ENHANCEMENT OF VIRULENCE OF MALLEOMYCES PSEUDOMALLEI

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Abstract
The virulence for mice of old strains of M. pseudomallei was generally low but varied with the route of inoculation. The LD50 was approximately 107 organisms intra-peritoneally (ip) and 104 intracerebrally (ic). Virulence was markedly enhanced by serial passage in mice by the ic route. After adaptation, the ic LD50 of 1 smooth and 2 rough isolates was < 10 organisms and the ip LD50 approximately 105 organisms. With additional serial ip passages, the ip LD50 for both types was < 100 organisms. Respiratory virulence of the smooth strain was likewise enhanced by serial ip passage in mice from > 107 organisms/LD50 before adaptation to< 30 after adaptation. Similar results were obtained with the rough strains. Adaptation to mice of both rough isolates was accompanied by a change in colonial morphology from the initial large rough type to smaller intermediate rough types which could be maintained for only a few passages in culture media. Reversion to the parent type was accompanied by decrease in virulence. The smooth isolate, on the other hand, showed no change in colonial morphology during the course of nearly 70 serial passages in mice. Two strains recently isolated from human infections were somewhat more virulent for mice than those above. Serial passage in mice increased their virulence to approximately 5 organisms/LD50. The high initial virulence of these 5 strains for hamsters by all routes of inoculation was not decreased by their adaptation to mice.