title: 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 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.
authors.affiliation: Programa de Pós Graduação em Ciências Médicas, Universidade Federal do Rio Grande do Sul (UFRGS), Brazil.
authors.forename: Djuli M
authors.initials: DM
authors.lastname: Hermes
authors.affiliation: Faculdade de Farmácia, UFRGS, Brazil.
authors.forename: Caroline
authors.initials: C
authors.lastname: Pormann Pitt
authors.affiliation: Unidade de Microbiologia, Hospital de Clínicas de Porto Alegre (HCPA), Brazil.
authors.forename: Larissa
authors.initials: L
authors.lastname: Lutz
authors.affiliation: Programa de Pós Graduação em Ciências Médicas, Universidade Federal do Rio Grande do Sul (UFRGS), Brazil.
authors.forename: Aline B
authors.initials: AB
authors.lastname: Teixeira
authors.affiliation: Programa de Pós-Graduação em Ciências Farmacêuticas, UFRGS, Brazil.
authors.forename: Vanessa B
authors.initials: VB
authors.lastname: Ribeiro
authors.affiliation: Programa de Pós Graduação em Ciências Médicas, Universidade Federal do Rio Grande do Sul (UFRGS), Brazil.
authors.forename: Bárbara
authors.initials: B
authors.lastname: Netto
authors.affiliation: Universidade Metodista de Porto Alegre, Brazil.
authors.forename: Andreza F
authors.initials: AF
authors.lastname: Martins
authors.affiliation: Infectious Disease Service, HCPA, Brazil.|Programa de Pós Graduação em Ciências Médicas, Universidade Federal do Rio Grande do Sul (UFRGS), Brazil.
authors.forename: Alexandre P
authors.initials: AP
authors.lastname: Zavascki
authors.affiliation: Serviço de Patologia Clínica, HCPA, Brazil.|Programa de Pós-Graduação em Ciências Farmacêuticas, UFRGS, Brazil.|Programa de Pós Graduação em Ciências Médicas, Universidade Federal do Rio Grande do Sul (UFRGS), Brazil.
authors.forename: Afonso L
authors.initials: AL
authors.lastname: Barth
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001202:Ascitic Fluid / Q000382:microbiology; D016470:Bacteremia; D001938:Brazil; D015780:Carbapenems / Q000494:pharmacology; D003550:Cystic Fibrosis / Q000382:microbiology; D003710:Demography; D024881:Drug Resistance, Bacterial; D005838:Genotype; D006801:Humans; D008826:Microbial Sensitivity Tests; D010641:Phenotype; D011112:Polymyxin B / Q000494:pharmacology*; D011552:Pseudomonas Infections / Q000188:drug therapy / Q000382:microbiology*; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000201:enzymology / Q000254:growth & development / Q000302:isolation & purification; D013183:Sputum / Q000382:microbiology; D001618:beta-Lactamases / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D015780:Carbapenems; D001618:beta-Lactamases; D011112:Polymyxin B