Dangerous bacteria (1,4)
I believe the answer is:
'dangerous bacteria' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for e coli that I've seen before include "cause of serious complaint" , "Some types of this bacterium are toxic" , "cause of ill-feeling" , "Bacterium found in the gut" , "Cause of infection" .)