Evolution of CTX-M-type β-lactamases in isolates of Escherichia coli infecting hospital and community patients

Escherichia coli isolates collected at our Institution from 1999 to 2003 (n = 20,258) were studied to evaluate the production of CTX-M-type extended-spectrum β-lactamases (ESBL). Isolates suspected of producing CTX-M enzymes were analyzed by the double-disk synergy test, hybridization with specific probes, PCR and direct DNA sequencing. Overall, 53 ESBL-positive isolates were found to carry CTX-M-type genes (blaCTX-M-1, n = 51; blaCTX-M-15, n = 2). The isolation of CTX-M-positive strains increased from 1 per year (1999) to 26 per year (2003). The first isolate carrying the blaCTX-M-15 gene appeared in 2003 and was obtained from a patient previously treated with ceftazidime. CTX-M-positive isolates were characterized by multi-drug resistance and were obtained both from inpatients (n = 29) and outpatients (n = 24). Most patients were over 60-year-old (n = 45), had underlying chronic diseases (n = 32), and had been hospitalized more than once (n = 33). Strains were frequently isolated from the urinary tract, often after recurrent infections. Our study demonstrates that CTX-M-producing isolates are increasing among E. coli strains. Adequate laboratory detection may help in choosing appropriate treatment and in limiting the spread of this resistance trait.