Mutation of lasA and lasB reduces Pseudomonas aeruginosa invasion of epithelial cells

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Abstract
Pseudomonas aeruginosais an opportunistic bacterial pathogen implicated in a variety of devastating conditions. Its flexibility as a pathogen is attributed to a myriad of virulence factors and regulatory elements that respond to prevailing environmental conditions. ExoS and ExoT are type III secreted effector proteins, regulated by the transcriptional activator ExsA, that can inhibit invasion of epithelial cells by cytotoxic strains ofP. aeruginosa. This study sought to understand why invasive strains, which can secrete both ExoS and ExoT, still invade epithelial cells. The results showed that LasA and elastase (LasB), which are regulated by the Las and Rhl quorum-sensing systems, modulatedP. aeruginosainvasion. Mutation oflasAand/orlasBreducedP. aeruginosainvasion, which was not fully restored by extracellularly added LasB,P. aeruginosaconditioned medium containing LasA and LasB, or EGTA pretreatment of cells. This indicated that protease effects on invasion involved factors additional to tight junction disruption and subsequent alterations to cell polarity. Upon mutation oflasAand/orlasB, steady-state levels of ExoS and ExoT were increased in culture medium ofP. aeruginosagrown under conditions stimulatory for these toxins. The increase in ExoS was significantly correlated with reduced invasion.In vitroexperiments showed that purified LasB degraded recombinant ExoS. Taken together, these studies suggest a mechanism by which invasive strains can synthesize inhibitors of invasion, ExoS and ExoT, yet still invade epithelial cells. By this mechanism, LasA and LasB decrease the levels of the toxins directly or indirectly, and thus reduce inhibition of invasion.