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

• Published in: Journal of medical microbiology Vol. 62; no. 8; pp. 1184 - 1189
• Main Authors: HERMES, Djuli M, PITT, Caroline Pormann, LUTZ, Larissa, TEIXEIRA, Aline B, RIBEIRO, Vanessa B, NETTO, Bárbara, MARTINS, Andreza F, ZAVASCKI, Alexandre P, BARTH, Afonso L
• Format: Journal Article
• Language: English
• Published: Reading Society for General Microbiology 01.08.2013
• Subjects: Anti-Bacterial Agents - pharmacology; Ascitic Fluid - microbiology; Bacteremia; Bacteriology; beta-Lactamases - metabolism; Biological and medical sciences; Brazil; Carbapenems - pharmacology; Cystic Fibrosis - microbiology; Demography; Drug Resistance, Bacterial; Fundamental and applied biological sciences. Psychology; Genotype; Humans; Infectious diseases; Medical sciences; Microbial Sensitivity Tests; Microbiology; Miscellaneous; Phenotype; Polymyxin B - pharmacology; Pseudomonas aeruginosa - drug effects; Pseudomonas aeruginosa - enzymology; Pseudomonas aeruginosa - growth & development; Pseudomonas aeruginosa - isolation & purification; Pseudomonas Infections - drug therapy; Pseudomonas Infections - microbiology; Sputum - microbiology; Pseudomonadales; Resistance; Antibiotic; Cyclic peptides; Polypeptide; Bacteria; Pseudomonadaceae; Pseudomonas aeruginosa; Antibacterial agent; Polymyxin B
• ISSN: 0022-2615; 1473-5644
• DOI: 10.1099/jmm.0.059220-0

One hundred and twenty-four Pseudomonas aeruginosa isolates were selected for antimicrobial susceptibility testing with anti-pseudomonal agents, MIC determination for polymyxin B and metallo-beta-lactamase detection (genes blaSPM, blaVIM-1, blaNDM-1 and blaIMP). 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 colony-forming 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: MIC50 and MIC90 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 blaIMP. There were no heteroresistant subpopulations in the carbapenem-susceptible group, but three isolates presented heterogeneous subpopulations. The PAP frequency ranged from 2.1×10(-4) to 6.9×10(-8). 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 2.1×10(-7) to 2.6×10(-4). 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.