Capsular polysaccharide is a major complement resistance factor in lipopolysaccharide O side chain-deficient Klebsiella pneumoniae clinical isolates

We have previously demonstrated the existence of Klebsiella pneumoniae clin ical isolates deficient in the lipopolysaccharide O side chain, the major f actor for resistance to complement-mediated killing in this bacterial speci es. These isolates are complement resistant, and their mechanisms to resist complement were investigated by selecting transposon-generated complement- sensitive mutants. One mutant with a drastically reduced capacity to grow i n nonimmune human serum carried the transposon inserted in an open reading frame of a gene cluster involved in capsule synthesis. This mutant produced less capsule, bound more molecules of the complement component C3, and was more sensitive to complement-mediated and opsonophagocytic killings than w as the parent strain. Four additional clinical isolates representing four d ifferent K serotypes were studied, and results showed that capsular polysac charide is a major complement resistance factor in these O side chain-defic ient isolates.