Strain-specific variation in the complement resistome of <i>Pseudomonas aeruginosa</i>

Bloodstream infections caused by <i>Pseudomonas aeruginosa</i> are associated with high mortality rates. The complement system, a key component of the innate immune response, plays a major role in eliminating <i>P. aeruginosa</i> from human blood. However, the sensitivity of <i>P. aeruginosa</i> strains to plasma varies widely, ranging from highly sensitive to persistent or fully resistant. Although most studies use model strains, the species genomic and phenotypic diversities suggest more complex interactions with complement than previously appreciated. In this study, we characterized the plasma resistome of <i>P. aeruginosa</i> using Tn-seq in three strains with varying levels of plasma sensitivity. A gain-of-function screen in the plasma-sensitive strain PA14 revealed numerous bacterial factors influencing plasma resistance, including components of the RetS-LadS/Gac/Rsm regulatory pathway and outer membrane porins. In the plasma-resistant strains CHA and YIK, Tn-seq analysis indicated that each strain relies on a distinct, limited set of proteins to evade complement-mediated killing. Despite these differences, we identified common mechanisms across all three strains. These include the production of exopolysaccharides (EPSs), the presence of surface appendages, and modifications in the O-specific antigen. Notably, we identified Ssg and Crc as shared contributors to plasma resistance. Although deletion mutants lacking <i>ssg</i> and/or <i>crc</i> exhibited reduced survival in plasma, a subpopulation of these mutants was able to persist during prolonged exposure. Overall, this work provides new insights into the complex interplay between <i>P. aeruginosa</i> and the human complement system in the context of bloodstream infections and raises concerns regarding the efficacy of therapies that target individual virulence factors.