Clinical evaluation of enteric media in the primary isolation of Salmonella and Shigella.

Over a 1-year period, media for the isolation of enteric pathogens were compared on 455 stool specimens. Fifty-three pathogens were isolated, of which 56% were Shigella sonnei and 13% were Sh. flexneri. Of these isolates, 90% were found on xylose-lysine-desoxycholate agar, 87% on Hekton enteric agar, and 80% on MacConkey without crystal violet with 2% agar and 0.007% neutral red, but only 28% were recovered on Salmonella-Shigella agar. Less than one-half of the shigellae were recovered after Selenite-F enrichment. On the other hand, enrichment was the most helpful method for isolating salmonellae. Studies on cultures from which mixed isolates were obtained indicated that numbers and chance distribution have an effect on the results obtained. The performance of Salmonella-Shigella agar in the isolation of enteric pathogens was inferior, and the effort involved to obtain those isolates was greater than for Hekton enteric and xylose-lysine-desoxycholate agars.