Since there are conflicting reports in the literature on a possible relationship between the K1 capsular polysaccharide (CP) content of Escherichia coli and its susceptibility to killing, we reexamined this issue in a strain that had a smooth lipopolysaccharide (LPS) phenotype (E. coli O18:K1:H7 Bort) and in a strain with a deep rough LPS phenotype (E412, spontaneously agglutinable: K1:H-). When cell-associated K1 capsular content was greater than 90 micrograms of K1 polysaccharide per 10(10) CFU, neither strain was lysed by 20% normal human serum. In contrast, at equivalent but lower levels of K1 CP content, E412 but not strain Bort was lysed by normal human serum. Thus, LPS phenotype is an additional surface determinant that affects bacterial susceptibility to killing. Organisms obtained from very early log phase, when cell-associated K1 CP is greatest, were significantly more virulent for mice than were bacteria harvested in stationary phase, when cell-associated K1 polysaccharide is lowest. We conclude that (i) there is a threshold level of K1 CP needed to confer protection from lysis by serum, and this is usually exceeded under standard growth conditions; (ii) at a given level of K1 CP the LPS phenotype is an important determinant of bacterial killing; and (iii) the loss of capsule at low pH may be an additional mechanism by which hosts defend against invasive infection by K1-encapsulated E. coli
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