Foodborne intoxication most likely linked to Bacillus cereus in a school canteen hot ready-to-eat meal.

On Tuesday, September 9, 2025, the Cantonal Laboratory of Basel City received reports of cases of vomiting following the consumption of a macaroni dish served at a school canteen. The outbreak involved 27 children, and the emetic symptoms started 40 - 90 minutes after lunch. Microbiological analyses of food samples obtained from the canteen kitchen revealed the presence of B. cereus at 1100 colony forming units (cfu)/g in leftovers of a béchamel sauce that was an ingredient of the macaroni dish. Nanopore long-read sequencing methodology and the BTyper3 tool were applied to identify B. mosaicus subsp. cereus biovar Emeticus (B. Emeticus) as the most likely etiological agent of this outbreak. Strain B. Emeticus BH-1 belonged to panC group III, sequence type (ST)164 and contained emetic toxin (cereulide) genes cesA, cesB, cesC, and cesD, but lacked enterotoxins nheA, hblD and cytK1 that cause diarrhea. This is one of few reports of B. Emeticus ST164 implicated in food intoxication. Based on the epidemiological and microbiological investigations, the macaroni dish was identified as the most likely vehicle of transmission in this outbreak, although definitive evidence was limited by the lack of retentionsamples of the full meal. Inadequate temperature control during food holding and serving in the school canteen likely led to B. cereus proliferation and toxin production and was the most probable risk factor that contributed to the outbreak. This outbreak emphasizes the importance of considering the presence of B. cereus for implementing appropriate procedures to ensure the safety of food produced within the catering sector. Our study highlights the added value of a combined approach that used epidemiological, microbiological and advanced next-generation whole genome sequencing (WGS) methods to identify the likely outbreak source and the etiological B. cereus strain.