Microbiological examination for dairy purposes. Part 3. Methods of detection and/or enumeration of specific groups of microorganisms. Section 3.8. Enumeration of presumptive Escherichia coli
British Standard (BS 4285 Section 3.8 1988): 4
A colony count technique and MPN technique are specified for enumerating presumptive Escherichia coli in milk and milk products. In the colony count technique a defined test portion or series of decimal dilutions is mixed with violet red bile lactose agar in Petri dishes, allowed to set, overlaid with the same medium and incubated for 24 h at 44 degrees C. Selected characteristic red colonies are then incubated for 24-48 h at this temp. in tubes of lactose bile brilliant green broth (LBBGB) and tryptone water. The number of presumptive E. coli is calculated from the percentage of colonies giving positive results in both the LBBGB (gas) and the tryptone water (deep red colour due to indole on addition of Kovacs reagent).