Staphylococcus aureus protein A--antibody-mediated haemagglutination for the analysis of cross-reactivity between phenol-water extracts of Bacteroides fragilis.

Passive haemagglutination (PHA) and Staphylococcus aureus protein A--antibody-mediated haemagglutination (SAPA-AMHA) were used to analyse the cross-reactions of rabbit antisera against four strains of Bacteroides fragilis. There was cross-reactivity between all the strains tested but strain-specific reactions were obtained with three strains. Two to 32-fold higher antibody titres were obtained with SAPA-AMHA than with PHA. The antigen concentration required in inhibition assays was up to 32-fold higher for PHA than for SAPA-AMHA. The latter is a simple and superior alternative to PHA for such studies.