Surviving serum: The Escherichia coli iss gene of extraintestinal pathogenic E. coli is required for the synthesis of group 4 capsule

Extraintestinal pathogenic Escherichia coli (ExPEC) strains constitute a serious and emerging clinical problem, as they cause a variety of infections and are usually highly antibiotic resistant. Many ExPEC strains are capable of evading the bactericidal effects of serum and causing sepsis. One critical factor for the development of septicemia is the increased serum survival (iss) gene, which is highly correlated with complement resistance and lethality. Although it is very important, the function of the iss gene has not been elucidated so far. We have been studying the serum survival of a septicemic strain of E. coli serotype O78, which has a group 4 capsule. Here, we show that the iss gene is required for the synthesis of capsules, which protect the bacteria from the bactericidal effect of complement. Moreover, we show that the deletion of the iss gene results in significantly increased binding of the complement proteins that constitute the membrane attack complex to the bacterial surface.