[koh-luh
n] 
| any of several bacilli, esp. Escherichia coli and members of the genus Aerobacter, found as commensals in the large intestine of humans and certain other animals, the presence of which in water indicates fecal pollution. |
colon bacillus n.
A rod-shaped bacterium, especially Escherichia coli, a generally nonpathogenic commensal found in all vertebrate intestinal tracts, but which can be virulent, causing diarrhea and other dysenteric symptoms. Its presence in water is an indicator of fecal contamination.