Target (MexB) and efflux based mechanisms decreasing the effectiveness of the efflux pump inhibitor D13-9001 in P. aeruginosa PAO1: uncovering a new role for MexMN-OprM in efflux of {beta}-lactams and a novel regulatory circuit (MmnRS) controlling MexMN expression [Mechanisms of Resistance]

Efflux pumps contribute to antibiotic resistance in Gram-negative pathogens. Correspondingly, efflux pump inhibitors (EPIs) may reverse this resistance. D13-9001 specifically inhibits MexAB-OprM in P. aeruginosa. Mutants with decreased susceptibility to MexAB-OprM inhibition by D13-9001 were identified and these fell into two categories; those having alterations in the target MexB (F628L and V177) and those with mutations in PA1438 (L172P substitution) which encoded a putative sensor kinase of unknown function. The alterations in MexB were consistent with reported structural studies of D13-9001 interaction with MexB. The PA1438_L172P alteration mediated a >150-fold upregulation of MexMN pump gene expression and >50-fold upregulation of PA1438 and the neighboring response regulator gene PA1437. We propose that these be renamed as mmnR/mmnS for MexMN Regulator and Sensor. MexMN was shown to partner with the outer membrane channel protein OprM and to pump several β-lactams, monobactams and tazobactam. Upregulated MexMN functionally replaced MexAB-OprM to efflux these compounds but was insusceptible to inhibition by D13-9001. MmnS_L172P also mediated a decrease in susceptibility to imipenem / biapenem that was independent of MexMN-OprM. Expression of oprD, encoding the uptake channel for these compounds was downregulated, suggesting that this channel is also part of the MmnSR regulon. RNA-seq of cells encoding MmnS_L172P revealed among other things an interrelationships between regulation of mexMN and genes involved in heavy metal resistance.

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