Establishing Streptomycin Epidemiological Cut-Off Values forSalmonellaandEscherichia coli

Abstract

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 4 mg/L harbored resistance genes. Of the Salmonella isolates exhibiting MICs 8 mg/L, 16 mg/L, and 32 mg/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) ≤32 or ≤16 mg/L. A cut-off value of WT ≤32 mg/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 ≤32 mg/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 4 mg/L, 8 mg/L, 16 mg/L, and 32 mg/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 ≤16 mg/L), 25% of the E. coli strains presenting MIC ≤16 mg/L would have been incorrectly categorized as belonging to the WT population. The authors recommend an ECOFF value of WT ≤16 mg/L for Salmonella and WT ≤8 mg/L for E. coli.

A Suitable Streptomycin-Resistant Mutant for Constructing Unmarked In-Frame Gene Deletions Using rpsL as a Counter-Selection Marker

“…In the 18 K. pneumoniae strains that had MICs≥6 µg/ml, streptomycin resistance genes could only be found in the 6 strains that had MICs≥12 µg/ml. This result was similar to the previous study in which the streptomycin epidemiological cut-off values were recommended as WT≤16 µg/ml for Salmonella and WT≤8 µg/ml for E. coli [2] . The aadA gene was found in the four Klebsiella strains exhibiting streptomycin MIC 12 or 16 µg/ml.…”

“…The aadA gene was found in the four Klebsiella strains exhibiting streptomycin MIC 12 or 16 µg/ml. The low-level resistance to streptomycin conferred by aadA can also be found in Salmonella and E. coli [2] , [7] , [44] . No streptomycin resistance was found to be caused by rpsL mutations in any of the 127 clinical strains of K. pneumoniae .…”

“…Streptomycin, the first aminoglycoside antibiotic, was reported in 1944 [1] and can inhibit protein synthesis by binding to the 30S ribosomal subunit. The most common mechanisms contributing to streptomycin resistance are aminoglycoside-modifying enzymes and target gene mutations [2] , while 16S rRNA methylases, which have recently emerged, confer high-level resistance to all clinically available aminoglycosides except streptomycin [3] – [5] . Adenylyltransferase encoded by aadA and phosphoryltransferase encoded by strA or strB have been demonstrated to be responsible for streptomycin resistance and are widely disseminated among Enterobacteriaceae [2] , [6] , [7] .…”

“…The most common mechanisms contributing to streptomycin resistance are aminoglycoside-modifying enzymes and target gene mutations [2] , while 16S rRNA methylases, which have recently emerged, confer high-level resistance to all clinically available aminoglycosides except streptomycin [3] – [5] . Adenylyltransferase encoded by aadA and phosphoryltransferase encoded by strA or strB have been demonstrated to be responsible for streptomycin resistance and are widely disseminated among Enterobacteriaceae [2] , [6] , [7] . On the other hand, streptomycin resistance caused by its target genes is mediated by mutations in 16S rRNA encoded by rrs or ribosomal protein S12 encoded by rpsL [8] .…”

Whole-Genome Sequencing for Detecting Antimicrobial Resistance in Nontyphoidal Salmonella

“…The NARMS program has long used Ն64 mg/liter to define resistance and detected it with a narrow dilution range (32 mg/liter to 64 mg/liter) around the breakpoint. Further testing using a broader dilution range (2 mg/liter to 64 mg/liter) revealed that Ն32 mg/liter better reflects the presence of resistance genes in Salmonella and E. coli (30), as has been found by others (31). Based on these data, the NARMS will change interpretive criteria for streptomycin resistance in Salmonella in future reports.…”