incorporated to detect 9 toxin-associated genes. The antibiogram of the B. cereus isolates against 13 antibiotics and biofilm-forming ability were determined by Kirby-Bauer disc diffusion method and submerged-biofilm assay respectively. A total of 34 B. cereus positive samples were identified with the bacterial loads ranging from 3.51 to 5.95 log CFU/g. High percentage (82.4%) of the isolates carried at least 1 toxin gene, with higher detection rates of most diarrheal genes (58.8–76.5%), than emetic toxin gene (14.7%). The percentage of bacterial isolates displaying resistance against ampicillin, penicillin and trimethoprim were in the range of 79.4–94.1%. High percentage was observed for sensitivity to antibiotics such as ciprofloxacin (94.1%), chloramphenicol (94.1%), gentamicin (88.2%), kanamycin (82.4%), vancomycin (82.4%) and tetracyline (73.5%). Twelve isolates were identified as multidrug-resistant (MDR) strains. The presence of antibiotic resistant B. cereus especially MDR strains in food commodities indicate the role of food systems as reservoir for resistance genes. High percentage (55.9%) of the isolates in this study were capable of biofilm formation, a phenotype that promotes recurrent contamination in food processing lines. In conclusion, RTE cooked rice samples may act as a vehicle for B. cereus dissemination and the characteristics displayed by the isolates reflect on their role in potential pathogenicity, due to the organism's resistance and multiple toxin genes.