Validation of Compact Dry EC Medium for the Enumeration of Escherichia coli in Fresh Fruit and Vegetable, Raw Meat and Raw Milk Cheese Samples at an Incubation Temperature of 42°C

Introduction: Canadian government regulations require the enumeration of Escherichia coli in food products such as fresh fruit and vegetables (FFV), raw ground meat, meat trim and cheese. The analytical methods used are MFHPB-27 (Direct Plating) for FFV and MFHPB-34 (E. coli Petrifilm™) for other food samples, both of which use a non-selective incubation temperature of 35°C. For these food products, in which considerable competitive bacterial flora may be present, the colony count is a challenge.  

Purpose: The Compact Dry EC (CDEC) has been validated by the manufacturer for enumeration of E. coli in foods at 35°C.  Because of its physical properties, the CDEC plate can be incubated at 42°C for inhibition of competitive microorganisms’ growth, which facilitates the determination of accurate counts of E. coli.

Methods: Selectivity of the CDEC, determination of its linearity compared to a reference medium and determination of its limit of quantification (LOQ) for the targeted food category has been evaluated. The selectivity study was conducted using 69 strains of coliforms and 50 non-E.coli and non-coliform strains. The linearity of CDEC was done on lettuce, raw ground beef and camembert cheese spiked at concentrations close to the regulatory level specific for E. coli. The performance of the CDEC was further evaluated using a total of 179 contaminated foods.

Results: The established LOQ (10 CFU/g for FFV, 50 CFU/g for other food products) meets the Canadian regulatory standards related to E. coli. The visual and statistical evaluation (linear regression, comparison of means and standard deviations) of the data obtained using CDEC at 42°C for 24 h showed that this medium can be an alternative approach for E. coli enumeration in FFV, raw meat and raw milk cheese.

Significance: CDEC allows for a 42°C incubation temperature, providing food laboratories with an alternative method for evaluating foods with considerable competitive flora.