Thirteen serogroups of Escherichia coli most frequently are responsible for urinary tract infections in humans. Knowledge of serogroup is required for epidemiological reasons and to help differentiate between new and reemergent strains in infected patients. Coagglutination and bacterial agglutination were compared as serodiagnostic tools for identifying groups of E. coli associated with these infections. In contradistinction to bacterial agglutination, coagglutination showed no cross-reactions for 12 of the 13 known strains obtained from the Center for Disease Control. A single antiserum, E. coli O4, caused bacterial agglutination and not coagglutination. In addition, staphylococcal cells that were coated with antiserum reacted with sheep erythrocytes that had been treated with soluble extract of E. coli. The latter reaction, cohemagglutination, was as specific and rapid as coagglutination; easily visible patterns of clumping developed within 1 min after sensitized staphylococci and E. coli reagent were mixed. These results revealed a method that is sensitive and reproducible for identifying serogroups of E. coli responsible for urinary tract infections
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