Improved microbiological assay for folic acid based on microtiter plating with Streptococcus faecalis.

We have developed an improved method, using 96-well microtiter plates, for the microbiological assay of folic acid. With this method, the tedium of conventional microbiological analysis is substantially decreased. Culture volumes have been reduced 33-fold, and pipetting procedures have been simplified. Assay time has been reduced to 14 h, and sensitivity has increased 10-fold (0.1 ng/mL). Analytical recoveries range from 98 to 104%. Intra-assay and interassay variabilities are less than 11%. The assay does not require extensive manipulation of inoculum. Day-to-day variability has been minimized by using saline aliquots of the bacterial suspension stored at 4 degrees C. The procedure is accurate, selective, and useful for direct measurement of folic acid in multivitamin formulations.