Frontiers in Marine Science (Jun 2014)
Colilert® applied to food analysis
Abstract
Colilert® (IDEXX) was originally developed for the simultaneous enumeration of coliforms and E. coli in water samples and has been used for the quality control routine of drinking, swimming pools, fresh, coastal and waste waters (Grossi et al., 2013). The Colilert® culture medium contains the indicator nutrient 4-Methylumbelliferyl-β-D-Glucuronide (MUG). MUG acts as a substrate for the E. coli enzyme β-glucuronidase, from which a fluorescent compound is produced. A positive MUG result produces fluorescence when viewed under an ultraviolet lamp. If the test fluorescence is equal to or greater than that of the control, the presence of E. coli has been confirmed (Lopez-Roldan et al., 2013). The present work aimed to apply Colilert® to the enumeration of E. coli in different foods, through the comparison of results against the reference method (ISO 16649-2, 2001) for E. coli food analysis. The study was divided in two stages. During the first stage ten different types of foods were analyzed with Colilert®, these included pastry, raw meat, ready to eat meals, yogurt, raw seabream and salmon, and cooked shrimp. From these it were approved the following: pastry with custard; raw minced pork; soup "caldo-verde"; raw vegetable salad (lettuce and carrots) and solid yogurt. The approved foods presented a better insertion in the tray, the colour of the wells was lighter and the UV reading was easier. In the second stage the foods were artificially contaminated with 2 log/g of E. coli (ATCC 25922) and analyzed. Colilert® proved to be an accurate method and the counts were similar to the ones obtained with the reference method. In the present study, the Colilert® method did not reveal neither false-positive or false-negative results, however sometimes the results were difficult to read due to the presence of green fluorescence in some wells. Generally Colilert® was an easy and rapid method, but less objective and more expensive than the reference method.
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