The virulence of 36 strains of was studied by intravenous injection into mice; the strains were found to fall into three broad groups, of high, medium and low virulence. This difference could not be related to any biochemical property studied, pyocine type nor antibiotic sensitivity pattern. It was found that 24 of the 30 strains of the high and medium virulence group produced large colonies on agar while 5 of the 6 strains of the low virulence group produced small colonies when plated under similar conditions. To test this observation a further series was collected and the strains were allocated to the high or low virulence category on colonial appearance. Mouse challenge with these fresh strains showed that the prediction was accurate. The large colony type had a faster growth rate on agar and in broth than the small. When a strain was grown in agar it was found to be more virulent than when harvested from broth ( = 0.02–0.01). Chromatography studies on slime derived from all three categories showed that they were apparently chemically similar. The amount of slime produced by a standard number of bacteria was measured for agar and broth grown cultures. The yields for the two media were approximately the same with large-colony forms. Small-colony forms produced similar amounts to the large forms on plates, but up to 15 times as much when grown in broth.

The ability to kill mice was derived in part from early toxic death; the remainder died of renal disease. Study of the initiation of kidney infection showed that large-colony types were more successful in maintaining their numbers in the kidneys over the first 5 hr. Strains grown on nutrient agar did not give higher counts in the kidney than those from broth in the first 5 hr but after 24 hr they grew faster, infected more kidneys and killed more mice.


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