Virulent and avirulent encapsulated variants of Staphyococcus aureus.

There are at least two serologically distinct capsular types of coagulase-positive Staphylococcus aureus. Until now, unequivocal evidence for encapsulation of the Smith diffuse variant was lacking. However, the data presented in this paper provide definitive details of encapsulation of the Smith strain. A marked difference in ld(50) values for the two serologically distinct capsular types of S. aureus was demonstrated. The paradoxical behavior of these two strains suggested that the host was resistant to one and was susceptible to the other. A survey of the carriage incidence in mice for staphylococci and staphylococcal capsular antibodies disclosed the presence of staphylococci and capsular antibodies in these animals. The capsular antibodies detected were reactive against only one of the capsular types of S. aureus. None of the sera from the mice surveyed possessed capsular antibodies against the Smith diffuse variant, but the average incidence for the capsular antibodies against the wound mucoid type was 46%. We postulated that the susceptibility of the mice to the Smith diffuse variant was caused by the absence of protective, type-specific capsular antibodies. Conversely, the resistance of the mice to the wound mucoid staphylococci may have been a result of the presence of type-specific capsular antibodies.