Development of a new chromogenic medium for the enumeration of Bacillus cereus in various ready-to-eat foods

Isolation and characterization of novel Bacillus strains with superior probiotic potential: comparative analysis and safety evaluation

Despite the current use of some Bacillus spp. as probiotics, looking for and introducing new efficient and safe potential probiotic strains is one of the most important topics in both microbiology and food industry. This study aimed to isolate, identify, and evaluate the probiotic characteristics and safety of some Bacillus spp. from natural sources. Thirty-six spore-forming, Gram-positive, and catalase-positive Bacillus isolates were identified in 54 samples of soil, feces and dairy products. Bacterial identification was performed using 16S rDNA sequencing. To evaluate the probiotic potential of isolates, the resistance of bacterial cells to simulated gastrointestinal tract (GIT) conditions, the presence of enterotoxin genes, their susceptibility to antibiotics, antimicrobial and hemolytic activities and biochemical profiles were investigated. The results revealed that eight sporulating Bacillus spp. isolates fulfilled all tested probiotic criteria. They showed a high growth rate, non-hemolytic and lecithinase activity, and resistance to simulated GIT conditions. These strains exhibited broad-spectrum antibacterial activity against pathogenic bacteria. In addition, they did not exhibit antibacterial resistance to the 12 tested antibiotics. The results of this study suggest that these isolates can be considered as candidates for functional foods and as safe additives to improve diet quality.

Performance Testing of Bacillus cereus Chromogenic Agar Media for Improved Detection in Milk and Other Food Samples

In this study, the performance of four alternative selective chromogenic B. cereus agar was compared to the reference mannitol-yolk polymyxin (MYP) agar (ISO 7932) using inclusion and exclusion test strains (n = 110) and by analyzing naturally contaminated milk and other food samples (n = 64). Subsequently, the panC group affiliation and toxin gene profile of Bacillus cereus senso lato (s.l.) isolates were determined. Our results corroborate that the overall best performing media CHROMagar™ B. cereus (93.6% inclusivity; 82.7% exclusivity) and BACARA^® (98.2% inclusivity, 62.7% exclusivity) are more sensitive and specific compared to Brilliance™ B. cereus, MYP and ChromoSelect Bacillus Agar. Both media allow unequivocal detection of B. cereus with low risks of misidentification. Media containing ß-D-glucosidase for the detection of presumptive B. cereus may form atypical colony morphologies resulting in a false negative evaluation of the sample. Naturally contaminated samples presented high numbers of background flora, while numbers of presumptive B. cereus were below the detection limit (<10 CFU g⁻¹ or mL⁻¹). Recovery after freezing resulted in the highest detection of B. cereus s.l. on BACARA^® (57.8%), CHROMagar™ B. cereus (56.3%) and MYP agar (54.7%). The panC/toxin profile combination IV/A was the most abundant (33.0%), followed by III/F (21.7%) and VI/C (10.4%). More panC and toxin combinations were present in 15.6% of samples when reanalyzed after freezing. In order to improve detection and confirmation of B. cereus s.l. in food samples, we recommend the parallel use of two complementary selective media followed by molecular characterization (e.g., panC typing combined with toxin gene profiling). When determining psychrotolerant or thermophilic members of the B. cereus group, the selective agar media should additionally be incubated at appropriate temperatures (5 °C, ≥45 °C). If high-risk toxin genes (e.g., ces or cytK-1) are detected, the strain-specific ability to produce toxin should be examined to decisively assess risk.