AbuO, a TolC-Like Outer Membrane Protein of Acinetobacter baumannii, Is Involved in Antimicrobial and Oxidative Stress Resistance

Abstract

Although Acinetobacter baumannii is well accepted as a nosocomial pathogen, only a few of the outer membrane proteins (OMPs) have been functionally characterized. In this study, we demonstrate the biological functions of AbuO, a homolog of TolC from Escherichia coli. Inactivation of abuO led to increased sensitivity to high osmolarity and oxidative stress challenge. The ⌬abuO mutant displayed increased susceptibility to antibiotics, such as amikacin, carbenicillin, ceftriaxone, meropenem, streptomycin, and tigecycline, and hospital-based disinfectants, such as benzalkonium chloride and chlorhexidine Outer membrane proteins (OMPs) are known to have a pivotal role in bacterial physiology, such as adherence, invasion, and serum resistance, maintenance of cell structure, and binding a variety of substances, including passive and active transport (1). The archetypical OMP, TolC, has been considered to be a multifunctional protein due to its involvement in cell membrane integrity, acid tolerance, expulsion of metabolites, export of siderophores that are required in iron acquisition, export of plasmid and chromosomally encoded toxins, such as hemolysin, colicin V, and microcins, and virulence, as evident from studies in Enterobacter, Borrelia, Salmonella, Vibrio, Legionella, Francisella,. In E. coli, TolC is promiscuous because it supports the functioning of multidrug resistance efflux pumps, such as AcrD, AcrEF, and MdtABC (the resistance nodulation cell division [RND] superfamily) (11-13), EmrAB and EmrKY (the major facilitator superfamily [MFS]), and MacAB (the ATP-binding cassette [ABC] superfamily) (14-17). Although the functions of TolC homologs in many Gram-negative bacteria, such as E. coli, Vibrio vulnificus, Stenotrophomonas maltophilia, Enterobacter cloacae, and Yersinia pestis (5, 18-21), have been elucidated, the biological functions of TolC in an important human pathogen, Acinetobacter baumannii, have remained enigmatic so far.Multidrug-resistant (MDR) Acinetobacter strains kill up to 50% of infected patients, despite treatment with last resort drugs, and the resistance rates of such strains continue to escalate globally (22, 23). Significant increases in the number of A. baumannii strains that are resistant to carbapenems, cephalosporins, aminoglycosides, and fluoroquinolones with a diverse antibiotic resistome have been reported from hospitals in the United States and other countries (24-27). In our previous study, we demonstrated the role of antibiotic resistance genes and efflux pumps in mediating antimicrobial resistance in A. baumannii isolates from Ohio (28, 29). To date, few OMPs have been implicated in carbapenem resistance when their expression is reduced: CarO, Omp33 to -36, and an OprD homolog (30-32).In a continuation of our efforts toward understanding the origin/network of multidrug resistance in Acinetobacter, in this study, using genetic and molecular approaches, we demonstrate the role of a putative OMP (a homolog of TolC, designated AbuO) in bacterial stress physiology in general and a...

Light Modulates Metabolic Pathways and Other Novel Physiological Traits in the Human Pathogen Acinetobacter baumannii

“…EmrAB can function as components of tripartite systems together with TolC, for example (e.g., EmrAB-TolC), and in this context, it has been recently shown that the TolC homolog AbuO is involved in tigecycline resistance in Acinetobacter baumannii (36). EmrAB is therefore an interesting candidate to explain the reduction in susceptibility to these antibiotics in response to blue light, in addition to induction of adeABC genes previously described under this condition (8).…”

Lineage-specific SoxR-mediated Regulation of an Endoribonuclease Protects Non-enteric Bacteria from Redox-active Compounds

“…Identification of AoSoxR Target Genes-To identify SoxRregulated genes, we used the A. oleivorans genome and transcriptome data indicating possible SoxR binding sites (24,(31)(32)(33). Although five possible candidate promoters (AOLE_ 16635 encoding a putative L-PSP endoribonucleases, AOLE_ 18450 encoding a membrane protein, argT, scpA, and hisZ) were found using a putative SoxR binding site, two of them (scpA and hisZ) were not induced by RACs.…”

“…Severe growth defects in the Acinetobacter oxyR mutant grown in hexadecane-amended conditions indicated that OxyR is important for the elimination of ROS during hexadecane biodegradation (30). It was recently reported that SoxR from Acinetobacter baumannii AYE can bind the promoter regions of two genes, abuO and abeD, which encode a TolC-like outer membrane protein and an RND-type membrane transporter, respectively (31,32). Our bioinformatics study suggested that Acinetobacter species, including A. oleivorans DR1, possess a SoxR binding site in the promoter regions of several genes, including a gene coding an L-PSP endoribonuclease, which is one of the five known categories of gene products controlled by SoxR in non-enteric bacteria (33).…”

An ompR-envZ Two-Component System Ortholog Regulates Phase Variation, Osmotic Tolerance, Motility, and Virulence in Acinetobacter baumannii Strain AB5075

“…2). To date, there is limited information regarding porins in A. baumannii, although a few have been investigated and were found to play roles in processes unrelated to osmotic regulation (i.e., nutrient acquisition, antibiotic resistance, oxidative stress, and pathogenesis) (32)(33)(34). Based on amino acid homology, we hypothesized that ABUW_2160 encoded an OmpC ortholog.…”