ABSTRACT: CO₂ production in aerobic bacteria was modeled from the time-courses of the in vitro activity of isocitrate dehydrogenase (IDH), bacterial protein, and the concentration of the carbon source in the cultures. The model was based on the concept of bisubstrate control of the IDH reaction throughout the exponential, steady-state, and senescent phases of the cultures. In the exponential phase, the measured rates of CO₂ production and the in vitro IDH activity were closely coupled, but in the senescent phase, they became uncoupled. The in vitro IDH activity remained high even after the culture\'s carbon source was exhausted, while the CO₂ production fell to low levels. Based on the hypothesis that this uncoupling was caused by internal substrate limitation, 2 mathematical models incorporating a bisubstrate enzyme kinetics algorithm were constructed and tested. The models predicted the rate of CO₂ production throughout the different phases of the cultures with an r² greater than 0.84.