Antigenic characteristics of enterotoxigenic and nonenterotoxigenic isolates of Bacteroides fragilis.

Fecal isolates of enterotoxigenic (44 isolates) and nonenterotoxigenic (25 isolates) Bacteroides fragilis were obtained from diarrheic calves (62 isolates), lambs (2 isolates), and pigs (5 isolates). Using a Microtiter whole-cell agglutination test and gel double-diffusion analysis, the isolates were reacted with nonabsorbed rabbit antisera prepared against 13 isolates of enterotoxigenic B fragilis (ETBF). Isolates of B fragilis were antigenically diverse. Thirty-seven (84%) of the 44 isolates of ETBF comprised 13 serogroups on the basis of reaction in the agglutination test. Fourteen (56%) of the 25 isolates of non-ETBF comprised 4 of the 13 groups. Compared with results of the gel-diffusion test, most isolates had a different agglutination test reaction pattern against the 13 antisera. Isolates of ETBF could not be distinguished from non-ETBF. Antigenic heterogeneity of B fragilis facilitated differentiation of individual isolates, a capability that may be useful in future epidemiologic and virulence studies.