Comparative epidemiological study of Pseudomonas aeruginosa isolates during two time periods in a tertiary care hospital

Abstract

Pseudomonas aeruginosa is a nosocomial pathogen of clinical importance carrying virulence and antibiotic resistance determinants. In the present study, phenotypic and genotypic characteristics of P. aeruginosa isolates collected at the University General Hospital of Patras (UGHP) in two time periods were investigated. Two groups of P. aeruginosa clinical isolates were compared collected with one year interval, in the UGHP. Group A consisted of 120 isolates recovered during one year (2004), whereas, group B included 240 recovered during a two year period (2006-2007). P. aeruginosa isolates were identified at the species level by standard methods (Oxiferm, BD, BBL). Antibiotic susceptibility testing was performed for the group A by the Etest for piperacillin, aztreonam, imipenem, tobramycin and ciprofloxacin, whereas, in group B by the agar disk diffusion method for piperacillin, carbenicillin, azlocillin, ticarcillin-clavulanic acid, ceftazidime, aztreonam, imipenemamikacin, tobramycin, netilmicin and ciprofloxacin, while MIC determination of colistin by the Etest (bioMerieux, Marcy l' Etoile, France), (CLSI 2008). The production of metallo-beta-lactamases (MBL) was investigated by the double strip Etest-MBL (bioMerieux). Serotyping was performed by 16 monovalent antisera. In group B, blaVIM , exoY, exoT, exoS and exoU genes were investigated by PCR and blaVIM genes were identified by sequencing analysis. PFGE (SpeI) was performed in all isolates of bothgroups. The main PFGE types of group B were characterized by MLST. Phenotypic and genotypic characteristics of P. aeruginosa were compared between groups. According to our results, phenotypic and genotypic analysis of isolates demonstrated the prevalence of multidrug-resistant P. aeruginosa (MDRPA) mainly serotype O11 clones, especially in the Internal Medicine during 2004 (group A) and in the ICU during 2006 and 2007 (group B). Fifty nine patients from group A (59/120), mainly hospitalized in the Internal Medicine, and 152 from group B (152/240), mainly in the ICU, were colonized or infected with MDRPA. High rate of MBL-positive isolates was observed in group A compared to group B (45% vs 33%), mainly in the Internal Medicine. All MBL-positive isolates of group B carried blaVIM- 2 and blaVIM-1 genes. Among isolates of group B, exoU gene was detected mainly in isolates from the ICU, whereas, exoS in isolates from other wards. In group A strains, four dominant PFGE types were characterized: A, B, C and D. In group B five PFGE types, a, d, b, c and s, different from the aforementioned pulsotypes, which were associated with ST235, ST111, ST253, ST309 and ST639. Significant polyclonality was observed in group A as compared to group B. In conclusion, this study describes two independent clonal outbreaks of MDRPA in the UGHP which occurred primarily in the Internal Medicine Ward (group A) and after one year interval in the ICU (group B).