Expression of capsular polysaccharide determines serum resistance in Escherichia coli K92

The amount of capsular polysaccharide expression has been shown to be the major determinant of serum resistance in Escherichia coli K1. E. coli K92, like K1, is a polymer of sialic acid molecules. It differs from K1 by containing both alpha (2.8) and alpha (2.9) linkages. Four strains of E. coli K92 were tested for serum resistance. Three strains were serum-resistant (50% normal human serum), one strain was moderately serum-sensitive. The serum-resistant strains expressed significantly more capsular polysaccharide than did the serum-sensitive strain. For each of the serum-resistant strains, six mutants were isolated by selection for resistance against infection with a K92-specific bacteriophage. All of the mutants expressed less capsular polysaccharide than the respective wild-type strains. All mutants were more sensitive to serum killing than the wild-type strains. In all groups, the mutants with lowest expression of capsular polysaccharide were highly serum-sensitive. Changes of outer membrane proteins or lipopolysaccharide patterns that were present in some mutants did not correlate with serum resistance properties of the mutants. Furthermore, it was investigated whether the presence of active serum had an influence on capsule expression. In the serum-sensitive strain, the presence of serum induced a significant and concentration-dependent increase of capsule expression. Serum had no effect on capsule expression by the serum-resistant strains. We conclude from the data that the expression of K92 capsular polysaccharide determines serum resistance in the strains examined.