SUMMARY. The relative importance of the classical and alternative complement pathways in serum bactericidal activity against strains of the common urinary O-serogroups has been assessed with strains that differ widely in their sensitivity to normal human serum. With most promptly serum-sensitive strains, rapid killing occurred, mediated by the classical pathway and, when this pathway was eliminated, delayed killing by alternative-pathway activity occurred. However, one strain of serogroup O1 was affected by the classical pathway only and a strain of serogroup O9 was killed rapidly by the alternative pathway. Strains with delayed sensitivity to normal human serum were largely, and in some cases exclusively, affected by the classical pathway.

When added to heat-inactivated serum, some strains showed no significant growth whereas the viable numbers of other test strains increased more than 50-fold in 3 h. Whether this variation is due to differences in nutritional requirements or sensitivity to some non-complement-dependent bacteriostatic mechanism remains to be determined.


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