Analysis of outer membrane components of Escherichia coli ML308 225 and of a serum-resistant mutant.

Escherichia coli ML308 225 in the early exponential phase of batch cultures was killed by human serum in a complement-dependent reaction. Under the same conditions, a mutant derived from this strain by multiple subcultures in human serum was resistant to killing by serum. The components of the outer membrane of both the parent and mutant strains were analyzed. The protein and phospholipid analyses were identical, but the mutant strain produced twice as much lipopolysaccharide as the parent strain. These results suggest that the extent of bacterial surface coverage by lipopolysaccharide determines the degree of sensitivity to serum.