Optimization of stool culture protocol in a tertiary hospital in Bangkok, Thailand: a prospective cohort study

Stool culture protocols are resource-intensive, and the necessity of specific selective media is frequently debated. We aimed to determine the prevalence of bacterial enteric pathogens in a Thai tertiary hospital and optimize routine culture protocols by evaluating various diagnostic methods.

We evaluated 597 diarrheal stool samples using standard protocols for Salmonella enterica, Shigella spp., Vibrio spp., Aeromonas spp., and Plesiomonas shigelloides. Additional protocols evaluated the detection of Campylobacter spp. (selective medium and membrane filtration) and enteric Yersinia spp. (cefsulodin-irgasan-novobiocin [CIN] agar). To estimate S. enterica method sensitivities, we applied Bayesian latent class analysis (LCA). Additionally, analytical limit of detection (LOD) studies using spiked stool matrices were performed for S. enterica, Campylobacter spp., and Yersinia spp.

Enteric pathogens were isolated from 12.7% of the samples, predominantly S. enterica (10.4%) and Campylobacter spp. (2.2%); no Yersinia spp. were detected. For S. enterica, LCA estimated sensitivities of Gram-negative (GN) broth (97.4%) and modified semisolid Rappaport Vassiliadis (MSRV) agar (89.4%), significantly outperforming other solid media. Campylobacter membrane filtration and selective agar showed near-perfect clinical agreement (99.8%); however, spiked LOD testing revealed that selective agar was 10⁴-fold more analytically sensitive. For Yersinia spp., both CIN and MAC agars achieved a LOD of 10⁴ CFU/ml in fresh stool.

GN broth and MSRV agar are needed to maximize S. enterica yield. Routine CIN agar for Yersinia spp. is not cost-efficient, and Campylobacter selective agar should replace membrane filtration to prevent false-negative reporting in low-shedding patients.