Colistin Resistance in Acinetobacter baumannii Is Mediated by Complete Loss of Lipopolysaccharide Production

Abstract

Infections caused by multidrug-resistant (MDR) Gram-negative bacteria represent a major global health problem. Polymyxin antibiotics such as colistin have resurfaced as effective last-resort antimicrobials for use against MDR Gram-negative pathogens, including Acinetobacter baumannii. Here we show that A. baumannii can rapidly develop resistance to polymyxin antibiotics by complete loss of the initial binding target, the lipid A component of lipopolysaccharide (LPS), which has long been considered to be essential for the viability of Gram-negative bacteria. We characterized 13 independent colistin-resistant derivatives of A. baumannii type strain ATCC 19606 and showed that all contained mutations within one of the first three genes of the lipid A biosynthesis pathway: lpxA, lpxC, and lpxD. All of these mutations resulted in the complete loss of LPS production. Furthermore, we showed that loss of LPS occurs in a colistin-resistant clinical isolate of A. baumannii. This is the first report of a spontaneously occurring, lipopolysaccharide-deficient, Gram-negative bacterium.Acinetobacter baumannii is an emerging, opportunistic, Gram-negative bacterial pathogen (19). It is associated with a range of nosocomial infections, including bacteremia, pneumonia, meningitis, and urinary tract infections. Outbreaks, especially in intensive care unit settings, have been identified in numerous countries around the world (23). The treatment of these infections is hampered by the rapid rise in prevalence of A. baumannii strains that are resistant to almost all available antibiotics, including ␤-lactams, fluoroquinolones, tetracyclines, and aminoglycosides (23). In these multidrug-resistant (MDR) strains, colistin (also known as polymyxin E) is often the only remaining treatment (15), although colistin-resistant clinical isolates have already been reported (7,10,21). Intriguingly, some A. baumannii isolates have been shown to display heteroresistance to colistin, where an apparently colistin-susceptible strain (based upon the MIC) harbors a small proportion of colistin-resistant cells (9, 16). Under selective pressure both in vitro (33) and in vivo (10), heteroresistant A. baumannii strains can rapidly give rise to strains with high-level colistin resistance.Colistin is a cationic polypeptide antibiotic that is composed of a cyclic decapeptide linked by an ␣-amide linkage to a fatty acyl chain (15). Its structure differs from that of polymyxin B by only a single amino acid; the two antibiotics demonstrate comparable activities against a range of Gram-negative bacteria (6). Polymyxins are proposed to exert their antibacterial effect on Gram-negative bacteria via a two-step mechanism comprising initial binding to and permeabilization of the outer membrane, followed by destabilization of the cytoplasmic membrane (37). While the exact mechanism of bacterial killing is not clearly defined, a critical first step in the action of polymyxins is the electrostatic interaction between the positively charged peptide and the negatively charged ...

Old drug, new findings: colistin resistance and dependence of Acinetobacter baumannii

“…baumannii [49]. Alterations in the lipid A biosynthesis genes (lpxA, lpxC, and lpxD) by amino acid substitutions, deletions, or insertion of ISAba1 are responsible for the loss of LPS [49,50].…”

“…Alterations in the lipid A biosynthesis genes (lpxA, lpxC, and lpxD) by amino acid substitutions, deletions, or insertion of ISAba1 are responsible for the loss of LPS [49,50]. A recent metabolomic study revealed that an LPS-deficient, colistin-resistant A. baumannii strain showed perturbation in specific amino acid and carbohydrate metabolites, particularly pentose phosphate pathway and TCA (tricarboxylic acid) cycle intermediates [37].…”

Dissemination and Genetic Structure of Carbapenemase Encoding Genes (bla<sub>OXA-23</sub> and bla<sub>OXA-24</sub>) in <i>Acinetobacter baumannii</i> from Southern Texas

“…Outbreaks of carbapenem associated multidrug resistant A. baumannii may be the most problematic issue to treat the infected patients since the only viable therapeutic option for this infection is polymyxins [9], but polymyxins cause neuro-and nephro-toxicity [27] and polymyxin selection pressure can readily develop polymyxin resistant A. baumannii [28] [29]. In this study, most of the isolates showed carbapenem associated multidrug resistance, which was similar as other areas of the United States and worldwide [9] [11] [30], and more sadly some of them conferred intermediate levels of polymyxin resistance.…”

Are we closing in on an "elusive enemy"? The current status of our battle withAcinetobacter baumannii

“…19 Studies relevant to the antibiotic treatment of A. baumannii implicate the loss of LPS with colistin resistance (through mutations in genes involved in LPS synthesis) and with the two-component system PmrAB. 20,21 At the same time, certain isolates that are colistin resistant display marked loss of virulence. 22 Proteomic analysis demonstrate that the cost of colistin resistance involves, among others such as chaperones and metabolic enzymes, loss of outer membrane proteins.…”