DETERMINATION OF MINIMUM INHIBITORY CONCENTRATIONS (MICs) FOR COLOSTIN SULPHATE AGAINST PATHOGENIC ESCHERICHIA COLI OF PIG ORIGIN: CHANGES IN RESISTANCE OVER THE LAST 15 YEARS
P. Richez, N. Keck and A. Tahara

Abstract
This study was performed to determine in-vitro Minimum Inhibitory Concentrations (MICs) for colistin sulphate against pathogenic Escherichia coli strains isolated from pigs and to assess changes in the antibiotic susceptibility of Escherichia coli in pigs, between 1989 and 2006. MIC determination analyses in Mueller Hinton agar used twenty-five strains isolated in France in 1989-1992, 25 in 1994-1996, 25 in 1999-2001 and 25 in 2002-2006. The results obtained showed a very low resistance level, despite the widespread use of colistin since its introduction on the veterinary market more than 40 years ago. Only three of the 100 strains tested showed resistance in more than 15 years of observation. Susceptibility remained unchanged between 1989 and 1992 and the most recent period (2004-2006). In particular, MIC50 values were remarkably stable over time: 0.43 ± 0.069 µg/ml in the 1989-1992 period and 0.36 ± 0.009 µg/ml 15 years later (2004-2006). The apparently higher value for MIC90 in the 2000-2001 period is an artefact due to a few strains with higher MIC values, but the corresponding values measured or observed for the other periods were essentially similar (0.86, [0.25-0.5] and 0.62 µg/ml for the 1989-1992, 1994-1996 and 2004-2006 periods, respectively).

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