Multidrug resistance efflux pumps frequently present low levels of basal expression. However, antibiotic resistant mutants that overexpress these resistance determinants are selected during infection. In addition, increased expression of efflux pumps can be induced by environmental signals/cues, which can lead to situations of transient antibiotic resistance. Herein, we have applied a novel high-throughput methodology in order to identify inducers able of triggering the expression of the Stenotrophomonas maltophilia SmeVWX and SmeYZ efflux pumps. To that goal, bioreporters in which the expression of the yellow fluorescent protein (YFP) is linked to the activity of either smeVWX or smeYZ promoters were developed and used for the screening of potential inducers of the expression of these efflux pumps using Biolog phenotype microarrays. YFP production was measured by flow cytometry and the levels of expression of smeV and smeY in the presence of a set of selected compounds were also determined by real-time RT-PCR. The expression of smeVWX was induced by iodoacetate, clioquinol and selenite, while boric acid, erythromycin, chloramphenicol and lincomycin triggered smeYZ expression. The susceptibility to antibiotics that are known substrates of the efflux pumps decreased in the presence of the inducers. However, the analyzed multidrug efflux systems did not contribute to S. maltophilia resistance to the studied inducers. To sum up, the use of fluorescent bioreporters in combination with Biolog plates is a valuable tool for identifying inducers of the expression of bacterial multidrug efflux pumps, and likely of other bacterial systems whose expression is regulated in response to signals/cues.
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