Transformation during mixed pneumococcal infection of mice.

The recent demonstration by others of transformation during peritoneal infection of mice by two genetically distinct pneumococcal strains supports the notion that transformation may be significant in pneumococcal infection in nature. These studies confirm the occurrence of transformation during mixed infection of mice and define some conditions for its occurrence and its significance. Mice were inoculated with deoxyribonucleic acid (DNA) donor (small type III capsule, low virulence, streptomycin-susceptible) and recipient (noncapsulated, low virulence, streptomycin-resistant) pneumococci, and the bacteremia in mice that died was evaluated. Transformants (large type III capsule, virulent, streptomycin-resistant) were isolated from up to 80% of mice that died from mixed peritoneal infection. Transformation occurred in mice that received donor and recipient 6 hr apart; hence, active DNA was released and competence developed during growth in vivo. Transformation was detected only with progressive infection by both strains, and then transformants were few in the blood and apparently were not responsible for the death of the animals. In doubly infected mice treated with streptomycin, transformation was enhanced; transformants numerically dominated the bacteremia and seemed to cause the death of the mice. Transformation was also demonstrated for the first time during infection of the respiratory tract.