Virulence for mice of Klebsiella strains belonging to the O1 group: relationship to their capsular (K) types

Eighty-two Klebsiella O1 strains belonging to various K types were tested for virulence for mice by intraperitoneal inoculation. They comprised 49 strains newly isolated from various clinical specimens, 31 reference strains, and 2 laboratory strains. Of 9 Klebsiella O1:K2 strains, 7 were highly virulent inasmuch as their 50% lethal doses per mouse were less than 10 CFU, whereas the other 2 strains were avirulent even when encapsulated just like the virulent strains as revealed by the quellung test. Klebsiella K2 reference strain B5055 and strain Chedid, which were maintained in vitro for a long time, were highly virulent. Of 8 Klebsiella O1:K1 strains, 2 showed moderate virulence, and the other 6 strains had relatively low or no virulence. We found no definite correlation between the virulence of Klebsiella K1 and K2 strains for mice and the kind of clinical specimens in which they originated. All of the Klebsiella O1 strains tested belonging to K types other than K1 and K2 showed very low or no virulence for mice.