Effect of serum on gram-positive cocci grown in the presence of penicillin.

Strains of Staphylococcus aureus, Streptococcus faecalis, Streptococcus bovis, and a strain of Streptococcus that produces filaments were grown on agar containing penicillin at concentrations of one-half to 1/10th the minimal inhibitory concentration. These penicillin-exposed organisms as well as untreated control organisms were incubated with human serum of plasma. Both serum and plasma produced a remarkable bactericidal effect on the filament-forming Streptococcus grown in the presence of penicillin, whereas the untreated control was only slightly affected. This response resembled that of gram-negative bacilli rather than that of gram-positive cocci. The growth of staphylococci exposed to penicillin was slightly inhibited in the presence of serum, whereas the growth of untreated staphylococci was stimulated. The streptococci, regardless of whether they were treated with penicillin or whether they were untreated, showed no change in growth pattern in the presence of serum.