Comparative genome analyses of Staphylococcus aureus from platelet concentrates reveal rearrangements involving loss of type VII secretion genes

Supplementary Material for Comparative genome analyses of Staphylococcus aureus from platelet concentrates reveal rearrangements involving loss of type VII secretion genes. Platelet concentrates (PCs) are a life-saving blood product used to treat patients with bleeding disorders and requires stringent quality control which involves screening for microbial contamination or treatment with pathogen reduction technologies. Although implementation of PC screening with automated culture methods is a gold standard practice that has significantly improved the safety of this blood product, some bacteria rarely escape detection and can cause septic transfusion reactions in susceptible recipients. The clinical outcome of patients receiving contaminated PCs depends on the bacterial load of the transfused product, the virulence of the contaminant organism and the immune status of the recipient. This study focuses on the comparative genomic analysis of five S. aureus strains isolated from contaminated PCs, including two strains that escaped detection during PC screening and caused transfusion reactions. S. aureus is a common PC contaminant and is considered a major threat to patients due to its large repertoire of virulence factors including antibiotic resistance, production of exotoxins and biofilm formation. Our genomic analysis describes rearrangements involving phage Sa3int, which enhances its virulence potential. The most important finding reported herein is the discovery of a truncated type VII secretion system gene operon in the CBS2016-05 strain. The type VII secretion system is highly conserved within S. aureus isolates, has a key role in the secretion of virulence factors, and contributes to maintaining cell membrane integrity as well as interbacterial competition. It was therefore surprising to find that a highly virulent strain, which was involved in a septic transfusion event, lacks this important secretion system. Our findings indicate that virulent S. aureus strains have other mechanisms to fulfill the functions of the type VII secretion system, which merits further investigation.