Acquisition of Resistance to Carbapenems in Multidrug-Resistant Clinical Strains of Acinetobacter baumannii : Natural Insertional Inactivation of a Gene Encoding a Member of a Novel Family of β-Barrel Outer Membrane Proteins

Abstract

The outer membrane proteins responsible for the influx of carbapenem ␤-lactam antibiotics in the nonfermentative gram-negative pathogen Acinetobacter baumannii are still poorly characterized. Resistance to both imipenem and meropenem in multidrug-resistant clinical strains of A. baumannii is associated with the loss of a heat-modifiable 29-kDa outer membrane protein, designated CarO. The chromosomal locus containing the carO gene was cloned and characterized from different clinical isolates. Only one carO copy, present in a single transcriptional unit, was found in the A. baumannii genome. The carO gene encodes a polypeptide of 247 amino acid residues with a typical N-terminal signal sequence and a predicted transmembrane ␤-barrel topology. Its absence from different carbapenem-resistant clinical isolates of A. baumannii resulted from the disruption of carO by distinct insertion elements. The overall data thus support the notion that CarO participates in the influx of carbapenem antibiotics in A. baumannii. Moreover, database searches identified the presence of carO homologs only in species of the genera Acinetobacter, Moraxella, and Psychrobacter, disclosing the existence of a novel family of outer membrane proteins restricted to the family Moraxellaceae of the class ␥-Proteobacteria.The emergence of antibiotic resistance among both pathogenic and opportunistic microbes resident in hospitals represents a serious and recurrent problem for the treatment of infections (22). It generates a continuous demand for new antimicrobial agents, whose application feeds the undesired vicious circle of selection and dissemination of new patterns of antibiotic resistance. The otherwise bizarre but still elegant mechanisms that underlie some of these patterns of resistance demonstrate that the potential of microbes to challenge eradication attempts remains almost unexhausted (26). Yet, attempts to reduce the dissemination of rapidly evolving antibiotic-resistant pathogens are best based on a detailed knowledge of the causes that promote these patterns of resistance (26).The genus Acinetobacter, recently reassigned to the family Moraxellaceae in the class ␥-Proteobacteria (30), is constituted by gram-negative, pleomorphic aerobic species commonly isolated from many sources in the environment, including drinking and static water, soil, sewage, food, and the skin of humans and animals (5). Certain strains of a particular species of the genus, Acinetobacter baumannii, now account for a large percentage of nosocomial infections, including pneumonia, bacteremia, skin and wound infections, and urinary tract infections. These strains are almost invariably multidrug resistant, having successfully resisted eradication attempts by the use of penicillins, aminoglycosides, cephalosporins, and even fluoroquinolones (5). It is due to this outstanding ability to rapidly respond to the challenge of new antibiotics that the emerging resistance to carbapenems among nosocomial strains of A. baumannii represents a major concern (22).The molecular b...

Role of ISAba1 and ISAba125 in governing the expression of bla ADC in clinically relevant Acinetobacter baumannii strains resistant to cephalosporins

“…Insertional inactivation of carO was not seen in any of the strains, which also concurs with the fact that all the strains had a very low imipenem MIC value, as it has been shown by Mussi et al (2005) that one of the factors leading to carbapenem resistance is the loss or disruption of this 29 kDa OMP.…”

“…ISAba1, which drives the expression of bla ADC , is also shown to be present in the opposite orientation (39-59) with bla ADC, as shown by et al (2006), hence ISAba125 present in the opposite orientation (39-59) may also act in a similar way to drive the expression of bla ADC . Mussi et al (2005) have proved that ISAba125 causes insertional inactivation of the CarO OMP in A. baumannii strains, leading to a carbapenem-resistant profile. The insertion element ISAba125, first identified in A. baumannii, is widely distributed and has also been associated with gene variants of the bla NDM carbapenemase in the family Enterobacteriaceae (Hornsey et al, 2011;Nordmann et al, 2012;Pfeifer et al, 2011).…”

“…Approximately 100 mg protein was loaded into each well. Insertions causing disruption of carO gene (29 kDa OMP) were checked in all the isolates using the primers described by Mussi et al (2005).…”

Multi-drug resistance profiles and the genetic features of Acinetobacter baumannii isolates from Bolivia

“…The carO gene fragment was not disrupted by any insertion element (as indicated earlier by Mussi et al [14]) and thus did not contribute to reduced carbapenem susceptibility.…”

“…PCR amplification for the insertions causing disruption of the carO gene (29kDa OMP) was also completed for all the isolates using primers as described by Mussi et al [14].…”