Frequency of failure to isolate Shigella spp. by the direct plating technique and improvement of isolation by enrichment in selenite broth

In order to clarify the failure to isolate Shigella spp. by direct plating, we compared frequencies of Shigella spp. isolation by direct plating and b y plating after enrichment in selenite broth. A total 67 strains were isola ted in this study. The strains of 38 (56.7%) were isolated only by direct p lating, and 25 (37.3%) strains were isolated by both direct plating and aft er enrichment. Four strains (6.0%) were isolated after enrichment but not b y direct plating. Since 6% of isolated Shigella spp. were not isolated by d irect plating, we recommend that direct plating and additional isolations f rom selenite broth should be performed. The significance concerning reducti on of concentration of sodium selenite in enrichment broth is discussed.