Use of glycerol-cryoprotected Lactobacillus casei for microbiological assay of folic acid.

A simple procedure for preparing glycerol-cryoprotected Lactobacillus casei cultures has been developed. L. casei grown in medium supplemented with low concentrations of folic acid (0.3 micrograms/L) is diluted with an equal volume of glycerol (800 mL/L) and stored at -20 degrees C. Growth response of the glycerol-cryoprotected L. casei to low concentrations of folic acid exceeded that of cultures maintained by monthly agar stab transfer. Also, growth for the zero-folate blanks was considerably less for the cryoprotected cultures. Assay of folate in several rat tissues correlated well (r = 0.999) with the standard microbiological assay. The growth rate of the culture depends on the inoculum size, and a heavy inoculum of cryoprotected L. casei may be used to complete the assay after only an overnight incubation.