Prevalence of imp type metallo-β-lactamase in carbapenem resistant pseudomonas aeruginosa and correlation of genotype to phenotype

Abstract

Background Pseudomonas aeruginosa is a ubiquitous organism that is easily recovered from moist environments. It is encountered as a nosocomial pathogen and has become one of the most serious causes of nosocomial bacterial infections particularly in the immunocompromised and burned hosts as well as in patients with cystic fibrosis. Increasingly, Pseudomonas aeruginosa that are multidrug or pan-resistant are being isolated from clinical specimens. Metallo-b-lactamases (MBLs) which belong to Ambler class B b-lactamase have emerged as a therapeutic threat, as they often confer high-level resistance to all b-lactams except aztreo-nam. Aim The aim of this study is to identify the number of isolates that are carbapenem resistant due to the presence of an IMP type MBL from a storage collection in Queensland and to review the characteristics and epidemiology of these isolates. The secondary aim is to analyse whether the MBL genotypes correlate well with phenotypic detection. Method Previously stored carbapenem resistant Pseudomonas aeruginosa clinical isolates were retrieved. The minimum inhibitory concentrations (MICs) to two carbapenems (meropenem and imipenem) were defined using E-test (bioMerieux, France). Template DNA was prepared by heat lysis and polymerase chain reaction (PCR) was performed using IMP-1A and IMP-1B primers. Phenotypic testing was done using the same subculture by the modified Hodge test (MHT) and double disc synergy testing (DDST) methods. Results A total of 69 isolates was included in this study of which 14.5% (10 isolates) were positive for IMP type MBL. Of the 10 isolates, five were collected from intensive care units, three from medical wards and one each from the surgical ward and outpatients department. Phenotypic testing of the 10 isolates with the IMP type MBL showed negative results on MHT but was positive for the DDST. Conclusion This is the first study from Australia looking at the prevalence of IMP type MBL in carbapenem resistant Pseudomonas aeruginosa . The prevalence of IMP type MBL is 14.5% in carbapenem resistant Pseudomonas aeruginosa isolates in Queensland from as early back as 2002. Laboratory screening is important in identifying and preventing the further spread of MBL-producing organisms. The DDST is a relatively simple and economic test for this purpose. A positive screening test will need molecular confirmation to identify the MBL involved.