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SUMMARY: The effects of low concentrations of O2 on fermentation in the cattle parasite Tritrichomonas foetus KV1 and its variant 1MR-100 were compared using membrane inlet mass spectrometry to measure simultaneously and continuously ethanol, CO2 and H2. In strain KV1 glucose-supported H2 and CO2 production were stimulated by O2 concentrations < 1·4 μM but were inhibited at higher concentrations. Damped oscillatory responses in H2 production indicated the operation of a feedback control system. Measurement of the O2-dependence of O2 consumption rates confirmed the presence of a high-affinity terminal oxidase (apparent Km = 1·6 μM-O2 at 37 °C) and substrate inhibition by O2 at > 8 μM-O2. Successive periods of exposure to O2 resulted in decreased O2 scavenging capacity, as indicated by increasing apparent Km values for O2. The variant strain 1MR-100 which lacks pyruvate: ferredoxin oxidoreductase and hydrogenase showed quite different characteristics: H2 production was not detectable, ethanol formation was inhibited by O2 (K1 = 1 μM) and O2-dependence of O2 consumption indicated that no high-affinity oxidase was present (apparent Km = 33 μM-O2). Progressive increases in respiration rates on repeated exposure to low O2 concentrations indicated a capacity for adaptation to aerobiosis.

The Effects of Oxygen on Fermentation in Tritrichomonas Foetus KV1 and its Variant 1MR-100 with Defective Hydrogenosomes