Identification and characterization of Shigella with decreased susceptibility to azithromycin in the United States, 2005 to 2014

Objectives: Our objectives were to identify Shigella isolates in the United States with decreased susceptibility to azithromycin (DSA) and characterize the genetic mechanisms responsible for this resistance. Methods: The National Antimicrobial Resistance Monitoring System (NARMS) at the US Centers for Disease Control and Prevention (CDC) collects and conducts broth microdilution antimicrobial susceptibility testing on Shigella to determine minimum inhibitory concentrations (MICs) for up to 15 drugs, including azithromycin. Isolates with decreased susceptibility to azithromycin were subjected to molecular methods (e.g., polymerase chain reaction [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, 1 (