[EVOLUTION OF BETA-LACTAM ANTIBIOTIC RESISTANCE IN ENTEROBACTER SPP. IN CROATIA]

Enterobacter spp. develops resistance to expanded-spectrum cephalosporins by induction or derepression of chromosomal AmpC β-lactamase, or production of extended-spectrum β-lactamases (ESBLs) or carbapenemases. The aim of the study was to analyze the mechanisms of resistance to expanded-spectrum cephalosporins and the evolution of resistance mechanism during the study period (2008–2011) on a collection of 58 randomly collected Enterobacter spp. strains from three hospital centers in Croatia and Bosnia and Herzegovina during 2008-2010. The antibiotic susceptibility was determined by broth microdilution method according to CLSI. Resistance genes were determined by PCR. Plasmids were characterized by PCR-based replicon typing (PBRT). The hypothesis of the study was that there will be multiple mechanisms of ceftazidime resistance involved, from inducible and derepressed AmpC β-lactamases to extended-spectrum β-lactamases and carbapenemases at the end of the study. The isolates from different centers were expected to express different phenotypes and mechanisms of resistance. The study showed the predominance of derepressed AmpC β-lactamases combined with ESBLs belonging to CTX-M family as a mechanism of resistance to expanded-spectrum cephalosporins. The emergency of MBLs was reported in the last year of the study in University Hospital Center Zagreb. The plasmids encoding ESBLs belonged to different incompatibility groups. This points out to the evolution of β-lactam resistance in Enterobacter spp. from derepressed AmpC β-lactamases and ESBL to carbapenemases.