Susceptibility patterns to extended-spectrum cephalosporins among Enterobacteriaceae harbouring extended-spectrum β-lactamases using the updated Clinical and Laboratory Standards Institute interpretive criteria

Abstract We examined the effect of applying the updated 2010 Clinical and Laboratory Standards Institute (CLSI) susceptibility breakpoints for extended-spectrum cephalosporins (ESCs) to detect extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae. In total, 202 ESBL-producing, plasmidic AmpC- and carbapenemase-negative isolates derived from separate patients were collected from three Greek hospitals during 2007–2011, including 150 Escherichia coli , 43 Klebsiella pneumoniae and 9 Enterobacter cloacae clinical isolates. ESBLs were detected using the ESBL CLSI confirmatory test and PCR assays. Sequencing analysis showed that 91 (45.0%) of the ESBL-producers carried the bla CTX-M-3 gene, 66 (32.7%) carried the bla CTX-M-15 gene and the remaining 45 (22.3%) carried the bla SHV-5 gene. Minimum inhibitory concentrations for cefotaxime, ceftazidime and cefepime were determined by the agar dilution method. Based on the new CLSI breakpoints, 13 (6.4%) of the ESBL-producers were susceptible to cefotaxime, 90 (44.6%) to ceftazidime and 112 (55.4%) to cefepime; as many as 145 (71.8%) were susceptible to at least one ESC. Among the 150 E. coli , 12 (8.0%), 87 (58.0%) and 79 (52.7%) were susceptible to cefotaxime, ceftazidime and cefepime, respectively, whilst among the 43 K. pneumoniae , 1 (2.3%), 3 (7.0%) and 25 (58.1%) were susceptible to the above ESCs, respectively. None of the nine E. cloacae were susceptible to cefotaxime and ceftazidime, but all except one were susceptible to cefepime. By implementation of the new 2010 CLSI breakpoints, a considerable proportion of ESBL-possessing Enterobacteriaceae would be reported as susceptible, mostly to ceftazidime and cefepime, leading to possible infection control and therapeutic implications.