Repair and growth of heat- and freeze-injured Escherichia coli O157:H7 in selective enrichment broths

Two Escherichia coli O157:H7 strains, ATCC 35150 and 43894, were heat injured in a beef infusion at 53°C for 40 and 50 min, respectively (1· 5–2·0 log₁₀cfu ml⁻¹of injury) and freeze injured at −25°C for 30 days (1 log₁₀cfu ml⁻¹of injury) as determined by plating on MacConkey agar with 0·60% bile salts #3 (Mac-BS) as the selective medium and on Brain Heart Infusion agar (BHIA) as the non-selective medium. Repair of injury was measured in five selective enrichment broths [buffered peptone water supplemented with vancomycin, cefsulodin, and cefixime (BPW-VCC), modified EC broth with novobiocin (mEC+n), enterohaemorrhagic E. coli enrichment broth (EEB), double modified TSB (dmTSB), and BCM^®E. coli enrichment broth (BCM^®-EB)] versus TSB as the non-selective control broth over 3 h incubation at 37°C and 42°C. Repair was measured as the increase in cfu ml⁻¹enumerated on Mac-BS with time vs the total cfu ml⁻¹(injured and uninjured cells) enumerated on BHIA. In mEC+n, EEB, and dmTSB some death of both heat- and freeze-injured cells occurred immediately during the 3 h incubation (decrease on BHIA plates), and there was either minimal or no repair of the injured cells at both temperatures. Efficient repair of heat injury was obtained with both BPW-VCC and BCM^®-EB, but the latter produced a growth rate and final cell concentration closer to TSB. In freeze-injury repair however, BPW-VCC gave poor results while repair in BCM^®-EB was equal to TSB. Both BCM^®-EB and BPW-VCC inhibited the growth of all Gram-positive and a select number of Gram-negative bacteria tested. The ability of the selective enrichment broth BCM^®-EB to resuscitate heat- and freeze-injured E. coli O157:H7 efficiently within 3 h, warrants further testing with other types of stress in both artificially and naturally contaminated foods.