1 D. Analysis aoac Official Method (a) coliform count. Place dry-film E. coli count plate, B, or coliform and escherichia coli coliform count plate, (b) (see ), on flat surface. Lift Counts in Foods top film and inoculate 1 mL test suspension onto center of film base. Dry Rehydratable Film Carefully place top film down onto inoculum. Distribute test suspen- (Petrifilm E. coli/ coliform Count Plate and sion over prescribed growth area with downward pressure on center Petrifilm coliform Count Plate ) methods of plastic spreader device (flat side down). Leave plate undisturbed First Action 1991 1 min to permit gel to solidify. Incubate plates 24 2 h at 35 1 C. Final Action 1994 In incubator, place plates in horizontal position, clear side up, in See Table for the results of the interlaboratory study sup- stacks not exceeding 20 units. Count plates promptly after incuba- porting the acceptance of the Method . tion period. After incubation is complete, plates may be stored A. Principle frozen ( 15 C) up to 7 days.
2 This should be avoided as routine practice. Use standard colony counter for counting purposes. Magni- See (see ). fier-illuminator may also be used to facilitate counting. Coliforms ap- B. Apparatus and Reagent pear as red colonies that have one or more gas bubbles associated See (b) (f) (see ). (within one colony diameter) with them. Count all colonies in countable E. coli count plates. Plates are similar to coliform count plates, range (15 150 colonies). Red colonies without gas bubbles are not (b) (see ), with addition of5-bromo-4-chloro-3-indolyl- -D- counted as coliform organisms. glucuronide, an indicator of glucuronidase activity. This allows coliforms (b) E. coli count. Use E. coli count plate and proceed as in (a). In- and E. coli to be read on the same plate. Petrifilm E. coli Count Plates (Micro- cubate an additional 24 2 h (48 4 h total). E. coli colonies appear as biology Products, 3M Center, Bldg. 275-5W-05, St. Paul, MN 55144, USA) blue colonies associated with gas bubbles; other coliforms appear as red meet these specifications.
3 Colonies with gas. C. Preparation of Test Suspension Reference: JAOAC 74, 635(1991). See (see ) and (see ). Revised: March 2002. 2002 aoac INTERNATIONAL. Table Interlaboratory study results for total coliforms and E. coli on dry-film count plates Product Level Mean log, cfu/g sr sR RSDr, % RSDR, %. Total coliforms on coliform count plates Ground turkey Low Medium High Fresh mushrooms Low Medium High Beef with gravy Low Medium High Cheese Low Medium High Wheat flour Low Medium High Nut meal Low Medium High Total coliforms on E. coli count plates Ground turkey Low Medium High Fresh mushrooms Low Medium High Beef with gravy Low Medium High Cheese Low Medium High Wheat flour Low Medium High Nut meal Low Medium High Total E. coli on E. coli count plates Ground turkey Low Medium High Fresh mushrooms Low Medium High Beef with gravy Low Medium High Cheese Low Medium High Wheat flour Low Medium High Nut meal Low Medium High 2002 aoac INTERNATIONAL.