Atypical Escherichia coli in streams.

In the examination of stream waters for fecal coliforms, pale yellow colonies regularly appeared on m-FC broth base medium plates. The yellow colonies may comprise 70% more of the colonies of an m-FC plate. More than 80% of these colonies were identified as Escherichia coli by the API 20E identification system and by serotyping. The atypical yellow E. coli strains were not environmentally stressed E. coli since the atypical colonies continued to be yellow on m-FC medium after growth in a nonselective medium. However, 50% of the atypical E. coli strains were o-nitrophenyl-beta-D-galactopyranoside positive, and 20% produced gas in EC medium at 44.5 degrees C. Failure to consider these atypical E. coli strain in water quality analyses could lead to a significant error in the estimation of water quality in some instances.