Genomic epidemiology of a Bacillus cereus bacteraemia outbreak linked to contaminated hospital laundry

This study investigated an increase in <i>Bacillus cereus</i> bacteraemia cases amongst hospitalized patients in Italy during the summer of 2023. To precisely characterize the outbreak, we performed extensive genomic investigation, including both short- and long-read sequencing technologies, combined with bioinformatics analysis. This genomic approach enabled us to identify the putative source of the outbreak and understand the transmission dynamics of this opportunistic pathogen within the hospital. Our investigation revealed a complex, polyclonal contamination pattern traced to contaminated hospital laundry. Whole-genome sequencing (WGS) analysis identified multiple <i>B. cereus</i> sequence types (STs) in both clinical and environmental samples, with three predominant STs (ST-2184, ST-163 and ST-365) found in both. These STs, previously implicated in bloodstream infection (BSI) outbreaks, raise concerns about their potential as healthcare-associated pathogens. SNP-based phylogenetic analysis provided strong evidence linking human and environmental isolates, with close genetic relatedness observed between isolates from patients and those from laundered scrubs, transport trucks and bed linens. The study highlights the potential for laundry-mediated pathogen transmission in healthcare settings and underscores the importance of stringent laundry hygiene practices. Improved laundry procedures successfully resolved the <i>B. cereus</i> bacteraemia outbreak. This study demonstrates the power of WGS-based methodologies for investigating and resolving outbreaks, informing targeted infection control measures and ultimately enhancing patient safety.