Insufficient oxygen diffusion leads to distortions of microbial growth parameters assessed by isothermal microcalorimetry

Abstract

Heat is directly related to the number of bacteria as well as the kinetics and stoichiometry of bioconversions. Approximately 100 000 aerobically growing bacteria or 100 myocardial cells produce generally enough heat being measurable in conventional isothermal microcalorimeters in real time. The most convenient methods to measure heat signals are isothermal microcalorimetry (IMC) experiments using closed unstirred ampoules in multichannel instruments. The convincing advantages of this simple method led to many applications in medicine, pharmacy, environmental sciences and control of food and tap water. However, serious concerns for the characterization of biological processes arise from the often unclear influence of potential oxygen depletion on the heat signal. In this article we show that oxygen bioavailability shapes the heat signal of IMC measurements significantly. Therefore, the balance between oxygen sink (microbial activities) and oxygen source (diffusion from gas phase) in relation to the spatial biomass distribution is quantitatively evaluated here for the first time. Thereby it is demonstrated that the oxygen bioavailability is a limiting factor for several IMC measurements. Based on these results, practical suggestions for calorimetric experiments with lower influence of oxygen bioavailability are derived. As rule of thumb, only the heat trace up to 95 J L⁻¹ at 37 °C should be evaluated, if there should be no influence of oxygen diffusion for the study of homogenously distributed biomass. Furthermore, experimental measures to reduce the influence of oxygen bioavailability on the heat signal are discussed.