A Type III Secretion Negative Clinical Strain of Pseudomonas aeruginosa Employs a Two-Partner Secreted Exolysin to Induce Hemorrhagic Pneumonia

Virulence of Pseudomonas aeruginosa is typically attributed to its type III secretion system (T3SS). A taxonomic outlier, the P. aeruginosa PA7 strain, lacks a T3SS locus, and no virulence phenotype is attributed to PA7. We characterized a PA7-related, T3SS-negative P. aeruginosa strain, CLJ1, isolated from a patient with fatal hemorrhagic pneumonia. CLJ1 is highly virulent in mice, leading to lung hemorrhage and septicemia. CLJ1-infected primary endothelial cells display characteristics of membrane damage and permeabilization. Proteomic analysis of CLJ1 culture supernatants identified a hemolysin/hemagglutinin family pore-forming toxin, Exolysin (ExlA), that is exported via ExlB, representing a putative two-partner secretion system. A recombinant P. aeruginosa PAO1ΔpscD::exlBA strain, deficient for T3SS but engineered to express ExlA, gained lytic capacity on endothelial cells and full virulence in mice, demonstrating that ExlA is necessary and sufficient for pathogenicity. This highlights clinically relevant T3SS-independent hypervirulence, isolates, and points to a broader P. aeruginosa pathogenic repertoire. [Display omitted] •P. aeruginosa clinical isolate CLJ1 lacks a T3SS but induces hemorrhagic pneumonia•CLJ1 infected primary endothelial cells display characteristics of membrane damage•CLJ1 elaborates a two-partner secreted Exolysin (ExlA) with membrane lytic activity•ExlA is required and sufficient to confer in vivo virulence to T3SS-negative strains Pseudomonas aeruginosa virulence is typically attributed to its type III secretion system (T3SS). Elsen et al. characterize a clinical P. aeruginosa strain, CLJ1, that induces fatal hemorrhagic pneumonia in humans and mice. CLJ1 lacks T3SS but elaborates a two-partner secreted membrane lytic toxin that is required and sufficient for virulence.