Loss of resistance to ingestion and phagocytic killing by O- and K- mutants of a uropathogenic Escherichia coli O75 : K5 strain

To determine the importance of the O75 O antigen and the K5 capsular antige n in resistance to phagocytosis and phagocytic killing, we used previously described O75(-) and K5(-) mutants from an O75(+) K5(+) wild-type uropathog enic Escherichia coli strain in phagocytosis assays with polymorphonuclear leukocytes (PMNs) and monocytes. At a 10-to-1 ratio of bacteria to phagocyt es and in the presence of 10% serum, the parental strain GR-12 was resistan t to both PMNs and monocytes over a 2-h incubation period. The O75(-) and K 5(-) mutants were similar in sensitivity to killing by both PMNs and monocy tes, decreasing in viability by 80% in the first hour. Yet, a significant d ifference in killing between the O75(-) and K5(-) mutants was observed in t he first 15 min of incubation. The K5(-) mutant decreased in numbers by alm ost 60%, while the O75(-) mutant increased in numbers similarly to GR-12 in the first 15 min. The difference in killing was found not to be due to the rate of opsonization. To further determine the mechanism of resistance, a fluorescence assay was used to differentiate attached and internalized bact eria. The K5 capsule hindered the association of both the wild-type strain and the O75(-) mutant in the initial incubation time with PMNs. In conclusi on, both the K5 capsule and O75 O antigen play crucial roles in resistance to phagocytosis over time.