title: Complexity of resistance mechanisms to imipenem in intensive care unit strains of Pseudomonas aeruginosa.
abstract: BACKGROUND
  Pseudomonas aeruginosa can become resistant to carbapenems by both intrinsic (mutation-driven) and transferable (β-lactamase-based) mechanisms. Knowledge of the prevalence of these various mechanisms is important in intensive care units (ICUs) in order to define optimal prevention and therapeutic strategies.
  
  
  METHODS
  A total of 109 imipenem-non-susceptible (MIC >4 mg/L) strains of P. aeruginosa were collected in June 2010 from the ICUs of 26 French public hospitals. Their resistance mechanisms were characterized by phenotypic, enzymatic, western blotting and molecular methods.
  
  
  RESULTS
  Single or associated imipenem resistance mechanisms were identified among the 109 strains. Seven isolates (6.4%) were found to produce a metallo-β-lactamase (one VIM-1, four VIM-2, one VIM-4 and one IMP-29). Porin OprD was lost in 94 (86.2%) strains as a result of mutations or gene disruption by various insertion sequences (ISPa1635, ISPa1328, IS911, ISPs1, IS51, IS222 and ISPa41). Thirteen other strains were shown to be regulatory mutants in which down-regulation of oprD was coupled with overexpressed efflux pumps CzcCBA (n = 1), MexXY (n = 9) and MexEF-OprN (n = 3). The lack of OprD was due to disruption of the oprD promoter by ISPsy2 in one strain and alteration of the porin signal sequence in another.
  
  
  CONCLUSIONS
  Imipenem resistance in ICU P. aeruginosa strains may result from multiple mechanisms involving metallo-β-lactamase gene acquisition and genetic events (mutations and ISs) inactivating oprD, turning down its expression while increasing efflux activities or preventing insertion of porin OprD in the outer membrane. This diversity of mechanisms allows P. aeruginosa, more than any other nosocomial pathogen, to rapidly adapt to carbapenems in ICUs.
authors.affiliation: Centre National de Référence de la Résistance aux Antibiotiques, Centre Hospitalier Régional Universitaire de Besançon, Besançon, France.
authors.forename: Damien
authors.initials: D
authors.lastname: Fournier
authors.forename: Charlotte
authors.initials: C
authors.lastname: Richardot
authors.forename: Emeline
authors.initials: E
authors.lastname: Müller
authors.forename: Marjorie
authors.initials: M
authors.lastname: Robert-Nicoud
authors.forename: Catherine
authors.initials: C
authors.lastname: Llanes
authors.forename: Patrick
authors.initials: P
authors.lastname: Plésiat
authors.forename: Katy
authors.initials: K
authors.lastname: Jeannot
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D015153:Blotting, Western; D005602:France; D014644:Genetic Variation; D005838:Genotype; D006779:Hospitals, Public; D006801:Humans; D015378:Imipenem / Q000494:pharmacology*; D007362:Intensive Care Units; D008826:Microbial Sensitivity Tests; D011552:Pseudomonas Infections / Q000382:microbiology*; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000032:analysis / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D015378:Imipenem; D001618:beta-Lactamases
keywords: Pseudomonas aeruginosa; active efflux; carbapenemases; imipenem resistance; porin OprD