Purpose: To improve the method, we compared the performance of MAC with three more selective conventional agar media [Hektoen-Enteric (HEA), Xylose-Lysine-Desoxycholate (XLD) and Salmonella-Shigella (SSA)], and with three chromogenic media designed to enhance the differentiation of Shigella [Biolog Shigella/Aeromonas rainbow (RBW), R&F Shigella sp. (R&F) and Hardy diagnostic HardyCHROM SS (HSS)].
Methods: Eighty-one Shigella isolates, representing the diversity within the genus, were tested along with 26 non-Shigella strains as potential growth competitors of Shigella. We compared bacterial counts obtained on the non-selective tryptic soy agar and on the 8 selective agars, after 24 h incubation at 35°C. Colony size and color were also used in the performance comparison.
Results: Sensitivity, which is a measure of the agar’s ability to recover and correctly identify Shigella, was highest on MAC agar followed by HEA, SSA, HSS and R&F. R&F recovered S. sonnei and most S. flexneri isolates but SSA was too inhibitory (< 10% recovery) for several S. sonnei isolates although it performed well for the other strains. All agars had good specificity (agar’s ability to exclude Shigella correctly) except for MAC, which had a low specificity of 36%.
Significance: Adding SSA and R&F to MAC in the post-enrichment section of the BAM method for Shigella should improve our ability to recover and identify Shigella from contaminated foods. We will use R&F, MAC and SSA in future enrichment studies for this pathogen.