Carolina 3M Petrifilm E.Coli/Coliform Count Plates - Manual (Page 9)

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Petrifilm EC plates appear red with no gas bubbles, coliform colonies
appear red with gas bubbles, and E. coli colonies appear blue with gas
bubbles.
Sometimes gas bubbles are trapped when the inoculate is spread on
Petrifilm plates. These bubbles are irregularly shaped and not associated
with a bacterial colony, while gas bubbles produced by lactose-fermenting
bacteria are associated with a colony (see fig. 11.)
Sample Plate
Figure 12.
Figure 11. Gas bubbles produced by colonies
Gas bubble not associated
with a colony