Evaluation of Shigella Species Azithromycin CLSI Epidemiological Cutoff Values and Macrolide Resistance Genes

Abstract

Azithromycin (AZM) has been recommended by the American Academy of Pediatrics for the treatment of shigellosis in children. In this study, 502 <i>Shigella</i> species isolated between 2004 and 2014 were tested for AZM epidemiological cutoff values (ECV) by disk diffusion. AZM MICs and the presence of the macrolide resistance genes [<i>erm</i>(A), <i>erm</i>(B), <i>erm</i>(C), <i>ere</i>(A), <i>ere</i>(B), <i>mph</i>(A), <i>mph</i>(B), <i>mph</i>(D), <i>mef</i>(A), and <i>msr</i>(A)] were determined for all 56 (11.1%) isolates with an AZM disk diffusion zone diameter of ≤15 mm. The distribution of AZM ECV MICs was also determined for 186 <i>Shigella</i> isolates with a disk zone diameter of ≥16 mm. Finally, pulsed-field gel electrophoresis (PFGE) was performed on 15 <i>Shigella flexneri</i> isolates with an AZM disk zone diameter of <16 mm from different years and geographic locations. Serotyping the 502 <i>Shigella</i> species isolates revealed that 373 (74%) were <i>S. sonnei</i>, 119 (24%) were <i>S. flexneri,</i> and 10 (2%) were <i>S. boydii</i> Of the 119 <i>Shigella flexneri</i> isolates, 48 (42%) isolates had an AZM disk diffusion zone diameter of ≤15 mm and a MIC of ≥16 µg/ml. With the exception of one isolate, all were positive for the macrolide resistance gene <i>mph</i>(A). <i>S. flexneri</i> PFGE showed four distinct patterns, with patterns I and II presenting with 92.3% genetic similarity. On the other hand, 2 (0.5%) of the 373 <i>S. sonnei</i> isolates had the AZM non-wild-type (NWT) ECV phenotype (those with acquired or mutational resistance), as the AZM MICs were ≥32 µg/ml and the isolates were positive for the <i>mph</i>(A) gene. Overall, our <i>S. flexneri</i> results are in concordance with the CLSI AZM ECV, but isolates with an AZM disk diffusion zone diameter between 14 and 15 mm should be carefully evaluated, as the <i>S. flexneri</i> AZM MIC for NWT isolates may need adjustment to 32 µg/ml. Our data on <i>S. sonnei</i> support that the AZM NWT ECV should be 11 mm for the disk diffusion zone diameter and ≥32 µg/ml for the MICs.