Serum sensitivity of a diversity of Escherichia coli antigenic reference strains. Correlation with an LPS variation phenomenon.

One hundred and ninety-four E. coli O, K and H antigen reference strains and some strains of certain O:K types were examined for resistance to the bactericidal effect of normal human serum by the method of Olling. A strain was defined as serum-resistant when less than 50% and serum-sensitive when greater than 90% bacteria were killed. Fifty-seven reference strains were serum-sensitive, 21 of them produced K antigens which apparently had no protective role. Thirty-seven reference strains were serum-resistant, and 13 of these produced no K antigen; thus the O antigens might by themselves be protective in these cases. K- mutants of the serum-resistant O8:K87 strains were serum-sensitive, and the O9:K9 strain changed from being serum-resistant to being partly resistant when the K antigen was lost; mutants of the O6:K13 strain with no or a small amount of K13 were less resistant than the original K+ strain; similar mutants of the O22:K13 reference strain did not show any decrease in serum resistance. Strongly related to 022 is 083, and results with several 083 strains pointed to the importance of this O antigen for serum resistance. In an O83:K14 strain a spontaneous variation occurred in LPS from a few to many repeating units; this variation was accompanied by a change from serum sensitivity to serum resistance.