Intestinal bacteria (5)
'intestinal bacteria' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for e coli that I've seen before include "Some types of this bacterium are toxic" , "Common species of bacterium" , "An infection found in food and water" , "cause of serious complaint" , "cause of ill-feeling" .)
(Another definition for ecoli that I've seen is " Bacterium".)