Sensitivity to bile salts of Shigella flexneri sublethally heat stressed in buffer or broth

Batch cultures of Shigella flexneri M4243 were grown at 37 degrees C in broth to early stationary phase, washed, and heated at 50 degrees C in 0.1 M phosphate buffer (pH 7.0). Cells were surface plated on a tryptic phytone glucose agar (TPGA), TPGA with 0.15 or 0.85% bile salts no. 3 (TPGA-BS 0.15 or TPGA-BS 0.85), or TPGA with 0.25 or 0.50% sodium deoxycholate (TPGA-DC 0.25 or TPGA-DC 0.50). Cells sampled after no heating produced colony counts on TPGA-BS 0.85 or on TPGA-DC 0.50 that were no more than about 0.5 log lower than for unheated cell samples plated on TPGA. Cells heated at 50 degrees C for 30 min produced colony counts on TPGA-DC 0.50 or on TPGA-BS 0.85 that were about 1.5 logs lower than on TPGA. Cells heated for 30 min and shifted to TPG broth at 37 degrees C to allow resuscitation required about 2 h to regain tolerance to 0.85% BS. However, heated cells resuscitated on solid TPGA at 35 degrees C before being challenged with overlays of TPGA-BS 0.85 or TPGA-DC 0.50 required 6 to 8 h on TPGA to regain tolerance to 0.85% BS or 0.50% DC. To regain tolerance to overlays of 0.15% BS or 0.25% DC, heated cells required resuscitation periods on TPGA of about 2 or 2 to 6 h, respectively. Cells heated in TPG broth and sampled after no heating produced colony counts on TPGA that were about 1.5 logs lower than for unheated cell suspensions, suggesting greater apparent injury when heat stressed in broth than in buffer.(ABSTRACT TRUNCATED AT 250 WORDS)