Emetic and Diarrheal Toxins Detection and Typing of Bacillus cereus in Food and Human Milk in Italy

BACKGROUND: Bacillus cereus, responsible for 1.4%-12% of all food poisoning outbreaks worldwide, produces several virulence factors, including toxins which are mainly found in raw and processed food, as starch-based and dairy products. It can cause two types of gastrointestinal diseases: emetic form, with nausea and vomiting which appears as soon as after food consumption, and diarrheal form, with abdominal cramps, which occurs after 8-16 h. Moreover, B. cereus is the most common pathogen in pasteurized human milk, a precious aid in neonatal care. The aim of this study was to investigate the prevalence of B. cereus toxins kinds using WGS and to detect in silico sequence types (ST) in isolates from food and human milk. METHODS: During B. cereus routine surveillance in Lombardy (Italy), a total of 41 isolates were collected from food (n = 16) and donor breast milk (n = 25) between 2022-2023, and then analysed with WGS. Libraries were prepared using the Illumina DNA Prep Kit and WGS was performed on the Illumina MiSeq. Using the BTyper tool, for each sample emetic and diarrheal toxins were identified and ST was assigned. RESULTS: All 41 isolates had at least one diarrheal toxin, specifically the nonhemolytic enterotoxin was found in all the isolates, the cytotoxin K was detected in 37% of isolates, and the hemolysin BL in 32%. Instead, the emetic toxin was found in only 3 samples isolated from food. Using in silico MLST, different STs were found with a higher prevalence of ST26 and ST144. CONCLUSIONS: Diarrheal toxins were detected in all samples underling that B. cereus food infections play an important role in the public health and in the food industry. These aspects are even more significant when the pathogen is isolated from human breast milk, as it can induce severe infections in newborns. KEY MESSAGES: • The presence of B. cereus toxins in food and human milk underlines the importance of its ongoing monitoring and analysis. • Human breast milk has a high risk of B. cereus contamination which is the most frequent cause for its discarding.