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D.J. Evans, I. Zolfaghar, R. Ronaghi, S.M. J. Fleiszig; Mutation of Pseudomonas Aeruginosa RetS Delays the Onset of Microbial Keratitis, but Enhances Ultimate Disease Severity . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1079.
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© ARVO (1962-2015); The Authors (2016-present)
We previously reported that the type III secretion system (TTSS) of Pseudomonas aeruginosa strain PA103 contributes to bacterial colonization and virulence in a murine model of corneal infection, and that this involves repression of phagocyte (PMN) infiltration of infected corneas by the TTSS effector ExoU. We have also shown that mutation of the retS (regulator of exopolysaccharide and type III secretion) gene in strain PA103 reduces corneal virulence at 24 h post–infection. In–vitro, retS mutants show loss of the TTSS, reduced twitching motility, and a decrease in association, invasion and survival within corneal epithelial cells. Others have shown reduced virulence of retS mutants in pneumonia infection models, and that retS positively and negatively affects the transcription of multiple virulence genes. For example, psl and pel operon genes, which contribute to exopolysaccharide synthesis and biofilm formation, are actually upregulated in retS mutants. We hypothesized that reduced corneal virulence of retS mutants follows loss of the TTSS.
Corneal pathology, bacterial colonization, PMN infiltration and corneal thickness were compared for wild–type PA103, a retS mutant (PA103retS::Tn5) and an exsA (TTSS) mutant (PA103exsA::omega) over 7 days post–infection with ∼106 CFU bacteria using the 6–h heal murine model of microbial keratitis.
Mutation of either retS or exsA did not affect 4 h colonization, but delayed disease progression at 48 h, as illustrated by reduced overall disease severity scores and colonization. Surprisingly, retS mutant infections then became more severe than exsA mutant infections. By day 7, colonization levels of retS mutants even surpassed those of wild–type (>2–fold, p = 0.028). The retS mutant caused more severe opacification of central corneas than both wild–type and exsA mutants at 2, 4 and 7 days post–infection. Histology of infected corneas at 2 days showed significant increases in PMN infiltration and thickness of central regions when infected with the retS mutant as compared to both the exsA mutant and wild–type.
These data suggest that RetS influences P. aeruginosa ocular colonization and virulence beyond its effects on the TTSS, and that RetS actually dampens pathological effects at later stages of the disease process. Enhanced corneal virulence of retS mutants after 48 h may reflect the upregulation of bacterial exopolysaccharide genes or of other factors associated with chronic colonization as shown in–vitro.
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