Superiority of MacConkey's agar over salmonella-shigella agar for isolation of Shigella dysenteriae type 1

Abstract

The efficiency of MacConkey's agar in the isolation of various types of Shigella was compared with that of salmonella-shigella (SS) agar during an extensive 18-month outbreak of disease caused by Shigella. In all, 1,580 isolates of Shigella were obtained from 12,307 rectal swab and fecal samples of patients with diarrhea and their contacts by direct plating onto MacConkey's and SS agars. Shigella dysenteriae type 1 and Shigella flexneri constituted 55 percent and 33 percent of all isolates, respectively, with a smaller number of Shigella boydii and Shigella sonnei. MacConkey's agar was superior to SS agar in the detection of S. dysenteriae type 1; 83 percent of the isolates were detectable on MacConkey's agar, compared with 40 percent on SS agar. In contrast, 84 percent of S. flexneri isolates were detectable on SS agar, compared with 51 percent on MacConkey's agar alone. These findings confirm that, for the culture of fecal specimens thought to contain S. dysenteriae type 1, one of the media used should be noninhibitory. Shigella was isolated at a consistently and significantly higher rate from fecal samples than from rectal swabs.