Capability of Selective Media to Detect Heat-Injured Shigella flexneri.

Heat-injured cells of Shigella flexneri 5348 were detected by using a two-media plating system: tryptic soy broth containing 2% agar plus 1% sodium pyruvate (TSBA + P) on which both injured and non-injured cells produce colonies and semisynthetic agar medium plus 0.15% bile salts #3 (SSBA) on which only non-injured cells grew. Subtracting the count on SSBA from that on TSBA + P gave an estimation of injury. Heating washed cells of S. flexneri in phosphate buffer gave about 3 log units of injury after 40 min at 52°C. A number of broths and agars specific for shigellae and gram-negative bacteria (2% agar was added to selective broths) were tested for their ability to supported colony formation of heat-injured S. flexneri . Only one medium, shigella broth containing novobiocin, gave recovery of heat-injured cells similar to that on TSBA + P. Lauryl tryptose broth, Levine EMB agar, and EMB agar were the least inhibitory to repair of injured cells (as compared to SSBA). Media that were more inhibitory to repair of heat-injured cells (but not as inhibitory as was SSBA) included tergitol 7 agar, desoxycholate lactose agar, brilliant green 2% bile broth, GN broth, Mac-Conkey agar, and Endo agar. Desoxycholate agar, violet red bile agar, SS agar, and desoxycholate citrate agar were more inhibitory than SSBA to repair. The addition of 1% sodium pyruvate to MacConkey agar, desoxycholate lactose agar, lauryl tryptose broth, tergitol 7 agar, EMB agar, and SSBA resulted in increased recoveries of heat-injured S. flexneri but did not lead to complete recovery (i.e., the counts on the pyruvate containing selective media did not give counts equal to TSBA + P). These results indicate that most media used for isolation of shigellae do not permit recovery of heat-injured cells, and special techniques must be incorporated into the isolation procedures to recover injured cells.