Efficacy of Petrifilm^™ VRB for Enumerating Coliforms and Escherichia coli from Frozen Raw Beef

Petrifilm^™ violet red bile (PVRB) compared favorably to the most probable number method (MPN) and violet red bile agar (VRBA) methods for enumerating coliforms from frozen raw ground beef. When comparing PVRB and VRBA incubated at 35°C, coliform enumeration displayed a linear relationship (correlation coefficient of 0.932). However, by analyzing 64 ground beef samples, PVRB enumerated 41% more coliforms/g than did VRBA. Two distinct colony types were observed on PVRB: (a) type I (butterfly in appearence) with a colony diameter equal to or greater than 1 mm and gas bubbles 2-4 mm in diameter touching the associated colony; and (b) type II with a colony diameter less than 1 mm in diameter and gas bubbles of the associated colony not necessarily touching the colony but within a colony diameter. The disparity between PVRB and VRBA for enumerating coliforms was attributed to non-coliforms representing approximately 50% of the type II coliform colonies. At 35°C, 83.7% of the type I colonies were Escherichia coli , whereas only 10.9%, of the type II colonies were E. coli . By elevating the incubation temperature from 35°C to 44.5°C, over 90% of the colonies in the counting dilution were type I of which 99.2% were E. coli . At 44.5°C, 39.4% of the type II colonies were E. coli ; however, this colony type represented only 9.5% of the total colonies on PVRB. Therefore, a reliable method for enumerating E. coli from raw meat was developed by counting only the type I colonies on PVRB incubated at 44.5°C.