Biotypes and O serogroups of Escherichia coli involved in intestinal infections of weaned rabbits: clues to diagnosis of pathogenic strains.

A total of 575 Escherichia coli strains isolated from weaned rabbits experiencing diarrhea in 119 French commercial farms were tested for O serogroups. The results showed a strong predominance of serogroup O103 strains. A sample of 126 strains were further checked for simplified biotypes by using five carbohydrate fermentation reactions. Of 72 O103 strains, 70 were shown to belong to biotypes characterized by a rhamnose-negative reaction. Four of nine serogroup O68 strains also showed this type of reaction. Thirty-nine strains, representative of the serotypes and biotypes found, were further tested for experimental pathogenicity in weaned rabbits and for antibiotic susceptibility. All the rhamnose-negative strains produced life-threatening watery or hemorrhagic diarrhea, whereas rhamnose-positive strains induced only mild diarrheic syndrome without any mortality or no clinical signs at all. Rhamnose-negative, highly pathogenic strains did not belong to related antibiotypes. We think that O serogrouping together with biotyping, or even rhamnose fermentation testing, may be an important clue in the diagnosis of enteropathogenic strains from rabbits in France, permitting rapid identification of highly pathogenic strains and leading to improved prognosis and treatment.