Effect of pili on susceptibility of Escherichia coli to phagocytosis.

The degree of piliation of 20 clinical isolates of Escherichia coli was correlated with their susceptibility to phagocytosis by human polymorphonuclear leukocytes. Piliation was quantitated by negative staining, and phagocytosis was quantitated by a monolayer technique. Ingestion was confirmed by electron microscopy. In the absence of source of opsonins, there was a positive correlation between the degree of piliation and susceptibility to phagocytosis (y = 0.83x + 19.58; correlation coefficient = 0.65; P < 0.01). Heavily piliated strains were no longer phagocytized after their pili were removed by ultraviolet irradiation. Phagocytosis was reduced 75% in the presence of 0.1 M d-mannose, an agent which competitively inhibits binding of pili to cell surfaces. l-Mannose, d-glucose, and d-galactose were much less inhibitory. The viability of piliated organisms was reduced by 1 log after 1 h of incubation with polymorphonuclear leukocytes. Addition of 10% fresh human serum increased both the rate and completeness of killing. These observations suggest that polymorphonuclear leukocytes may interact with the pili of E. coli to promote phagocytosis. This phenomenon may have clinical relevance in situations where normal opsonic activity is poor, such as the renal medulla.