engleski

Effect of inoculum size on the antibiotic susceptibilities of Enterobacteriaceae producing SHV and CTX-M extended-spectrum beta-lactamases

EFFECT OF INOCULUM SIZE ON THE ANTIBIOTIC SUSCEPTIBILITIES OF ENTEROBACTERIACEAE PRODUCING SHV AND CTX-M EXTENDED-SPECTRUM BETA-LACTAMASES B. Bedenić1, 2, J. Vraneš1, 3, N. Beader1, 2, V. Plečko1, 2, I. Matulić4, S. Kalenić1, 2 1School of Medicine, University of Zagreb, Zagreb ; 2Department of Clinical and Molecular Microbiology, Clinical Hospital Center Zagreb 3Zagreb Institute of Public Health, Zagreb ; 4Trg Bana Jelačića 3 (retired) ABSTRACT Background and aim Many extended-spectrum β -lactamases (ESBL) producing isolates of E. coli and K. pneumoniae are susceptible in vitro to amoxycillin- clavulanate (AMC), ceftazidime-clavulanate (CAZ/cl), and piperacillin-tazobactam (TZP) but MICs increase substantially when higher inoculum is applied. The aim of this study was to determine the effect of inoculum size on the susceptibility of E. coli and K. pneumoniae isolates with well characterized ESBLs to amoxycillin (AMX), AMC, ceftazidime (CAZ), CAZ/cl, piperacillin (PIP), TZP, imipenem (IMI) and meropenem (MEM). Material and methods Minimum inhibitory concentrations (MIC) were determined by broth microdilution method using inocula that differed 100 fold in density according to CLSI5. The inocula contained 105 CFU/ml and 107CFU/ml approximately. Results Inoculum effect for CAZ/cl was detected in 52% of SHV-2 producing K. pneumoniae strains followed by AMC (43%) and TZP (38%). SHV-5 producing K. pneumoniae strains showed the most pronounced inoculum effect with CAZ/cl (57%) and AMC (55%) and to lesser extent with TZP (44%). Inoculum effect was observed for AMC, CAZ/cl and TZP in 71% of SHV-12 producers. E. coli producing SHV-5 β - lactamase showed the most pronounced inoculum effect with AMC (61%) followed by CAZ/cl (51%) and TZP (22%). Strains producing CTX-M β -lactamases had a marked inoculum effect with CAZ/cl (71%), AMC (57%) and TZP (50%). AMC and CAZ/cl were associated with inoculum effect against all type of ESBL producers: SHV-2, SHV-5, SHV-12 and CTX-M. TZP was less affected by the inoculum size then AMC, and CAZ/cl particulary with CTX-M producers. The activity of TZP was mostly compromised in the presence of high density of SHV-5 producing K. pneumoniae. Conclusions Carbapenems were the most stable compounds to inoculum effect regardless of the type of ESBL. Key words: inoculum effect, extended spectrum β -lactamases, carbapenems, β - lactam/inhibitor combinations Form of presentation: poster Correspondence: Branka Bedenić, School of Medicine, University of Zagreb, Zagreb ; Department of Clinical and Molecular Microbiology, Clinical Hospital Center Zagreb, tel: ++385 1 2367304, fax ++385 1 4590 130, e- mail:branka.bedenic@zg.htnet.hr

inoculum effect; Enterobacteriaceae; SHV beta-lactamas; CTX-M beta-lactamases

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