Interpretation of microbial bioassays using kinetic parameters

A method analogous to that used for enzyme kinetics was applied to estimate maximum growth rates and oxygen depletion rates as measures of microbial toxicity. Selected toxicants including phenol, formalin and CuSO4 were tested against an axenic culture of E. coli and a mixed culture derived from soil to determine reproducibility of the test procedures. Optical density was used as a measure of growth over time. Oxygen uptake over a short term (less than 20 min.) and over extended time (greater than 4 hrs.) was also monitored. Microbial growth rate constants (g) and oxygen depletion rate constants (k) were calculated and the EC50 values for toxicity were taken as one half the maximum growth rate or oxygen depletion rates, respectively. The method proved a reasonable and reliable measure of toxicity.