The importance of the
las system, particularly the possession of
lasI, in ocular virulence was confirmed using isogenic mutants of PAO1. The mice infected with mutants lacking
lasI showed reduced virulence, and the virulence of the
lasI mutant was fully restored by complementation with a functional
lasI gene.
P. aeruginosa quorum-sensing mutants lacking
lasI or
rhlI have been reported to be less virulent in a mouse model of
P. aeruginosa burn infection
28 29 and a model of acute
P. aeruginosa pulmonary infection.
26 The observation that the
lasR mutant did not have a statistically significant reduction in virulence (apart from reduced bacterial numbers in the eye) agrees with a previous report in which there was no significant difference reported between the 50% infective doses of PAO1 and the
lasR mutant during corneal infection.
31 The lack of infection caused by
lasI mutants could be due to direct effects of the AHL produced by the product of this gene,
N-(3-oxododecanoyl)homoserine lactone, which can stimulate a significant induction of mRNAs for the cytokines interleukin (IL)-1α and IL-6 and the chemokines macrophage inflammatory protein (MIP)-2, monocyte chemotactic protein-1, MIP-1β, inducible protein 10, and T-cell activation gene 3, in addition to cyclooxygenase (Cox)-2 expression in the skin of mice.
42 This may be more likely than a direct effect on gene transcription of those genes regulated by the Las system, because the
lasR mutants were relatively unaffected in their virulence. Alternatively,
lasR but not
lasI mutants may be compensated for in vivo by the presence of the RhlRI quorum-sensing system. A proteomics analysis of quorum-sensing mutants of
P. aeruginosa has been conducted.
43 Generally, mutations in either
lasR or
rhlR produced effects on the extracellular proteins released in vitro similar to those of mutants
lasI or
rhlI, respectively. However,
lasI mutants produced much less endopeptidase (PrpL) activity compared to the
lasR mutants or wild-type PAO1 and much less of the two-partner secretion exoprotein PA0041 than the
lasR mutant only.
43 This may indicate a role for this (PrpL) endopeptidase activity in corneal infection. Indeed, it has been shown that PrpL endopeptidase is identical with protease IV of
P. aeruginosa.
44 Mutants of
P. aeruginosa that lack protease IV (PrpL endopeptidase) show much reduced corneal virulence,
45 and
P. putida containing a plasmid encoding protease IV causes increased corneal virulence.
46