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


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