Characterization of extended-spectrum beta-lactamases in Enterobacteriaceae causing nosocomial infections in a Zagreb University Hospital.

The bacteria producing extended-spectrum beta-lactamases (ESBLs) are increasingly reported. production of ESBLs by Gram-negative bacteria is the major mechanism of resistance to oxymino-cephalosporins and aztreonam. the aim of the present study was to characterize ESBLs produced by Enterobacteriaceae, collected during 2003-2005 in a University Hospital in Zagreb, and to determine the risk factors associated with nosocomial infections due to them. 76 isolates of Enterobacteriaceae were included in the study. Antibiotic susceptibility testing was performed by disk-diffusion and broth microdilution method according to CLSI. beta-lactamases were characterized by PCR and sequencing of bla(ESBL )genes. plasmids were extracted by alkaline lysis method and digested with EcoRI enzyme. Most of the strains displayed CAZ phenotype meaning a higher level of resistance to ceftazidime compared to cefotaxime and ceftriaxone. 50 strains produced SHV-ESBL, 28 tem and 8 CTX-M beta-lactamase. Sequencing of bla(SHV )genes from representative strains revealed SHV-5 beta-lactamase in 6 strains whereas sequencing of bla(CTX-M )genes identified CTX-M-3 beta-lactamase in 3 and CTX-M-15 in 5 strains. Strains were assigned to groups from A to f according to plasmid fingerprinting. The spread of SHV-5-producing strains throughout the hospital units could be due to selective pressure of ceftazidime which is widely prescribed in our hospital thus favoring survival of strains possessing a mutation at the Ambler position 240 responsible for ceftazidime and aztreonam resistance.