Intestinal bacterium (1,4)
I believe the answer is:
e coli
'intestinal bacterium' 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 illness" , "Common species of bacterium" , "Cause of infection" , "Some types of this bacterium are toxic" , "Type of bacteria causing food poisoning" .)