Transcriptome Analysis Reveals that Multidrug Efflux Genes Are Upregulated To Protect Pseudomonas aeruginosa from Pentachlorophenol Stress

Abstract

Through chemical contamination of natural environments, microbial communities are exposed to many different types of chemical stressors; however, research on whole-genome responses to this contaminant stress is limited. This study examined the transcriptome response of a common soil bacterium, Pseudomonas aeruginosa, to the common environmental contaminant pentachlorophenol (PCP). Cells were grown in chemostats at a low growth rate to obtain substrate-limited, steady-state, balanced-growth conditions. The PCP stress was administered as a continuous increase in concentration, and samples taken over time were examined for physiological function changes with whole-cell acetate uptake rates (WAURs) and cell viability and for gene expression changes by Affymetrix GeneChip technology and real-time reverse transcriptase PCR. Cell viability, measured by heterotrophic plate counts, showed a moderately steady decrease after exposure to the stressor, but WAURs did not change in response to PCP. In contrast to the physiological data, the microarray data showed significant changes in the expression of several genes. In particular, genes coding for multidrug efflux pumps, including MexAB-OprM, were strongly upregulated. The upregulation of these efflux pumps protected the cells from the potentially toxic effects of PCP, allowing the physiological whole-cell function to remain constant.

Adaptations in microbiological populations exposed to dinitrophenol and other chemical stressors

“…Microbiological stress responses have been observed on exposure to many environmentally relevant chemicals, including phenols [1], chlorophenols [2,3], dinitrophenols [4,5], toluene [6], methyltert-butyl ether [7], explosive 2,4,6-Trinitrotoluene [8], explosive cyclotrimethylenetrinitramine [9], and dibenzo-p-dioxin [10]. Microorganisms that are effective at degrading pollutants can be stressed by exposure to these chemicals, as has been observed for Methylocystis sp.…”

The Challenge of Efflux-Mediated Antibiotic Resistance in Gram-Negative Bacteria

“…Unless mutations disrupt the nalC (PA3721) gene, which encodes a TetR family regulator (727) that strongly represses the adjacent PA3720-PA3719 operon, baseline amounts of ArmR are not expected to influence MexAB-OprM production in wild-type cells (406). However, chemostat experiments with strain PAO1 showed that various chlorinated phenols, including the environmental contaminant pentachlorophenol, can induce the expression of the PA3720-PA3719 and mexAB-oprM operons through reversible, noncovalent binding to the NalC protein (807,808). Recent data, however, showed that pentachlorophenol stimulates MexABOprM production, surprisingly, in an ArmR-independent (although MexR-dependent) manner (809).…”

The crystal structure of MexR from Pseudomonas aeruginosa in complex with its antirepressor ArmR

“…The best-characterized efflux system in P. aeruginosa is MexAB-OprM, which displays an expansive substrate profile that includes not only antibiotics and biocides, but organic solvents, dyes, detergents, and homoserine lactones involved in quorum sensing (3). Although pentachlorophenol exposure appears to up-regulate mexAB-oprM (6), expression of these efflux genes is generally considered constitutive in wild-type (WT) strains. However, mutations in any of three regulatory genes, mexR (7,8), nalC (9,10), or nalD (11), have been shown to cause hyperexpression of mexAB-oprM and increased resistance to various medically relevant antimicrobials (11,12).…”