Colonial morphology of Escherichia coli on Tergitol-7 medium.

Escherichia coli cultures grown on Tergitol-7 medium, with 2,3,5-triphenyltetrazolium chloride added, produced three main types of colonies: rough, intermediate, and mucoid. These colonies were yellow to amber in color and produced slight yellow zones in the medium. Rough colonies were flat, dry, and spreading, with a cut-glass appearance. Intermediate-type colonies varied considerably, but could be divided into two general subtypes. Intermediate-rough colonies had the cut-glass appearance characteristic of rough colonies, but were much more compact and raised, with irregular edges. Intermediate-smooth colonies had a slight cut-glass appearance, but were smooth and entire. Mucoid-type colonies also appeared in two subtypes. Mucoid A colonies were mucoid hemispheres. Mucoid B colonies, after incubation at 37 C for 24 hr, appeared as small, intermediate colonies. However, during a 24-hr holding period at room temperature, mucuslike material proliferated around the colonies. A fourth type of colony was red with blue surrounding medium. Only mucoid-type cultures could not be serologically O-grouped.