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The composition of the membrane-bound electron transport system of Haemophilus parainfluenzaeunderwent modification in response to the terminal electron acceptor in the growth medium. H. parainfluenzaewas able to grow with O2, nitrate, fumarate, pyruvate, and substrate amounts of nicotinamide adenine dinucleotide (NAD) as electron acceptors. When O2served as the electron acceptor and its concentration was lowered below 20 μm, the bacteria formed more cytochromes b, c, a1, a2, and othan were present in the cells grown at 150 to 200 μmO2. Nitrate and nitrite reductase activities also appeared during growth at the low O2concentrations in the absence of added nitrate. Cytochrome levels in cells grown anaerobically with fumarate, pyruvate, or NAD as terminal acceptors were similar to those formed in cells grown at low O2concentrations. Cells grown with nitrate had higher levels of cytochromes c, b, and o, and of nitrate and nitrite reductases, than did cells grown with the other acceptors. The formation of cytochrome oxidase a2was repressed by the presence of nitrate in the growth medium. The critical O2concentration (the O2concentration at which the rate of O2uptake becomes demonstrably dependent on the O2concentration) was about 100 μmin cells grown with nitrate and about 15 μmin cells grown with the other acceptors. A mutant of H. parainfluenzaewas found to make about 10% as much cytochrome cas the wild type, and its formation of cytochrome a2was not repressed by nitrate. The critical O2concentration of the mutant was high when it was grown with nitrate, suggesting that the high levels of cytochrome cand the absence of cytochrome a2from the wild type are not responsible for the high critical O2concentration. The modifications of the respiratory system induced by changing the terminal electron acceptor were inhibited by the presence of chloramphenicol, which suggests that protein synthesis is involved.

Protoheme is essential for the growth of some strains of Bacteroides melaninogenicus. At low concentrations in the growth medium, protoheme determines the doubling time, total cell yield, and amount of cytochrome per bacterium. At high protoheme concentrations, the doubling time, total cell yield, and amount of enzymatically reducible cytochrome appear to remain nearly constant, and protoheme is accumulated by the cell. The accumulated protoheme can support the growth of the bacterium for at least eight generations in a protoheme-free medium. When growth and cytochrome content are proportional during growth at low protoheme concentrations, the bacteria incorporate 10 to 20% of the total available protoheme into a membrane-bound respiratory system. This respiratory system includes cytochrome c, a carbon monoxide-binding pigment, and possibly flavoproteins. The pigments can be reversibly reduced by reduced nicotinamide adenine dinucleotide or endogenous metabolism and can be oxidized anaerobically by fumarate or by shaking in air. Electron transport is inhibited by 2-n-nonyl-4-hydroxy-quinoline-N-oxide.