Regulation of the MexAB-OprM Multidrug Efflux System of Pseudomonas aeruginosa: Involvement of Pentachlorophenol and Plant Chemicals
Starr, Lisa Michelle
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Pseudomonas aeruginosa is a common soil organism as well as an opportunistic human pathogen. Treatment of P. aeruginosa infections is often complicated by innate resistance to a variety of antimicrobials mediated by multidrug efflux systems. The MexAB-OprM efflux system is constitutively expressed in wildtype strains and contributes to innate antimicrobial resistance, while hyperexpression of the system results in acquired high levels of resistance. MexAB-OprM is hyperexpressed in nalC mutants containing mutations in the gene encoding NalC, a repressor of a two-gene operon, PA3720-armR. armR encodes a protein modulator of MexR, a repressor of mexAB-oprM expression. Previous reports showed that genes encoding the MexAB-OprM efflux system are upregulated in response to pentachlorophenol (PCP), a phenolic compound that is a common environmental contaminant. Induction of mexAB-oprM and PA3720-armR by PCP was confirmed using RT-PCR, and MexAB-OprM was shown to be involved in PCP resistance. An electromobility shift assay (EMSA) showed that PCP interacts with NalC, interfering with its binding to the PA3720-armR promoter region and thereby promoting PA3720-armR expression. Nonetheless, the increase in ArmR did not drive mexAB-oprM expression suggesting that PCP induction of this efflux operon occurred via a different mechanism. A direct PCP-MexR interaction could not be demonstrated using an EMSA. PCP exposure did, however, reduce expression of nalD, encoding a second repressor of mexAB-oprM, which might explain the PCP-promoted increase in mexAB-oprM expression. PCP is unlikely to be the intended inducer(s)/substrate(s) for this system but probably resembles these. Several compounds related to PCP were tested for an interaction with NalC but all were negative in EMSAs. Plants produce a variety of phenolic compounds, which are often antimicrobial and, so, root extracts of various plants were tested for an ability to interact with NalC and interfere with promoter binding. Extracts from Boehmeria tricuspis, Uncaria tomentosa and Ixiolirion tataricum were shown to interact with NalC, suggesting that plant compounds may be the intended inducers/substrates for NalC/MexAB-OprM.