The importance of the K1 capsule in invasive infections caused by Escherichia coli.

We examined 534 clinical isolates of Escherichia coli for sensitivity to rough lipopolysaccharide-specific and K1-specific phages. Twenty-eight percent of bacteremic isolates were sensitive to rough-specific phages. Forty-two percent of these strains, against only 20% of bacteremic isolates insensitive to rough-specific phages, had K1 capsule (P less than 0.001). K1-positive strains were usually resistant to phagocytic killing, whereas strains lacking the K1 capsule were more likely to be killed regardless of capsular type. Eighty-two percent of strains were typable with O-specific, 57% with K-specific, and 74% with H-specific antisera. Sixty percent of E coli were agglutinated by only 10 O-specific antisera. K1 was the most common capsular type, followed by K5, K2, and K12, whereas four H antigens accounted for nearly half of the H-typable strains. We conclude that (1) the combination of rough-specific and K1-specific phage sensitivity defines functionally similar groups of bacteria and (2) a polyvalent vaccine against invasive E coli is possible given the relatively limited number of invasive O:K:H serotypes.