A third mechanism of serum resistance in Escherichia coli

15 serum-resistant strains of E. coli group III characterized by binding of both C3 and factor H in the immunofluorescence test were studied in respect of the mechanism on which serum resistance is based in these strains. Serum resistance in 7 strains were found to be established by one of the mechanisms first described by Joiner et al. or by Kubens et al. The classification of these strains should therefore be altered. The binding and consumption of C5 as well as the binding of C9 was investigated for the remaining 8 strains. All strains were found to bind the two complement components which are part of the membrane attack complex (MAC) without causing cell death. These results suggest that resistance in strains of group III is based on a third mechanism which shows similarities to data obtained for other species but has not yet been described for E. coli.