POST-PASTEURIZATION PERSISTENCE OF ENTEROTOXIGENIC MULTI-DRUG RESISTANT BACILLUS CEREUS IN THE DAIRY CHAIN

Abstract

Milk is a crucial global food source, yet raw milk serves as vehicle for foodborne pathogens, including spore-forming bacteria that challenge thermal processing. The effectiveness of standard batch pasteurization (63°C/30 min) against the public health risk posed by Bacillus cereus in raw milk from diverse Egyptian sources remains ambiguous. Fifty raw milk samples from farms, shops, smallholders, and street vendors were analyzed pre- and post-laboratory pasteurization for physicochemical quality and microbial load. Surviving B. cereus isolates (N=11) were characterized by five enterotoxin genes and their resistance to ten antibiotics. The findings revealed significant differences in the chemical composition within raw milk samples, where milk from dairy farms and small holdings exhibiting higher fat and protein levels compared to milk from street vendors and dairy shops. Street vendors' milk showed the poorest physicochemical quality, with the highest levels of added water (8.8%) and the highest microbial contamination, including a significantly higher B. cereus count (0.8 ± 0.3 log₁₀ CFU/mL). Pasteurization effectively lowered aerobic plate counts by 99.99% and eliminated coliforms and Enterobacteriaceae but was completely ineffective against aerobic spores and B. cereus counts, which remained unchanged. Genotypic profiling of the B. cereus isolates (N=11) revealed a high prevalence of enterotoxin genes, primarily nhe (81.8%) and bceT (72.7%). Furthermore, these isolates showed high resistance to penicillin (100%), trimethoprim-sulfamethoxazole (90.9%), and cefoxitin (81.8%). The study concludes that pasteurization alone is insufficient to eliminate the critical public health risk posed by virulent and multi-drug resistant B. cereus in raw milk, underscoring a critical need for improved pre-processing hygiene and alternative control strategies.