Isolation and lactose metabolism of Streptococcus lactis variety tardus from southwestern Virginia

Abstract

Forty-three cultures of Streptococcus lactis variety tardus were isolated from the one hundred and sixteen lactic streptococci cultures in raw milk samples from five areas of southwestern Virginia. Three of the sampled areas contained a population of S. lactis var. tardus that composed 61.5, 90.0 and 95.2 per cent of the total lactic streptococci population. Identification of the S. lactis var. tardus cultures was accomplished by silicic acid column chromatography of the spent lactose-phosphate-yeast extract media. Two cultures of S. lactis var. tardus were isolated from parent§_. lactis cultures that were being transferred in litmus milk rehydrated with distilled water during a four- to ten-month period. The cellular growth of S. lactis var. tardus as measured turbidimetrically proceeded at a faster rate on glucose than it did on lactose. The chromatographic analysis of acid end products proved that S. lactis var. tardus converted glucose to lactic acid while lactose was converted to acetic, formic and lactic acids in a ratio of 1:1:1 or 1:1:2, respectively. Under shake-culture conditions with a nitrogen atmosphere (anaerobic) the pattern of fermentation products changed so that 60 per cent of the total acid produced was lactic, about 20 per cent was formic and 10 to 15 per cent, acetic acid. Under aerobic shake-culture conditions the acid pattern was altered so that 70 per cent acetic acid was formed, no formic and over 20 per cent was lactic acid. Gas chromatography proved that the production of ethanol increased under anaerobic conditions and decreased to almost zero under aerobic conditions. An hourly analysis of the plf and acid production of growing cells of S. lactis var. tardus on lactose indicated that acetic and formic acids are produced under static aerobic conditions before the medium reaches pH 5, and below pH 6 lactic acid accumulates. The inhibition of acid production occurred when lactate or acetate was present in the lactose medium at the beginning of the fermentation by the growing cells of S. lactis var. tardus.