The Relationship Between Electron Transport Activity as Measured by CTC Reduction and CO₂ Production in Mixed Microbial Communities

Abstract

CTC (5-cyano-2,3-ditolyl tetrazolium chloride) is a redox indicator that facilitates the detection of microbial electron transport activity due to the fluorescence and water insolubility of the reduced CTC-formazan (CTF). The goal of this work was to establish the relationship between the CTC response (both the numbers of CTF-containing cells and the fluorescence intensity of CTF per cell) and respiration in mixed microbial communities. To obtain CTF-containing cell numbers over a range of respiration rates, aerobic bioreactors with on-line CO₂ monitoring were batch fed ground wheat at slow, intermediate, and fast retention times. Samples were taken before and after feeding, and throughout starvation cycles. Each sample was treated with 25 mm CTC, and either supplemented with 10% R2A, or left unsupplemented. CTF-containing cell numbers showed a weak and inconsistent response to transient pulses in respiration, and decreased during long-term starvation at all three retention times. The degree of starvation within the microbial community could be estimated using the ratio of supplemented to unsupplemented CTF-containing cell population. Total fluorescence intensity per cell was consistently higher at peaks of CO₂ production, but did not decrease as dramatically as total cell numbers did in response to starvation. The results indicate the importance of concurrent examination of both the numbers and total fluorescence intensity of CTF-containing cells.