Evaluation of heteroresistance to polymyxin B among carbapenem-susceptible and -resistant Pseudomonas aeruginosa

Abstract

One hundred and twenty-four Pseudomonas aeruginosa isolates were selected for antimicrobial susceptibility testing with anti-pseudomonal agents, MIC determination for polymyxin B and metallobeta-lactamase detection (genes bla SPM , bla VIM-1 , bla NDM-1 and bla IMP ). According to the imipenem and/or meropenem susceptibility profile, a set of randomly selected isolates (12 isolates carbapenem-susceptible and 12 isolates carbapenem-resistant) were evaluated for heteroresistance to polymyxin B. Heteroresistance testing was performed by plating the isolates onto increasing concentrations of polymyxin B (from 0 to 8.0 mg l -1). The population analysis profile (PAP) was defined as the ratio of the number of colony-forming units on the plate with the highest concentration of polymyxin B at which bacterial growth occurred against the number of colonyforming units on the plate without antibiotic. Isolates presenting subpopulations that exhibited growth at polymyxin B concentrations ¢2 mg l -1 were considered heteroresistant. Isolates containing subpopulations that grew at polymyxin B concentrations at least twice as high as the original MIC but ,2 mg l -1 were considered heterogeneous. Antimicrobial susceptibility testing results indicated a variable degree of susceptibility: high levels of resistance to gentamicin (30.6 %) and imipenem (29.0 %); low levels of resistance to aztreonam (1.6 %) and ciprofloxacin (4.8 %). All isolates were susceptible to polymyxin B: MIC 50 and MIC 90 were 1 mg l -1 and 2 mg l -1 , respectively. Thirty-seven isolates (30 %) were carbapenem-resistant. Four isolates resistant to carbapenems were positive for bla IMP . There were no heteroresistant subpopulations in the carbapenem-susceptible group, but three isolates presented heterogeneous subpopulations. The PAP frequency ranged from 2.1¾10 . In the carbapenem-resistant group, one isolate was heteroresistant. Six isolates in this group presented heterogeneous subpopulations. In the resistant population, the PAP frequency ranged from 2.1¾10 "7 to 2.6¾10 . In this study, polymyxin B heteroresistance in P. aeruginosa was uncommon and occurred in only one carbapenem-resistant isolate, despite the fact that several isolates presented heterogeneous subpopulations with increased polymyxin B MICs.

Pharmacokinetics and Pharmacodynamics of Aerosolized Antibacterial Agents in Chronically Infected Cystic Fibrosis Patients

“…Previous colistin therapy significantly increased the number of bacteria exhibiting colistin heteroresistance in A. baumannii (95). Although polymyxin heteroresistance was uncommon, several isolates presented heterogeneous subpopulations with increased and, thus, borderline susceptible MICs, which might cause treatment failures (92); only four successive in vitro passages in the presence of colistin increased the proportion of resistant subpopulations from 0.000023% to 100% (93).…”

“…Carbapenem or polymyxin-colistin heteroresistance has been described for Acinetobacter baumannii and P. aeruginosa, occurring with frequencies ranging from 2 ϫ 10 Ϫ4 to 7 ϫ 10 Ϫ8 ; heteroresistance to both drug classes was rare (89)(90)(91)(92), as was heteroresistance to colistin in multidrug-resistant A. baumannii (93,94). Although polymyxin heteroresistance was uncommon, several isolates presented heterogeneous subpopulations with increased and thus borderline susceptible MICs, which might cause treatment failures (92,95).…”

“…Although polymyxin heteroresistance was uncommon, several isolates presented heterogeneous subpopulations with increased and thus borderline susceptible MICs, which might cause treatment failures (92,95). Heteroresistance to carboxy-and ureidopenicillins, cephalosporins, carbapenems, aminoglycosides, colistin, fluoroquinolones, as well as the piperacillin-tazobactam and ticarcillin-clavulanate combinations in P. aeruginosa has been reported (96)(97)(98)(99)(100)(101).…”

Susceptibility Testing for the Polymyxins: Two Steps Back, Three Steps Forward?

“…Manual BMD, however, is laborious and not performed in many routine clinical microbiology laboratories, which often rely on diffusion or automated systems for susceptibility testing. Moreover, and somewhat paradoxically in view of the drugs' poor diffusion in agar, diffusion methods (both disk and gradient strips) have been found to have unacceptably high levels of false susceptibility (or very major errors [VMEs]) in multiple studies (12,(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) (Table 1). Both the CLSI and EUCAST have now advised against diffusion methods for the polymyxins, removing disk diffusion interpretive criteria for them from their guidance documents (9,15,22).…”

The Evaluation of Four In Vitro Susceptibility Testing Methods for Colistin on Carbapenenm-Resistant Acinetobacter baumannii

“…In the 29 selected isolates of CRAB, the heteroresistance and heterogeneity were determined by population analysis profile (PAP) according to the reported method (13). Briefly, solutions containing 7 distinct bacterial inoculums ranging from 10 2 to 10 8 CFU/mL were prepared.…”