Summary: The influence of oxygen on growth and cytochrome synthesis was investigated in four species of After anaerobic growth all species contained cytochrome cytochrome or , cytochrome and a carbon monoxide-binding pigment. Cytochrome functions in an electron transport system from lactate to fumarate since the lactate-fumarate dismutation reaction, unlike lactate oxidation via methylene blue, was strongly inhibited by 2--heptyl-4-hydroxyquinoline--oxide (HOQNO). No species grew on agar plates under aerobic conditions but all grew in aerated media, though at different rates and with diminished synthesis of all cytochromes. In and which grew slower under aerobic conditions than and cytochrome synthesis was more repressed by oxygen. Large amounts of pyruvate accumulated under aerobic conditions whereas acetate, propionate and succinate were not formed. The bacteria are therefore dependent on oxidative phosphorylation for energy under aerobic conditions. For an value (g dry wt bacteria/g-atom oxygen taken up) of 15 was measured, suggesting a P/O value of 1 to 2. Absence of cytochrome synthesis resulting in the loss of oxidative phosphorylation may explain the absence of aerobic growth on agar plates.


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