Haemophilus parasuis was grown in cystine-supplemented tryptone-yeast extract medium (TYE) sparged with CO$ sb2$-containing gas mixtures. The organism was found to metabolize glucose via acetate- and succinate-producing pathways and a complete tricarboxylic acid cycle was absent. As compared with the oxygen-sufficient organism, oxygen-restricted H. parasuis synthesized higher amounts of high and low potential cytochromes c, exhibited higher activities of the enzymes of the succinate-producing pathway and excreted less acetate and more formate and succinate. The low potential cytochromes c were reduced by NADH and oxidized by fumarate and were situated on both sides of the HOQNO inhibition site of the low potential portion of the electron transport chain, but did not seem strictly required for the NADH:fumarate oxidoreductase activity. It is suggested that the low potential cytochromes c might act as an electron buffer permitting the regeneration of NAD under conditions of scarcity of the final electron acceptor.

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