Optimizing Shigella isolation: A multi-site evaluation of laboratory culture methods for Shigella detection, speciation, and serotyping with different transport media and sample types in the Enterics for Global Health study

Shigella is a leading cause of diarrhea and dysentery in children under 5 in low- resource settings and several vaccines are in development. Due to its fastidious nature, Shigella can be difficult to culture and eventual vaccine trials will need to optimize the isolation of Shigella to ensure efficient sample sizes. In the recently concluded Enterics for Global Health study (EFGH) Shigella Surveillance study, we compared Shigella culture isolates rates between rectal swab vs. whole stool; between two swabs vs. one, and between Cary Blair (CB) vs. modified-Buffered Glycerol Saline (mBGS) transport media to identify the optimal methods for Shigella recovery by microbiologic culture. Among 9,476 children aged 6-35 months enrolled in the EFGH study from seven country sites, Shigella isolation rates did not differ significantly between CB (7.8%) and mBGS (7.9%) (p=0.545). Using two swabs improved detection rates (9.3%) compared to one, (7.9%) (p<0.001). Among the 2048 children from Bangladesh and the Gambia where both rectal swabs and whole stool were collected from the same children, rectal swabs were found to be non-inferior to whole stool for Shigella culture (12.4% and 12.7%, respectively with a difference of -0.29% (95% confidence interval -0.83% to 0.24). To optimize Shigella recovery for future multi-country vaccine trials, we recommend collecting two flocked rectal swabs in CB or mBGS media with strict adherence to transit conditions--an approach proven feasible across EFGH sites.