To what do microaerophilic bacteria owe their special,
microaerobic growth capabilities? To derive energy as obligate,
nonphototrophic aerobes, microaerophilic bacteria
must oxidize substrate and involve oxidative phosphorylation.
Experimentally, considerable variation in the choice
in limiting-O2 conditions has resulted in different assumptions
about “microaerobic” growth conditions and, often,
quite disparate inferences. For microaerophilic bacteria, the
key requirement of microaerobic physiology is conceptually
straightforward: a DOT insufficient to sustain the cytochrome
aa3 (oxidase) activity of conventional aerobic
organisms. However, DOT is normally measured potentiometrically
in aqueous phases whereas the lipid phases of
biological membranes in which O2 is relatively more soluble
serve as an O2 reservoir for respiratory, integral membrane
complexes, such as terminal oxidases. While some
aerobic bacteria lack cytaa3, and/or all cytc-dependent respiration
and instead utilize quinol oxidases, such as cytbo,
the various aerobic cytochrome oxidases all share similar
O2 affinities, with apparent Michaelis constants (Km(O2)
values) near 0.5 μM DOT. For culture of microaerophilic
bacteria then, true microaerobic growth conditions are typically
those below 1 μM DOT. Vexingly, for the experimental
analysis of microaerobic growth, the 1 μM DOT threshold is
the baseline for potentiometric measurements with conventional
O2-specific electrodes. So, inferential, relatively complicated
methods to determine submicromolar DOT values
have been historically employed in growth studies involving
true microaerobic physiological conditions.