Difficulties in the serodiagnosis of infection with the fragilis group of Bacteroides.

Bacteroides antibodies were studied in sera from 74 patients infected with the fragilis group of Bacteroides and 74 healthy control persons, by immunofluorescence of 26 different serotypes of the fragilis group. Antibodies were present at titres of 10-320 in 65 (88%) patients and 50 (68%) controls (p less than 0.01). Titres of greater than or equal to 80 were demonstrated in sera of 38 (51%) patients and 5 (7%) controls (p less than 0.01). Specific IgM antibodies were detected in sera of 42 (57%) patients at a geometric mean titre (GMT) of 30, and 8 (11%) controls at a GMT of 11 (p less than 0.01). High antibody titres as well as specific IgM were found in 32 (43%) patients, while none of the controls showed such a combination (p less than 0.01). The majority of positive patients' sera (57%) reacted with five or more serotypes, whereas most positive control sera (51%) reacted against only one or two serotypes (p less than 0.01). A selected combination of serotypes not reacting with the control sera showed positive reactions with 52 (70%) patients' sera. These findings may be useful in devising schemes for the serodiagnosis of infection caused by the fragilis group of Bacteroides. However, there are indications of geographic variation in prevalence of serotypes, which may prevent the development of a single universal scheme.