Samples with colonies of Enterobacteriaceae
51. After incubation each set of duplicate plates shall be examined for colonies characteristic of Enterobacteriaceae (purple colonies 1-2 mm in diameter). All characteristic colonies on each plate shall be counted and the arithmetic mean of the duplicate plates taken.
The sample provisionally fails if either –
(a)any arithmetic mean is above 30(1); or
(b)three or more arithmetic means are above 10;
in which case the following procedure shall be followed to establish whether or not the colonies are Enterobacteriaceae.
(1)
An arithmetic mean of 30 is equivalent to 3 × 102 colony forming units per gramme of original sample