Organic vs. Conventional Milk: Uncovering the Link to Antibiotic Resistance in Bacillus cereus sensu lato

Bacillus cereus sensu lato (B. cereus s.l.) comprises mesophilic and psychrotolerant bacteria commonly found in natural environments as well as in organic and conventional milk. Due to their potential toxigenicity and antibiotic resistance, these bacteria pose a significant threat to consumer health. Organic milk production, which prohibits the use of antibiotics and artificial additives, may influence the composition of microbiota between milk types. This study aimed to compare the antibiotic resistance profiles and enterotoxic potential of B. cereus s.l. isolates from organic and conventional milk. The results indicate that, although conventional milk contains on average 3 times fewer B. cereus s.l. isolates, it has 10-15% more resistant isolates to selected beta-lactams, macrolides, and aminoglycosides. Regarding drug resistance, 21% of B. cereus s.l. isolates were multidrug-resistant, and as many as 42% were non-susceptible to two classes of antibiotics. Even among the sensitive isolates, bacteria from conventional milk exhibited on average 2.05 times higher MICs (minimal inhibitory concentrations) for beta-lactams, 1.49 times higher for erythromycin, 1.38 times higher for vancomycin, and 1.38 times higher for azithromycin. Antibiotic resistance was mostly associated with the origin of the isolates. These findings underscore the need for ongoing monitoring of antibiotic resistance and enterotoxicity among opportunistic B. cereus s.l. strains, which may pose challenges for public health and veterinary medicine. The results highlight that selective pressure associated with antibiotic use can drive resistance development in bacteria that are not the primary targets of antimicrobial therapy.