5. After incubation each set of duplicate plates shall be examined for colonies characteristic of Enterobacteriaceae (purple colonies 1–2mm 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.
An arithmetic mean of 30 is equivalent to 3×102 colony forming units per gramme of original sample