The behaviour of strains of of capsular serotype K21 and strains of producing a structurally related polysaccharide (colanic acid) was analysed by phagocytic and serum-killing assays. The cell-surface characteristics of these strains and of non-capsulate strains derived from them were also investigated by partitioning experiments in aqueous two-polymer phase systems. The possession of K21-type capsule by or colanic-acid polysaccharide by conferred a strong negative charge on capsulate bacteria. Negatively charged bacteria of producing colanic-acid capsules, however, like non-capsulate , were susceptible to uptake by polymorphonuclear leukocytes. In contrast, K21 polysaccharide conferred on klebsiellae considerable resistance to phagocytic uptake. The finding that ingested non-capsulate derivative strains of were less rapidly degraded by phagocytes than strains suggested that other components of the cell surface of , notably lipopolysaccharide, may be involved in protection against phagocytic killing. The presence of colanic-acid capsules on conferred little resistance to the bactericidal activity of human serum or phagocytic uptake and did not protect against intracellular killing by polymorphonuclear leukocytes.


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