Validation of the Soleris E. coli method for detection and semi-quantitative determination of Escherichia coli in foods

The performance of the Soleris E. coli method was compared with that of the ISO 7251 most probable number (MPN) and detection reference methods for Escherichia coli. The Soleris E. coli method is a growth-based, rapid, automated system composed of temperature-controlled incubation chambers and photodiode-based optical detection devices for measurement of color changes in a prepared medium vial. A dilution of the test sample homogenate is inoculated directly into the vial. Products of E. coli metabolism alter the color of the medium over time, and this change is monitored by the Soleris instrument. The test is used in a dilute-to-specification or specification monitoring manner in which the result is positive or negative around a desired cutoff (in CFU/g) determined by the dilution and volume of sample homogenate added to the vial. Alternatively, the test is used for zero tolerance determinations (e.g., absence in 25 g) by performing an off-line pre-enrichment step followed by transfer of a portion of the pre-enrichment culture to the Soleris vial. Six E. coli strains originating from food sources were inoculated individually into six food commodities: frozen green beans, Echinacea powder, cocoa powder, sweetened condensed milk, pasteurized liquid egg, and shredded mozzarella cheese. Uninoculated samples were included in each trial. The results obtained by the ISO 7251 detection method and the Soleris E. coli method were shown to be in agreement by Chi-square analysis when the presence of E. coli was determined in 25 g of sample. Results from the Soleris E. coli dilute-to-specification method and the ISO 7251 MPN method were found to be in agreement by probability of detection statistical analysis. In inclusivity testing, 52 of 53 E. coli strains were detected within 24 h. Only a non-thermoduric strain of serotype O157:H43 was not detected. In exclusivity testing, all 31 strains tested produced negative results. Results of ruggedness experiments show that accurate results can be obtained even when the operating temperature of the Soleris instrument is set beyond normal tolerances. The internal and independent laboratory studies demonstrated that the Soleris E. coli method could be successfully utilized as an alternative to the reference methods, with a significant time savings of 2 to 3 days.