Scientific Publications by FDA Staff

FEMS Microbiol Lett 2016 Feb;363(4):fnw009

Using whole-genome sequencing to determine appropriate streptomycin epidemiological cutoffs for Salmonella and Escherichia coli.

Tyson GH, Li C, Ayers S, McDermott PF, Zhao S

For Enterobacteriaceae such as Salmonella spp. and Escherichia coli, no unified interpretive resistance criteria exist for streptomycin, an epidemiologically important antibiotic. As part of the National Antimicrobial Resistance Monitoring System (NARMS), we had previously used a minimum inhibitory concentration (MIC) of >/=64 mug/mL as an epidemiological cutoff value (ECV) to define non-wild-type isolates. To identify whether this ECV correlated with genetic determinants of resistance, we performed whole-genome sequencing of 463 Salmonella and E. coli isolates to identify streptomycin resistance genotypes. From this analysis we found that using a streptomycin resistance breakpoint of >/=64 mug/mL classified over 20% of strains possessing aadA or strA/strB resistance genes as wild-type. Therefore, to improve the concordance between genotypic and phenotypic data, we propose reducing the phenotypic cutoff values to >/=32 mug/mL for both Salmonella and E. coli, to be used widely as ECVs to categorize non-wild-type isolates.