Evaluation of Dry Medium Film (Petrifilm VRB) for Coliform Enumeration

A new system, Petrifilm VRB, was compared to two commonly used methods for enumerating coliforms: (a) colony counts on violet red bile agar (VRBA) and (b) MPN determinations with lauryl sulfate tryptose broth (LST) and brilliant green lactose bile broth (BGLB). Petrifilm VRB consists of a bottom film coated with VRB nutrients and a top film coated with a cold-water-soluble gelling agent and a tetrazolium indicator dye. Gas bubbles from lactose fermentation produced around colonies differentiated coliforms from other gram-negative organisms. Coliforms from 120 raw milk samples were enumerated by the Petrifilm VRB, VRBA and MPN methodologies. Randomly selected colonies were chosen from each methodology for characterization. Counts determined with Petrifilm VRB were comparable to counts with VRBA, and both Petrifilm VRB and VRBA compared similarly to the MPN method. Data examined by plotting log₁₀ coliform counts from Petrifilm VRB vs. log₁₀ coliform counts from VRBA resulted in a regression line with a slope of 0.959 and an intercept of -0.076 which was close to the line of equality (slope of one and intercept of zero) for the two methods. No method selected or excluded any one type of microorganisms, and VRBA and Petrifilm VRB had equivalent repeatability.