Growth studies on the lactic streptococci: III. Observations on continuous growth behaviour in reconstituted skim-milk

Abstract

SummaryThe behaviour ofStreptococcus cremorisAM1and AM2, andStreptococcus lactisML8in continuous growth in reconstituted skim-milk medium under controlled conditions of pH has been investigated.At 20°C under aerobic conditions small quantities of oxygen were found to stimulate the growth rate ofStr. lactisML8between pH 5·05 and 5·45. Growth between pH 5·45 and 6·35 became erratic due to the accumulation of hydrogen peroxide. Above pH 6·35 hydrogen peroxide could not be detected in the continuously growing culture and below pH 5·05 the growth rate of the culture was not affected by small quantities of oxygen. An effect of direct oxygen toxicity on growth, distinct from that of hydrogen peroxide accumulation, was observed over the limited pH range investigated. After 48 h growth in the presence of small concentrations of oxygen the effect disappeared.At 30°C under anaerobic conditions a region in the dilution rate curves was observed withStr. cremorisAM1and AM2andStr. lactisML8which suggested the occurrence of a growth-limiting component. This region of slow growth could be correlated with diminished soluble nitrogen values, and was eliminated by enrichment of the medium with soluble nitrogen.An unusual feature of the investigation was the dilution rates, indicative of extremely fast specific growth rates, experienced forStr. lactisML8. It was thought unlikely that wall growth contributed appreciably to the dilution rate as a similar phenomenon was not observed withStr. cremorisAM1or AM2.