Fluorescent-antibody studies on selected strains of Bacteroides fragilis subspecies fragilis.

Antisera against seven strains of Bacteroides fragilis subspecies fragilis were produced from dense suspensions of whole cells. These sera exhibited high agglutination titers with homologous antigens. Reciprocal cross-reactions in agglutination tests with each immunizing strain yielded lower titers. Both the indirect and direct fluorescent-antibody techniques were used to evaluate these reagents in the serological identification of 24 defined strains of B. fragilis subspecies fragilis. Subspecies and even strain specificities were noted with particular antisera. A pooled antiserum and conjugate were prepared and studied. Study results showed that specific and high-titered antisera against strains within this subspecies can be produced by the methods described herein and that possibly more than one serotype exists within the seven strains studied. The development of more antibody pools will be necessary to encompass a wider antigenic coverage before the fluorescent-antibody technique can be relied upon altogether for serologically identifying isolates of B. fragilis subspecies fragilis. Test data showed that the indirect method of fluorescent-antibody staining with whole antiserum is an excellent means of identifying strains of this organism.