Comparison of selective agar media and enrichment broths for recovering heat-stressed Escherichia coli O157 : H7 from ground beef

Media for detecting and enumerating healthy as well as heat-injured cells o f Escherichia coli O157:H7 in foods are highly desired. This study was cond ucted to evaluate the performance of eight selective and two non-selective direct plating agar media for their ability to recover E. coli O157:H7 cell s from unheated and heated ground beef, and to compare the ability of five enrichment broths to recover E. coli O157:H7 cells from heated ground beef. Ground beef was incoulated with E. coli O157:H7 and heated at 56 degrees C for up to 30 min. Each agar was evaluated for its ability to support colon y formation by E. coil O157:H7 surviving heat treatment, and each enrichmen t broth was evaluated for its ability to recover low numbers of surviving c ells. Of the selective media tested modified eosin methylene blue agar (MEM B) and Rainbow(TM) agar O157 supported recovery of significantly (P less th an or equal to 0.05) higher numbers of heat-stressed cells of E. coli O157: H7, regardless of heating time. CHROMagar(TM) O157 sorbitol MacConkey agar (SMA) supplemented with cefixime and potassium tellurite (CT-SMAC), and SMA supplemented with cefixime and rhamnose (CR-SMAC) performed less favorably , even in recovering cells of E. coil O157:H7 that had not been subjected t o heat stress. SMA and BCM(TM) O157:H7 agar were similar to CT-SMAC and CR- SMAC in their ability to recover E, coli O157:H7 from heated beef. Tryptone bile X-glucuronide (TBX) agar performed significantly better than these me dia, but was inferior to MEMB agar and Rainbow(TM) agar O157:H7. Enrichment using tryptone soya broth with novobiocin or a procedure using brain-heart infusion and tryptone phosphate broths recovered the highest population of heat stressed E. coli O157:H7. EZ Coli(TM) enrichment broth was inferior t o other broths in resuscitating injured cells and supporting subsequent gro wth. (C) 1998 Academic Press.