Incidence of Extended-Spectrum--Lactamase-Producing Escherichia coli and Klebsiella pneumoniae Isolates That Test Susceptible to Cephalosporins and Aztreonam by the Revised CLSI Breakpoints

The incidence of aztreonam and cephalosporin susceptibility, determined using the revised CLSI breakpoints, for extended-spectrum--lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae isolates was evaluated. Our analysis showed that results for aztreonam and/or>1 cephalosporin were reported as susceptible or intermediate for 89.2 % of ESBL-producing E coli isolates (569/638 isolates) and 67.7 % of ESBL-producing K. pneumoniae isolates (155/229 isolates). Extended-spectrum--lactamase (ESBL)-producing Entero-bacteriaceae strains represent a challenging problem for health care providers, particularly in acute-care and long-term-care fa-cilities and more recently in community-acquired infections (1– 5). ESBL enzymes are capable of inactivating penicillins, aztreo-nam, cephalosporins, and-lactamase inhibitors, which limits the number of effective antibiotics for treatment (1, 2, 4, 5). The presence of an ESBL is suspected in Escherichia coli and Klebsiella pneumoniae infections when resistance to one or more of the extended-spectrum cephalosporins (ESCs) (cefotaxime, ceftazidime, ceftriaxone, or cefepime) is detected (1, 2, 4). Based on pre-2010 guidelines from the Clinical and Laboratory Stan