Carbon-limited chemostat cultures of Klebsiella aerogenes NCTC 418 consumed more oxygen per unit of cell synthesis when growing on mannitol or glycerol than when growing on glucose; and since the “maintenance” requirements were similar, this suggested that the extra reducing equivalents present in these compounds were oxidized wastefully. By comparison with carbon-limited cultures, carbon-sufficient cultures that were ammonia-, sulphate- or phosphate-limited generally consumed considerably more oxygen per unit of cell synthesis, particularly at low growth rates. Thus, according to the theory of Pirt, these carbon-sufficient cultures had a greatly increased “maintenance energy” requirement but nevertheless used the remaining energy with a much increased efficiency compared with carbon-limited cultures. This, we suggest, is a false conclusion which stems from the basic assumption that the maintenance requirement does not change with growth rate. Thus we propose an alternative theory which allows for this possibility, and present evidence to show that it may be applicable to both carbon-limited and carbon-sufficient chemostat cultures. Finally we offer an explanation of the high “maintenance” rate of oxygen consumption found with carbon-sufficient cultures, and consider this phenomenon in relation to the loose coupling between respiration and growth extant in most microbial cultures.
Klebsiella aerogenesContinuous culture Growth yield Maintenance energy Bioenergetics