Establishing streptomycin epidemiological cut-off values for Salmonella and Escherichia coli.

This study was conducted to elucidate the accuracy of the current streptomycin epidemiological cut-off value (ECOFF) for Escherichia coli and Salmonella spp. A total of 236 Salmonella enterica and 208 E. coli isolates exhibiting MICs between 4 and 32 mg/L were selected from 12 countries. Isolates were investigated by polymerase chain reaction for aadA, strA, and strB streptomycin resistance genes. Out of 236 Salmonella isolates, 32 (13.5%) yielded amplicons for aadA (n=23), strA (n=9), and strB (n=11). None of the 60 Salmonella isolates exhibiting MIC 4mg/L harbored resistance genes. Of the Salmonella isolates exhibiting MICs 8mg/L, 16mg/L, and 32mg/L, 1.6%, 15%, and 39%, respectively, tested positive for one or more genes. For most monitoring programs, the streptomycin ECOFF for Salmonella is wild type (WT) Oen32 or Oen16mg/L. A cut-off value of WT Oen32mg/L would have misclassified 13.5% of the strains as belonging to the WT population, since this proportion of strains harbored resistance genes and exhibited MICs Oen32mg/L. Out of 208 E. coli strains, 80 (38.5%) tested positive for aadA (n=69), strA (n=18), and strB (n=31). Of the E. coli isolates exhibiting MICs of 4mg/L, 8mg/L, 16mg/L, and 32mg/L, 3.6%, 17.6%, 53%, and 82.3%, respectively, harbored any of the three genes. Based on the European Committee on Antimicrobial Susceptibility Testing guidelines (ECOFF Oen16mg/L), 25% of the E. coli strains presenting MIC Oen16mg/L would have been incorrectly categorized as belonging to the WT population. The authors recommend an ECOFF value of WT Oen16mg/L for Salmonella and WT Oen8mg/L for E. coli. -® Copyright 2012, Mary Ann Liebert, Inc. 2012