Formate stimulated growth of the microaerophilic bacterium subsp. at reduced dissolved oxygen tensions (d.o.t). At a d.o.t. of 2 kPa the mean doubling time of growing in a complex medium supplemented with formate was 2 h. Growth did not occur in the absence of O. When was cultured in a complex medium with formate, growth and formate consumption slowed down after a rise of the d.o.t. from 2 to 15 kPa. At the same time the potential respiration rate diminished greatly. The effects of exposure to a d.o.t. of 15 kPa could still be reversed after 4 h by shifting the d.o.t. back to 2 kPa. The decrease in the potential respiration rate could be related to a loss of formate dehydrogenase activity. Its susceptibility to a high d.o.t. was greater when formate was present in the growth medium than in its absence. Formate oxidase had a low affinity for O and HO was a product of the oxidation of formate by O. Formate dehydrogenase appeared to be a membrane-bound enzyme. Of the possible physiological electron acceptors tested, only FAD gave rise to a detectable, although low, activity of formate dehydrogenase. grown in the complex medium with formate contained cytochromes of the - and -type. Cytochrome was membrane-bound; cytochrome was largely recovered in the soluble fraction. A CO-binding pigment was identified as a cytochrome of the -type. possessed cytochrome peroxidase activity. Cytochrome oxidases of a known type could not be detected unequivocally in a (reduced-plus-CO reduced) difference spectrum. An important role in the microaerophilic nature of is ascribed to HO.


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