FACTORS INFLUENCING THE RATE OF FERMENTATION OF STREPTOCOCCUS LACTIS

Abstract

The rate of lactic fermentation by streptococci can be measured simply by titration when the cells are obtained by centrifugation and suspended in a buffered glucose solution of pH 7. It is important that the cells are young (12-24 hrs.) and more conc. than normal growth would make them. All cells in these expts. were tested in 2% phosphate buffer + 2% glucose + 0.3-0.5% peptone. The rate of fermentation had its optimal pH at 7. Deviation to the alkaline side retarded less than acid deviation. Fermentation took place most rapidly in a buffer containing 4% of phosphates; the largest amounts of final acidity were produced with 6%. As long as more than 0.2% of glucose was present, the rate of fermentation was not affected by the sugar conc. Na lactate (1%) retarded slightly; 4% reduced the rate to less than half the normal rate. Continued agitation by a current of air or O2 retarded fermentation, while N2 increased the rate. Little difference was found between cells of different age, ranging from 2 hrs. to 2 days, if the cells were kept young by very frequent transfer; if old cells were transferred to a fresh medium, the fermenting capacity was doubled when the cells came out of lag, and decreased again after the number of cells had doubled. The centrifuged cells can be stored in a phosphate buffer without glucose at about + 2[degree] C for 4 days without loss of fermenting capacity, and with little loss of viability.