Rates of Biotransformations Microbial Growth

We begin by considering the relationship of cell numbers to time for a growing population limited by a substrate like / -cresol. In response to a new growth opportunity, the cell numbers increase exponentially, and this period of so-called exponential growth can be described using  

Put another way, during exponential growth the microbial population will double in number for every time interval, t = (In 2)1 fx. 

KiM is the Monod constant (mol L-1) equivalent to the chemical concentration of i at which population growth is half maximal 

KiM is often denoted with IQ in the microbiology literature, but here we use the subscript iM to emphasize Monod growth on i is rate-limiting. This formulation yields a hyperbolic dependency of n on [i] (Fig. 17.15 and case 1 in Box 17.1). That is, when 

1 is present at low levels ([i ] Km), its concentration limits the rate of increase in cell numbers. However, when there is a surplus level of the food chemical, other factors limit the rate of population increase. 

---

Hence, sometimes phenomena associated with enzyme kinetics control the rate of biotransformations. If suitable enzymes are present in the microbial community, for example due to consumption of structurally related growth substrates, then we may see immediate degradation of compounds of interest like BQ when they are added to these metabolically competent microbial communities (Fig. 17.17). For such cases, if the abundance of the bacteria is varied, the rate of removal changes accordingly. Consequently, the removal of BQ could be described by a second-order rate law (Smith et al., 1978) ... 

Because the rates of biotransformation are influenced by a variety of factors related to the size, growth, and substrate utilization rate of the microbial populations involved, they often exhibit a more complex dependence upon substrate concentration than do the rates of abiotic transformation in homogeneous solution (D Adamo et al, 1984). Under many circumstances, however, these more complex relations often simplify to being pseudo first-order with respect to substrate concentration, particularly when the latter is substantially lower than that required to support half the maximum rate of growth of the organisms of interest. At concentrations well above this level, transformation rates may be independent of substrate concentration (Paris et al, 1981). 

A. Because of the small sizes of many microbial cells, especially bacterial cells, compared with those of plants and animals, micro-organisms are able to grow relatively quickly. This is because small cells have a high surface-area volume ratio, which in turn influences the potential to take up nutrients from the environment. A necessary corollary to promote rapid growth is that microbial cells have high rates of cellular metabolism, leading to fast rates of biotransformation of substrates undertaken by whole cells of micro-organisms. 

Microbial Biotransformation. Microbial population growth and substrate utilization can be described via Monod s (35) analogy with Michaelis-Menten enzyme kinetics (36). The growth of a microbial population in an unlimiting environment is described by dN/dt = u N, where u is called the "specific growth rate and N is microbial biomass or population size. The Monod equation modifies this by recognizing that consumption of resources in a finite environment must at some point curtail the rate of increase (dN/dt) of the population ... 

The strict separation of microbial growth and biotransformation offers some advantages. Thus, each step can be optimized individually, and a negative influence of the substrate or its product excluded. Furthermore, the hydroxylation rate in water-suspended mycelia sometimes... 

Figure 17.1 Sequence of events in the overall process of biotrans-formations (1) bacterial cell containing enzymes takes up organic chemical, /, (2) i binds to suitable enzyme, (3) enzyme / complex reacts, producing the transformation product(s) of /, and (4) the product(s) is(are) released from the enzyme. Several additional processes may influence the overall rate such as (5) transport of / from forms that are unavailable (e.g., sorbed) to the microorganisms, (6) production of new or additional enzyme capacity [e.g., due to turning on genes (induction), due to removing materials which prevent enzyme operation (activation), or due to acquisition of new genetic capabilities via mutation or plasmid transfer], and (7) growth of the total microbial population carrying out the biotransformation of /. ...

Yeast was the first microbial host used by mankind for biotransformation of raw materials, and it marked the early developments of industrial biotechnology. Initially, Saccharomyces cerevisiae and closely related species were used because of their high fermentative capacity and based on the vast experience from alcoholic beverage production. While a high fermentation rate is favorable for the production of bioethanol and other primary metabolites, it implicates disadvantages for growth-coupled production. Consequently, a number of other yeasts have been developed for the production of biofuels, biochemicals, lipids, or recombinant proteins. 

The models discussed above are simplifications in the real world, numerous factors influence the growth and biotransformation capabilities of microbial populations. Moreover, natural microbial populations are extraordinarily diverse and contain microbes having a wide range of growth and uptake parameters. Nevertheless, even the most complex models that attempt to predict biodegradation rates are usually based on the uptake and growth expressions described above, and may be recognized as variations of biofilm, batch, or continuous models. 

---