The sensitivity of strains to 80% normal human serum (NHS) and the relative importance of the classical and alternative complement pathways was assessed in relation to K1, K5, and O antigen carriage. Strains of each of the common O-serogroups, O, O2, O4, O6, O7, O9, O18 and O75, smooth strains not typable (NT) with these antisera and auto-agglutinable (AA) strains were studied. Of the 166 strains studied, 37 carried the K1 antigen and 45 the K5 antigen. The variation in sensitivity to NHS between different O-serogroups reported previously was confirmed. Although carriage of the K1 and K5 antigens varied with O-serogroup, this did not explain the differences either between or within O-serogroups. Strains with the K1 or K5 antigen were significantly more resistant to the alternative complement pathway than strains without these antigens. However, this appeared to be more related to the O-serogroups with which they were associated; 37 of 50 O2, O4, O6 and AA strains were affected by complement through both pathways but 20 of 30 O7, O18 and O75 strains were affected by the classical pathway alone and 16 of 20 O9 and NT strains were affected by the alternative pathway alone.


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