Effect of Storage Times and Temperatures on Escherichia coli Isolation

Introduction: Beef released to commerce in the US and other countries must be tested and deemed free of Escherichia coli O157. Confirmation of the presence of E. coli O157 occurs at centralized laboratories and can often require the transport of enrichment broths for substantial distances and times. It is therefore important that the type of enrichment broth used, time between screening and confirmation, and storage temperatures of broths prior to confirmation do not negatively impact isolation of E. coli O157.

Purpose: Determine if storage times and temperatures affect the likelihood of isolating E. coli O157 from potential positive broths.

Methods: A cocktail of rifampicin resistant E. coli O157 were inoculated into three commercially available broths with no, low or high background beef microflora and enriched for 12 and 24 h. Enrichments were stored at 4, 10 or 20° C for 7 days and detection using IMS and counts of E. coli O157 were determined daily.

Results: The concentration and isolation of E. coli O157 from a potential positive enrichment broth stored at 4°C or 10° C was not significantly affected by the type of enrichment broth, concentration of background microflora, enrichment time, or the storage of the enrichment broth with maximum reductions of 0.59 log­ CFU/ml observed during the seven day storage period. Isolation of E. coli O157 from high background samples stored at the abuse temperature of 20°C was problematic.

Significance: Storage of enrichment broths at 4 or 10°C had minimal negative effect on the concentration or the likelihood of isolating E. coli O157 from potential positive enrichment broths during seven days storage. Whilst a 100% conversion of potential positives to confirmed positives may be difficult to achieve, it is clear that the transport and storage protocols currently used by Australian beef processors do not affect the likelihood of isolating E. coli O157.