GROWTH-KINETICS OF SUSPENDED MICROBIAL-CELLS - FROM SINGLE-SUBSTRATE-CONTROLLED GROWTH TO MIXED-SUBSTRATE KINETICS

Growth kinetics, i.e., the relationship between specific growth rate a nd the concentration of a substrate, is one of the basic toot in micro biology. However, despite more than half a century of research, many f undamental questions about the validity and application of growth kine tics as observed in the laboratory to environmental growth conditions are still unanswered For pure cultures growing with single substrates, enormous inconsistencies exist in the growth kinetic data reported. T he law quality of experimental data has so far hampered the comparison and validation of the different growth models proposed and only recen tly have data collected from nutrient-controlled chemostat cultures al lowed us to compare different kinetic models on a statistical basis. T he problems are mainly due to (i) the analytical difficulty in measuri ng substrates at growth-controlling concentrations and (ii) the fact t hat during a kinetic experiment, particularly in batch systems, microo rganisms alter their kinetic properties because of adaptation to the c hanging environment. For. example, for Escherichia coli growing with g lucose, a physiological long-term adaptation results in a change in K- s for glucose from some 5 mg liter(-1) to ca. 30 mu g liter(-1). The d ata suggest that a dilemma exists, namely, that either ''intrinsic'' K -s (under substrate-controlled conditions in chemostat culture) or mu( max) (under substrate-excess conditions in batch culture) can be measu red but both cannot be determined at the same time. The above-describe d conventional growth kinetics derived from single-substrate-controlle d laboratory experiments have invariably been used for describing both growth and substrate utilization in ecosystems. However in nature, mi crobial cells are exposed to a wide spectrum of potential substrates, many of which they utilize simultaneously tin particular carbon source s). The kinetic data available to date for growth of pure cultures in carbon-controlled continuous culture with defined mixtures of two or m ore carbon sources (including pollutants) clearly demonstrate that sim ultaneous utilization results in lowered residual steady-state concent rations of all substrates. This should result in a competitive advanta ge of a cell capable of mixed-substrate growth because it can grow muc h faster at low substrate concentrations than one would expect from si ngle-substrate kinetics. Additionally the relevance of the kinetic pri nciples obtained from defined culture systems with single mixed, or. m ulticomponent substrates to the kinetics of pollutant degradation as i t occurs in the presence of alternative carbon sources in complex envi ronmental systems is discussed. The presented overview indicates that many of the environmentally relevant apects in growth kinetics are sti ll waiting to be discovered established and exploited.