Growth studies on the lactic streptococci: IV. Some observations on redox potential
Summary Redox potential changes at 30°C in anaerobic reconstituted skim-milk cultures of Streptococcus cremoris AM1 and AM2 and Streptococcus lactis ML8 have been investigated in batch cultures and continuous cultures maintained at constant pH by the addition of fresh medium. Str. lactis ML8 was more strongly reducing than either Str. cremoris AM1 or AM2. The positive Eh values which can arise in dense continuous cultures of lactic streptococci are not likely to affect either the lactic acid formation or proteolytic activity of the resultant bacterial concentrate. An electrolytic reduction cell is described which allows variations in redox potential in cultures to be controlled simply and effectively.
Growth studies on the lactic streptococci: III. Observations on continuous growth behaviour in reconstituted skim-milk
Summary The behaviour of Streptococcus cremoris AM1 and AM2, and Streptococcus lactis ML8 in 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 of Str. lactis ML8 between 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 with Str. cremoris AM1 and AM2 and Str. lactis ML8 which 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 for Str. lactis ML8. It was thought unlikely that wall growth contributed appreciably to the dilution rate as a similar phenomenon was not observed with Str. cremoris AM1 or AM2.
EFFECT OF FUNGI ON THE OXIDATION‐REDUCTION POTENTIALS OF LIQUID CULTURE MEDIA
BIOLOGICAL OXIDATIONS and reductions which deal with tlle energy relationships essential to all forms of life have been studied by many investigators, all aiming at an ultimate understanding of these complex processes. Since the release or building up of energy involves the transfer of electrons, it is possible to measure these phenomena in biological systems by oxidation-reduction indicators or electrical apparatus in which electron transfers result in color changes or in the building up of an electrical charge at suitable electrodes. Investigations on the effects of plants on oxidationreductioln potentials (Eh) have been confined chiefly to bacteria and yeasts. Hewitt (1933) has given an extensive review of the effects of bacteria on electrode potentials, listing over 65 references dealing with this suibject. Many. more results have been reported on oxidation-reduction phenomena in relation to bacterial metabolism since Hewitt's report was published. Potter (1912) showed that yeast cells produced redulcing conditions in their surrounding medium. Using both indicators and electrical methods of measuring Elh, Cannan, Cohen, and Clark (1926) followed the potential changes produced by yeast cells suspended in buffer solutions. They concluded that yeast cells produce "hydrogen donators " which reduce tthe media and result in the lowering of the oxidation-reduction potentials. Aubel, Aubertin, and Genevois (1929) showed that anaerobic yeast cultures had an Eh of about 160 to 200 millivolts. In a study of the relation of metabolic processes to the oxidation-reduction potentials, Kluyver and Hoogerheide (1934) found that anaerobic fermentation began at an Eh of about 90 millivolts while aerobic fermentation began at a potential of about 160-250 millivolts. They found it possible to determine the dominant type of metabolism (i.e., respiration or fermentation) carried on in a yeast culture by observing the oxidation-reduction potential. In a recent report by Tang and Lin (1936) the results are given of an investigation on the effects of the unicellular algae Chlorella vulgaris and C. pyrenoidosa on the oxidation-reduction potentials and pH of a synthetic nutrient solution. As shown with bacteria and yeasts, these algae also lowered the Eh of the media when respiration was the dominant type of metabolism-i.e., when the potentials were measured in darkness. In the presence of light, when presumably photosynthesis was the dominant process, the El values became more positive.