Selection of mouse virulent non-motile strains of Escherichia coli by the soft agar technique.

Using strain SME-12 of Escherichia coli and its variants A and B showing large round, diffuse and compact-type colonial morphology, respectively, in soft-agar medium, their capsule showed that although these strains were similar in toxicity, the parent strain exhibited a large capsule, a large cell volume and a high mouse virulence. Variant A had no capsule, its cell volume was remarkably lower than that of the original strain, and was avirulent; variant B had no capsule and displayed the lowest cell volume and mouse virulence. With 193 fresh isolates of E. coli, the majority of colonies of 16, 152 and 25 strains were of large round, diffuse and compact types, respectively. Fifteen strains of pure growth type from each of the three groups were tested for mouse virulence. The majority of strains showing large round-type growth was virulent, diffuse-type strains displayed a low virulence, while no mouse was killed by compact-type strains.