This study investigated an increase in Bacillus cereus 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 B. cereus 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 B. cereus 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.

Listeria monocytogenes (Lm) is a serious public health foodborne pathogen cause of listeriosis, usually in elderly, pregnant and immunocompromised people, linked to consumption of contaminated food, especially ready-to-eat (RTE) products. Different protocols can be used to detect Lm, and ISO11290-1:2017 is the reference method in Europe. Through molecular techniques such as whole genome sequencing (WGS) it is possible to discriminate between Lm strains, which are unequally distributed between clinical cases, food or food related environments, probably also due to enrichment step bias towards some Lm serogroup (IIa) compared to IVb. In the present work a set of Lm strains, detected in clinical cases and food, was investigated to define Lm strains growth ability after incubation in Half Fraser broth, and Genome Wide Association Studies (GWAS) applied to correlate the growth phenotype traits to presence of relevant genes. GWAS enabled the identification of a more relevant cassette of genes associated to a holin region of bacteriophage A118 and the determination of the distribution of relevant genes, highlighted from GWAS analysis within a population of Lm IVb and IIa.