SUMMARY: Serum-susceptible (SS) were induced to resistance (SR) to complement-mediated killing by fresh human serum (FHS) by a small- factor(s) from guinea-pig blood in 3 h at 37 °C, but not in the presence of bacteriostatic concentrations of chloramphenicol or neomycin, indicating that proteins mediated the acquisition of resistance. SDS-PAGE protein profiles of lysates of equal numbers of gonococci showed only two qualitative differences between SR and SS organisms, both in minor components (a protein A of about 205 kDa in the former and not the latter and vice versa for a protein B of about 16 kDa). Many proteins, however, including the three principal outer-membrane proteins, were present in larger amounts in SR gonococci. The lack of major changes in proteins when resistance is acquired was confirmed by immunoblotting the two protein profiles with the IgG of hyper-immune rabbit anti-SR and anti-SS sera, of rabbit anti-SR serum after absorption by SS organisms and of FHS used alone and after absorption with SS organisms. The IgM of FHS, which is responsible for most of the bactericidal activity, showed only faint reactions with a few proteins common to both SS and SR gonococci and no reactions when the FHS was absorbed with SS gonococci. This is in contrast to the strong and different reactions given with lipopolysaccharide (LPS) components of SS and SR organisms, which, prepared from the former organisms, neutralize the bactericidal activity of FHS. Hence, the relatively small protein changes accompanying induction are less likely to be directly responsible for serum resistance than the more profound LPS changes.


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