Identification and Characterization of Shigella with Decreased Susceptibility to Azithromycin in the United States, 2005 to 2014

Abstract Objectives To identify Shigella isolates in the U.S. with decreased susceptibility to azithromycin (DSA) and characterized the genetic mechanisms responsible for this resistance. Methods The National Antimicrobial Resistance Monitoring System (NARMS) at Centers for Disease Control and Prevention (CDC) collects and conducts broth microdilution antimicrobial susceptibility testing on Shigella to determine minimum inhibitory concentrations (MIC) for up to 15 drugs, including azithromycin. Isolates with decreased susceptibility to azithromycin were subjected to molecular methods (PCR, whole genome sequencing, and plasmid typing/transformation) to identify the genetic mechanisms of resistance. Results A total of 118 isolates with decreased susceptibility to azithromycin were tested; 65 (55%) isolates contained only mphA, one ( Conclusions Additional studies are needed to understand the effect on treatment outcomes, epidemiology, and possible additional mechanisms responsible for decreased susceptibility of azithromycin in Shigella.