Reliability of Selective Media for the Enumeration and Estimation of Escherichia coli

Suitability of selective plating media for recovering heat- or freeze-stressed Escherichia coli O157:H7 from tryptic soy broth and ground beef

“…E. coli O157:H7 strain CA-1 appears to be the most heat resistant of the O157:H7 strains, while strain 505B appears to be the most sensitive. Roth et al (14) also observed differences in recovery of heatinjured E. coli strains on selective media. Extreme caution must therefore be exercised in interpreting results of studies with stress-injured organisms, especially when only one strain is being evaluated and selective media are used for enumeration.…”

“…The inability of MSMA to recover heat-stressed E. coli O157:H7 is attributed, in part, to the presence of bile salts. Roth et al (14) reported that the presence of bile salts no. 3 in nutrient agar reduced the number of viable heat-treated E. coli SA 603 cells recovered by 50%.…”

“…Our study has shown that substitution of lactose with sorbitol in EMB agar enhances the recovery of heat-and freeze-stressed E. coli O157:H7 cells, as well as heat-stressed E. coli non-O157 cells, even when 0.5% NaCl is present as a minor selective agent. It is not known if lactose inhibits repair of stressed E. coli O157:H7 cells, but Roth et al (14) reported that lactose broth does inhibit the growth of some strains of E. coli. It is well established that heat-stressed microbial cells exhibit increased sensitivity to NaCl (3,4,8).…”