In vitro bactericidal activity of Streptococcus pneumoniae and bactericidal susceptibility of Staphylococcus aureus strains isolated from cocolonized versus noncocolonized children.

Streptococcus pneumoniae is bactericidal to Staphylococcus aureus in vitro. To determine whether this in vitro effect accounts for the inverse relation between S. pneumoniae and S. aureus colonization reported in previous epidemiologic studies, we compared S. pneumoniae and S. aureus strains from cocolonized children to those from noncocolonized children. Cocolonizing pneumococci were less bactericidal and cocolonizing staphylococci less susceptible to this effect; however, the magnitude of the effect was small. Thus, in vitro killing is not the major determinant of the pattern of cocolonization.