Abstract
In recent years, effective treatment of infections caused by Acinetobacter baumannii has become challenging due to the ability of the bacterium to acquire or up-regulate antimicrobial resistance determinants. Two component signal transduction systems are known to regulate expression of virulence factors including multidrug efflux pumps. Here, we investigated the role of the AdeRS two component signal transduction system in regulating the AdeAB efflux system, determined whether AdeA and/or AdeB can individually confer antimicrobial resistance, and explored the interplay between pentamidine resistance and growth conditions in A. baumannii ATCC 17978. Results identified that deletion of adeRS affected resistance towards chlorhexidine and 4’,6-diamidino-2-phenylindole dihydrochloride, two previously defined AdeABC substrates, and also identified an 8-fold decrease in resistance to pentamidine. Examination of ΔadeA, ΔadeB and ΔadeAB cells augmented results seen for ΔadeRS and identified a set of dicationic AdeAB substrates. RNA-sequencing of ΔadeRS revealed transcription of 290 genes were ≥2-fold altered compared to the wildtype. Pentamidine shock significantly increased adeA expression in the wildtype, but decreased it in ΔadeRS, implying that AdeRS activates adeAB transcription in ATCC 17978. Investigation under multiple growth conditions, including the use of Biolog phenotypic microarrays, revealed resistance to pentamidine in ATCC 17978 and mutants could be altered by bioavailability of iron or utilization of different carbon sources. In conclusion, the results of this study provide evidence that AdeAB in ATCC 17978 can confer intrinsic resistance to a subset of dicationic compounds and in particular, resistance to pentamidine can be significantly altered depending on the growth conditions.
Highlights
Acinetobacter baumannii causes a range of disease states including hospital-acquired pneumonia, blood stream, urinary, wound and bone infections, and is responsible for epidemic outbreaks of infection worldwide [1]
The adeRS operon of A. baumannii ATCC 17978 was disrupted through the introduction of an ERY resistance cassette to produce a ΔadeRS strain
RNA-seq results were confirmed by quantitative Real-Time PCR for nine genes that displayed different levels of expression in ΔadeRS compared to WT; a good correlation between methods was observed (S1 Fig)
Summary
Acinetobacter baumannii causes a range of disease states including hospital-acquired pneumonia, blood stream, urinary, wound and bone infections, and is responsible for epidemic outbreaks of infection worldwide [1]. Such infections are often very difficult to treat due to the multidrug resistant (MDR) character of isolates displayed by this organism [2, 3]. Of particular interest is the AdeABC system which affords resistance to diverse antibiotics, biocides and dyes [10, 13,14,15], and has gained attention due to its high incidence of over-expression across many MDR A. baumannii clinical isolates, primarily from incorporation of point mutations in the genes encoding its positive regulator, AdeRS [6, 8, 13, 16]. Deletion of adeC in the A. baumannii strain BM4454 did not affect resistance towards two substrates of the pump suggesting that AdeAB can utilize an alternative outer membrane protein (OMP) to efflux antimicrobial compounds [13]
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