Occurrence of CTX-M-1-producing Escherichia coli in pigs treated with ceftiofur.

for 108 Enterobacteriaceae were 0.25 –1 mg/L (mode 0.5 mg/L) for the susceptible population and 32 to .128 mg/L (mode 32 mg/L) for the resistant population. For the 64 P. aeru-ginosa isolates, the MICs of colistin were 1 – 4 mg/L (mode 4 mg/L) for the susceptible population and 8 – 16 mg/L (mode 8 mg/L) for isolates considered as resistant. When determined by Etest, there was a shift towards 2-fold lower MICs (one dilution) for 37.3% of the isolates and 4-to 8-fold lower MICs (two to three dilutions) for 6.5% of the isolates. Overall, 91.2% of results were in agreement i.e. within a 2-fold dilution. Susceptible and resistant Enterobacteriaceae were clearly delineated with MICs 1 mg/L for the susceptible population and 32 mg/L for the resistant population, resulting in 100% agreement between the two methods. In contrast, the range of MICs for P. aeruginosa (1 –16 mg/ L) overlapped the susceptibility breakpoint. The 2-fold shift towards lower MICs and lack of an 'intermediate' category resulted in 7 of 12 colistin-resistant P. aeruginosa isolates being misclassified as susceptible by the Etest. This is, however, not significant: a clinically relevant approach has to take into account the site of the infection, the colistin level obtained at this site and the MIC for the strain. Despite this, the 91.2% agreement (+ a 2-fold dilution) observed in comparison with the reference agar dilution method indicates that the colistin Etest is a useful method for detecting colistin-resistant Gram-negative bacteria. Colistin is more often used in France than in other countries, both locally and parenterally, in part because of the high prevalence of multidrug-resistant bacteria. However, antibiotic susceptibility testing is still performed in many laboratories by the disc diffusion technique, which is highly unreliable for colistin. We strongly recommend that colistin MICs should be determined by Etest for all patients with severe infections caused by multidrug-resistant strains or in cases of clinical failure, and we emphasize the need to transmit these results to the clinician.