Molecular and Epidemiological Investigation of an Outbreak of Imipenem Resistant Acinetobacter baumannii in a University Hospital in Korea

Abstract

Acinetobacter baumannii is an aerobic, glucose non-fermentative, Gram-negative coccobacillus which is widely distributed in the hospital environment. It is an important opportunistic pathogen causing a variety of nosocomial infections. Several outbreaks of A. baumannii in intensive care units (ICUs) of hospitals have been documented. During January 2004 to December 2004, the medical ICU and surgical ICU of Chosun University Hospital (CHU), Gwangju, Korea (Republic), a 650 bed tertiary care centre, had experienced an apparent outbreak due to the imipenem-resistant A. baumannii. To determine the source of the epidemic and the risk factors, and to know the clonality and features of outbreak strains, environmental sampling from the ICU, a case-control study, genotyping of A. baumannii by pulsed-field gel electrophoresis (PFGE) and biochemical test and PCR assay for resistance determinants were performed. Thirty imipenem resistant A. baumannii (IRAB) clinical isolates, 9 imipenem susceptible A. baumannii (ISAB) clinical isolates and 8 IRAB isolated from environmental culture (E-IRAB) collected during that outbreak period were analyzed in this study. Environmental culture revealed widespread contamination of IRAB in the ICU environment, especially, from suction- , tracheostomy-, and patient's ventilator-associated surfaces. IRAB were isolated from health care worker's (HCW) hands also. This finding was matched well with the finding of the risk factors which were identified by multiple logistic regression analysis were mechanical ventilation practice and time at risk. So it seemed like that multiple objects in hospital environments, especially, ventilator- and suction-related systems were important reservoir of IRAB and the spreading of IRAB was mediated by HCW's contaminated hands. Because ICU environment and HCW's hands were contaminated, prolonged stay in ICU may have contributed for the easy acquisition of IRAB, which is supported by the fact that time at risk was the important risk factor. PFGE analysis result showed most IRAB and E-IRAB were type A, a major PFGE type in this study. Three other minor types were also observed but only in a few sporadic cases. ISAB showed 8 types which were well differentiated from IRAB types. To determine whether the outbreak strains had carbapenemase, modified Hodge test (MHT) were carried out for the 47 A. baumannii isolates. All of MHT positive isolates were tested by imipenem-EDTA double disk synergy test (DDST) and several kinds of carbapenemase gene PCR. The MHT showed positive results for all of 30 IRAB and 8 E-IRAB while all of 9 ISAB were negative for MHT. The DDST showed positive results in 5 of 30 (16.7 %) of IRAB and one of eight (12.5%) of E-IRAB. Five of 30 (16.7 %) of IRAB showed blaIMP and 1 of 30 (3.33 %) of IRAB showed blaVIM PCR positive. PCR results for other carbapenemase gene such as SPM-1, GES-1, GIM-1, blaOXA-23, blaOXA-24, and blaOXA-58 were negative for all of 30 IRAB. Dot blot hybridization test by fluorescence labeled probe for blaOXA-2 gene was carried out and only 3 out of 47 isolates (6.38 %) were positive. It may be possible that another antibiotic resistant determinant genes or other resistance mechanism have contributed to the imipenem resistance of these outbreak strains. To know the prevalence of integrase genes in our samples, integrase gene PCR was carried out. Most of them were class 1 integrase gene positive while all of them were class 3 integrase gene negative. The positive rates of class 2 integrase gene were 36.7%, 22.0%, and 37.5% of IRAB, ISAB, and E-IRAB respectively. To control the outbreak, several infection control strategy including cross-transmission prevention protocols, and restriction of the use of carbapenem was implemented. As cross-transmission prevention protocols, continued ICU personnel educational programs, thorough hand-washing procedure, rigorous open surveillance of adequate compliance with barrier precautions, proper cleaning protocols, and housekeeping procedures were used. Subsequently, steady reduction of attack rate of imipenem resistant A. baumannii into the basal rate was observed.