Trends in quinolone susceptibility of Enterobacteriaceae among inpatients of a large university hospital: 1992–98

Objectives   To assess trends in quinolone susceptibility of Enterobacteriaceae isolated in a large university hospital.
Methods   Between 1992 and 1998, bacterial isolates were collected each year during a 3-month period to evaluate annual changes in susceptibility. In addition, the activities of fluoroquinolones (pefloxacin, norfloxacin, ofloxacin, ciprofloxacin) against nalidixic acid-resistant strains were determined by disk diffusion and MIC methodologies during the first and last year of the study.
Results   The susceptibility of Enterobacteriaceae to nalidixic acid was unchanged between 1992 and 1998 (86% versus 85%). However, at the species level, the susceptibility rates to nalidixic acid decreased for Escherichia coli from 92% to 89%, and for Enterobacter cloacae from 87% to 82%. In contrast, there was a 10% increase in the nalidixic acid susceptibility rates for Klebsiella pneumoniae (74% versus 83%), which was thought to be due to the control of the spread of epidemic extended-spectrum β -lactamase (ESBL)-producing strains. The overall susceptibility of the Enterobacteriaceae to the fluoroquinolones remained high during the study period, greater than 90% in the case of ciprofloxacin. However, nalidixic acid-resistant Escherichia coli showed decreased susceptibility to ciprofloxacin between 1992 and 1998, as reflected by a decrease in median zone diameter (26 mm to 19 mm), an increase in MIC₅₀ (0.25 mg/L to 1 mg/L) and a shift in MIC distribution (unimodal in 1992 to bimodal in 1998). This has resulted in the reduced susceptibility of Escherichia coli to fluoroquinolones between 1992 and 1998 (pefloxacin, 95–90%; ciprofloxacin, 99–95%).
Conclusions   The susceptibility of Escherichia coli to quinolones has decreased, and the level of susceptibility of the resistant strains has increased over the 7-year study period.