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

ABSTRACT

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 (<1%) isolate contained only ermB, and 51 (43%) isolates contained both mechanisms. Seven isolates contained IncFII plasmids with mphA, ermB, or mphA and ermB, whereas one isolate contained an IncB/O plasmid with mphA. One (<1%) isolate that contained neither mphA nor ermB contained mutations in rrlH, rplD, and rplV genes and an insertion in rplV, the functions of which are not yet known. CONCLUSION: Additional studies are needed to understand the effect on treatment outcomes, epidemiology and possible additional mechanisms responsible for the decreased susceptibility of azithromycin in Shigella.