engleski

Nursing homes as reservoirs of carbapenem-resistant Acinetobacter haumannii

Introduction and purpose Acinetobacter baumannii is a glucose non-fermentative Gram-negative cocco-bacillus considered a relevant nosocomial pathogen occurring particularly in intensive care (ICU) as well as burn therapy units. The most frequent health-care-associated infections are urinary tract infections, bacteremia, surgical site infections and ventilator associated pneumonia. A. baumannii strains have emerged as resistant to almost all antimicrobial agents, including carbapenems. β-lactamase-mediated resistance is the most common mechanism for carbapenem resistance in this species. However, Acinetobacter may develop resistance to carbapenems through various combined mechanisms including decreased permeability, altered penicillin binding proteins (PBPs) and, rarely, efflux pump overexpression. The emergence of carbapenem resistance in Acinetobacter spp is a significant public health concern leaving few therapeutic options remaining. Carbapenemases found in Acinetobacter belong to molecular class D (OXA enzymes) or class B (metallo-β-lactamases of IMP, VIM or SIM family). Recently a multicenter study on carbapenem-resistance in A. baumannii was conducted in Croatia. OXA-24/40 was found to be the most prevalent group of acquired oxacillinases in Croatian hospitals. Nursing homes are known to be reservoir for multiresistant bacteria including ESBL, plasmid-mediated AmpC and carbapenemase producing Gram-negative bacteria. Recently emergence of carbapenem-resistance in A. baumannii isolates from nursing homes in Zagreb was observed. The aim of the study was to analyze the occurrence and genetic background of carbapenem-resistance in A. baumannii from nursing homes in Zagreb. Methods In total, 18 isolates were collected from nursing homes in Zagreb and Pula from various clinical specimens from 1st March 2013 till 24.01. 2014. Antibiotic susceptibilities were determined by broth microdilution. Combined disk test with clavulanic acid and EDTA was applied to detect extended-spectrum β-lactamases (ESBL) and metallo-β-lactamases (MBL) respectively. Detection of ESBLs was perfored with addition of cloxacillin in the medium to inhibit chromosomal ampC β-lactamase of Acinetobacter spp. BlaMBL and blaOXA genes were detected by multiplex PCR. PCR mapping was performed with primers for ISAba1, combined with forward and reverse primers for blaOXA-23, blaOXA-51 and blaOXA-24 genes. Genotyping of the strains was performed by determination of sequence groups by multiplex PCR. Results Eleven isolates originated from urine samples including catethers, five from wound swabs, and one from tracheal aspirate and nasal swab, respectively. All strains were found to be resistant to ceftazidime, cefotaxime, ceftriaxone, cefepime, piperacillin/tazobactam and ciprofloxacin. All but one strain were resistant to imipenem, meropenem and gentamicin. No resistance to colistin was observed. Colistin was the most potent antibiotic with MIC90 of 1 mg/L. Combined disk test with addition of clavulanic acid revealed the production of ESBL in 5 out of 18 strains as shown in Table 1. Combined disk test with EDTA was positive in 14 strains indicating production of MBL. PCR revealed the blaOXA-23 genes in 12, blaOXA-24 in five, blaVIM-1 in ten and blaNDM-1 in one strain as shown in Table 1 and Fig. 1. BlaVIM genes were found in ten strains, one of which coharboured blaOXA-24 gene and nine blaOXA-23 genes and blaNDM genes in one strain which possessed additional OXA-23 β-lactamase. Sequencing of blaOXA-23, blaOXA-24, blaVIM and blaNDM amplicons from respresentative strains revealed the presence of blaOXA-23, blaOXA-72, blaVIM-1 and blaNDM-1 genes. Two strains possessed only the intrinsic blaOXA-51 gene which was upregulated by ISAba1 in one strain. ISAba1 was found upstream of blaOXA-51 genes in all but two strains as shown in Fig. 2. BlaCTX-M-1 and blaTEM-1 genes were found in one and nine strains, respectively. Sequencing of blaTEM and blaCTX-M genes revealed TEM-1 in all nine tested strains and CTX-M-15 β-lactamase in one tested strain. Strains positive for TEM-1 had elevated MICs of ampicilin/sulbactam. Thirteen strains with OXA-23 and OXA-24 β-lactamases belonged to ICL II whereas three strains positive for blaOXA-24 and two only for blaOXA-51 gene belonged to ICL I. Conclusions: • The study found nursing homes to be important reservoir of colistin-only susceptible isolates of A. baumannii. • In contrast to hospitals where OXA-24/40-like β-lactamases and OXA-58 were the most prevalent OXA-23 like β-lactamases are the dominant group in the nursing homes. • OXA-58-like β-lactamase which is the most widespread group was not found. Aquistion of blaMBL genes in A. baumannii strains was observed. • Production of TEM-1 β-lactamase was associated with resistance to ampicillin/sulbactam as reported previosly. • Nursing homes can act as a source of dissemination of blaOXA genes in the environment and the possible influx to the hospital environment. Further analysis which would include more nursing homes throughout Croatia will be conducted. Colistin remains the antibiotic of choice for the treatment of infections caused by multidrug-resistant A. baumannii from nursing homes although colistin resistance was observed among hospital isolates.

nursing homes ; acinetobacter baumannii ; OXA-23

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