A nonchromatographic assay for sorbitol in chemically complex growth media containing sorbose

Summary Microbiologists who lack gas-chromatographic equipment cannot easily study the microbial conversion of sorbitol to sorbose, because there is no other proven method for quantitatively measuring sorbitol in complex growth media. The purpose of this study was to determine if the polyol assay suggested by Mäkinen in 1980 could be used to measure sorbitol depletion in chemically complex bacteriological-culture media containing sorbose. After we pretreated the culture medium to remove interfering phosphate complexes and slightly modified Mäkinen's assay, we were able to detect 0.1 mg/ml differences in sorbitol concentrations that varied between 0.9 and 1.5 mg of sorbitol/ml. The presence of sorbose in the complex medium did not affect the assay. Growing cultures ofGluconobacter oxydans were used to test this assay, because these bacteria reportedly oxidize sorbitol to sorbose and quantitatively release the sorbose into the growth medium. When samples of the culture medium removed during growth were centrifuged to remove cells and precipitated to remove interfering substances then tested with the mofidied Mäkinen assay, these cultures showed a sorbitol depletion rate of 4.9 (±0.1) mg/ml h^–1. The Fehling's assay on the same cultures showed a sorbose accumulation rate of 5.12 (±0.01) mg/ml h^–1. We concluded that the modified Mäkinen phosphate-interference assay can be satisfactorily used to quantitatively screen cultures for their ability to oxidize sorbitol.