Frozen storage of Escherichia coli O157: H7 in vegetable broth

Rejuvenated cells of Escherichia coli O157: H7 (105 cfu/ml) were frozen in a commercial vegetable broth and stored (−18 °C) for up to 168 h. The pathogen was recovered and enumerated on three bacteriological media. A selective medium (Violet Red Bile Agar or VRBA), a non-selective medium (Plate Count Agar or PCA) and a solid-repair medium (overlaid VRBA on surface plated-PCA) were used. Frozen storage for 24 and 168 h reduced the viable population (Vp) of the pathogen by 56% (P<0.05) and 96% (P<0.01) and damaged about 22% (P<0.05) and 45% (P<0.01) of the survivors, respectively. The average rate of change of the Vp and the non-damaged population (NDp) of E. coli O157: H7 was about 12% and 10% of the respective rate of change for the first 24 h of frozen storage. The solid-repair medium increased the recovery of the pathogen throughout the frozen storage period. An analysis of variance showed that frozen storage significantly (P=0.01) decreased cell recovery. A series of paired t-tests indicated that media not always affected cell recovery (P⩽0.05).