Serum stimulation and repression of flow immunofluorescence staining of bacteria

A flow cytometer was used to measure the fluorescence intensity of Bacillus anthracis spores, B. subtilis spores and Escherichia coli stained in suspension with specific rabbit fluorescein-conjugated antibody. The effect of normal sera and a number of other additives on the binding of conjugate to the surface of the homologous bacteria was assessed by measuring the median fluorescence intensity of the bacterial population in the reaction mixture. Non-ionic detergent depressed binding of one conjugate (anti-E. coli) by up to 22%. Bovine serum albumin, gelatin, foetal calf serum and normal rabbit serum did not affect the median fluorescence value for these 3 bacterial species by more than 14%. Normal serum from 5 goats reduced the specific staining of B. anthracis by up to two-thirds. Anti-B. anthracis antibodies were detected in goat serum by indirect immunofluorescence microscopy, and it is inferred that these goat antibodies were in competition with fluorescein conjugate for the bacterial antigens. Normal goat and sheep serum stimulated the specific staining of B. subtilis and E. coli measured by the cytometer; in the case of goat serum previous heating of the serum to 56 degrees C resulted in repression of staining of E. coli. Since anti-E. coli antibody was detected in this normal sera by indirect immunofluorescence assays, it is proposed that repression was caused by anti-bacterial antibodies and stimulation by a separate factor, heat-labile in the case of goat serum. The stimulatory factor was also apparently inactivated by increasing the NaCl concentration, suggesting that stimulation depends heavily on charge interactions. Preliminary evidence is presented that the stimulatory factor may be anti-antibody, possibly of the IgA or IgG class.