Serum resistance in different serotypes of Escherichia coli.

Resistance to complement-mediated serum activity is an important virulence factor in E. coli isolated from extraintestinal infections. Because there are no reports about the percentage of serum-resistant E. coli strains in common O serogroups, the study was carried out using Taylor's method (75% serum) for the determination of serum resistance of 253 E. coli strains, which had been isolated from urinary tract infections. The strains belonged to 8 common serogroups (O1, O2, O4, O6, O9, O16, O18, and O75) with a frequency of 6 to 24%, 218 (86%) were encapsulated. Among 26 different K antigens, K1 and K5 could be found in 32 and 33%. 25% of all strains investigated were found to be serum-resistant. The percentage of serum-resistant strains was between 11% and 63% in the different O serotypes, the highest frequency was found in O6 (63%) and O2 strains (43%). Among all serum-resistant strains carrying 13 different K antigens, K1 and K5 were the most common ones, with a percentage of 62% altogether. Serum resistance can be expected in strains from urinary tract infections, with a quite varying frequency depending on the O serotype, certain K antigens and other factors.