Bacillus cereus s.l. from ready-to-eat products in Poland: Classical to whole-genome sequencing approach

The Bacillus cereus group ( B. cereus s.l. ) comprises related Gram-positive, spore-forming bacteria commonly found in the environment and implicated in food poisoning via toxins such as cereulide, hemolysin, non-hemolytic enterotoxin BL, and cytotoxin K. This study continues earlier monitoring (2016–2020) by analyzing 20,275 food samples in Poland between 2021 and 2024, forming the largest B. cereus surveillance to date. Disqualification rates due to B. cereus were 1.25% (2021), 0.76% (2022), 1.13% (2023), and 0.73% (2024). High genetic diversity, numerous virulence and resistance genes, and diverse toxin profiles were identified among 700 isolates: A (18.4%), B (0.7%), C (12.4%), D (23.4%), E (3.7%), F (28.6%), G (5.4%), H (0.4%), I (1%), and K (0.1%). Forty selected isolates underwent whole-genome sequencing to assess resistance, virulence, and biofilm-related genes. Two novel MLST sequence types (ST3379, ST3380) were discovered. Biofilm production (crystal violet assay in BHI) showed increased formation over time: after 24 h, 50% were non-producers, 20% weak, 12.5% moderate, 17.5% strong; after 48 h, these proportions shifted to 10%, 20%, 12.5%, and 57.5%, respectively. The most common phenotypic–genotypic profile was type 14 (beta-lactam resistance: bla +, bla2 +, fosB +), followed by types 8 ( bla +, bla2 +) and 16 ( bla +, bla2 +, fosB + with beta-lactam and erythromycin resistance). These findings highlight the need for continued food monitoring for B. cereus s.l. in the context of food safety and public health. • A total of 94.4% of the 700 B. cereus s.l. isolate abhor hbl , nhe , cytK , ces genes • Identified new K toxin profile was found ( hbl , nhe , ces ) • Identification of 24 sequence types and 5 clonal complexes within the isolates • Two previously unreported STs were identified namely 3379 and 3380