Roles of Porin and -Lactamase in -Lactam Resistance of Pseudomonas aeruginosa

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Abstract
Pseudomonas aeruginosa demonstrates high intrinsic resistance to most β-lactam antibiotics. Two factors that are interrelated appear to be important in this intrinsic resistance: an inducible, chromosomally encoded type Id β-lactamase and low outer-membrane permeability. β-Lactamase-noninducible mutants are supersusceptible to many β-lactam agents, whereas constitutively derepressed mutants are considerably more resistant even to so-called β-lactamase-stable β-lactams. For the latter mutants, by analysis of kinetics, it can be demonstrated that synergy betweens low permeation across the outer membrane and slow hydrolysis of the β-lactamase-stable β-lactams can explain resistance. Wild-type P. aeruginosa allows outer membrane permeation of β-lactam agents at rates 1%–8% of those measured for Escherichia coli. The majority of trans-outer-membrane channels formed by P. aeruginosa porin protein F are too small to allow passage of β-lactam antibiotics. Nevertheless, this porin is apparently a conduit for β-lactams, since protein F-deficient mutants have small changes in susceptibility to certain β-lactam agents. This low outer-membrane permeability acting in synergy with β-lactamase is probably responsible for intrinsic β-lactam resistance in P. aeruginosa.

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