Contribution of Resistance-Nodulation-Cell Division Efflux Systems to Antibiotic Resistance and Biofilm Formation in Acinetobacter baumannii

Abstract

Acinetobacter baumannii is a nosocomial pathogen of increasing importance due to its multiple resistance to antibiotics and ability to survive in the hospital environment linked to its capacity to form biofilms. To fully characterize the contribution of AdeABC, AdeFGH, and AdeIJK resistance-nodulation-cell division (RND)-type efflux systems to acquired and intrinsic resistance, we constructed, from an entirely sequenced susceptible A. baumannii strain, a set of isogenic mutants overexpressing each system following introduction of a point mutation in their cognate regulator or a deletion for the pump by allelic replacement. Pairwise comparison of every derivative with the parental strain indicated that AdeABC and AdeFGH are tightly regulated and contribute to acquisition of antibiotic resistance when overproduced. AdeABC had a broad substrate range, including β-lactams, fluoroquinolones, tetracyclines-tigecycline, macrolides-lincosamides, and chloramphenicol, and conferred clinical resistance to aminoglycosides. Importantly, when combined with enzymatic resistance to carbapenems and aminoglycosides, this pump contributed in a synergistic fashion to the level of resistance of the host. In contrast, AdeIJK was expressed constitutively and was responsible for intrinsic resistance to the same major drug classes as AdeABC as well as antifolates and fusidic acid. Surprisingly, overproduction of AdeABC and AdeIJK altered bacterial membrane composition, resulting in decreased biofilm formation but not motility. Natural transformation and plasmid transfer were diminished in recipients overproducing AdeABC. It thus appears that alteration in the expression of efflux systems leads to multiple changes in the relationship between the host and its environment, in addition to antibiotic resistance.

A Novel Genome-Editing Platform for Drug-Resistant Acinetobacter baumannii Reveals an AdeR-Unrelated Tigecycline Resistance Mechanism

“…Besides trm, we found other genes that were mutated in one of the two independently evolved strains (see Table S3 in the supplemental material) and that were previously linked to tigecycline resistance in A. baumannii, such as rpsJ (S10) or adeN (42)(43)(44)(45). However, to our knowledge the identified mutations in the RNase E 23S rRNA pseudouridylate synthase and in the promoter of the MATE family efflux pump AbeM (see Table S3) were never linked to tigecycline resistance, and further studies are required to verify their impact on tigecycline resistance.…”

The Efflux Pump SmeDEF Contributes to Trimethoprim-Sulfamethoxazole Resistance in Stenotrophomonas maltophilia

“…A differential effect of deleting or overexpressing efflux pumps on the susceptibility to antimicrobials has been reported in other cases. For instance, overexpression of the efflux pump genes adeIJK increases Acinetobacter baumannii MICs for ticarcillin, aztreonam, cephalothin, and ceftriaxone, among other antibiotics, whereas the deletion of adeJ decreases the MICs for ticarcillin and aztreonam only without changing the MICs for cephalothin and ceftriaxone (18). We believe this could be due to the different affinities of the efflux pumps for their substrates.…”

Pathogenic Acinetobacter: from the Cell Surface to Infinity and Beyond

“…Although bioinformatic studies have identified genes predicted to encode proteins required for the biogenesis of Tfp in A. baumannii, only Acinetobacter nosocomialis strain M2 has been shown conclusively to produce functioning Tfp (66,67), which is glycosylated by a TfpO-like oligosaccharyltransferase (30). Many A. baumannii isolates have been found to be naturally transformable and to exhibit twitching motility (63,(68)(69)(70), two classic Tfp-associated phenotypes, which strongly indicates their presence. Tfp-like structures were also identified on A. baumannii ATCC 17978 (71); furthermore, mutants in predicted Tfp biogenesis components of A. baumannii ATCC 17978 exhibited impaired biofilm formation (71) but the major pilin subunit, PilA, has not been shown to be surface exposed and/or associated with the pilin structures observed.…”