Microbial product formation

Figure 5.41. Concentration/time plot of the three basic types of microbial product formation (I) growth association, (II) mixed growth association, and (III) nongrowth association (Gaden, 1955).

Figure 5.44. Schematic representation of the numerical Kono approach to microbial product formation expressed as the general formulas of the rates of growth and production rp, including different growth phases (1, induction 2, transient 3, exponential and 4, declining), according to Equ. 5.126 and Table 5.2. (Adapted from Kono and Asai, 1968a-c, 1969a-c) (a) both and kp2 have a positive value. The dotted lines take into account a linear growth phase, as shown in Fig. 5.19. (b) kp >0, kp2 = 0. (c) kpi = 0, kp2 > 0. (d) /cpi > 0, kp2 < 0.

The microbial product-formation rate in a single CSTR is (Pirt, 1975)... 

The kinetic aspects of product formation in continuous culture are usually dealt with using the simple, formal kinetic approach of the Luedeking-Piret equation (Equ. 5.122). A modified kinetic approach to microbial product formation uses the concept that Vp not only is proportional to the actual biomass concentration but also depends on a second carbon source S2 (Hegewald et al., 1978) ... 

In any quantitative assessment of growth and/or product formation, it is essential to link formation of microbial biomass and products with the utilisation of substrate and nutrients. In the case of microbial biomass production, the total amount of cell mass yield formed is often proportional to the mass of substrate utilised. Mathematically this is coefficient expressed as the corresponding ratio, or yield coefficient ... 

The rate of product formation, rfi, depends upon the state of the cell population, environmental condition, temperature, pH, media composition and morphology with cell age distribution of the microorganism.2 3 A similar balance can be formulated for microbial biomass and cell concentration. The exponential phase of the microbial growth in a batch culture is defined by ... 

Initial scale-up of microbial biotransformation is conveniently run with multiple flasks without extensive reaction optimization. A typical flask fermentation is performed at 28 °C, 250 rpm with 100 mL culture in a 500 mL Erlenmeyer flask, although other settings will work fine too. Three parameters need to be investigated before scale-up the time for adding the substrate, the optimal substrate concentration and the time course of product formation. Optimization of other factors, such as medium composition and pH, growing cells versus resting cells [74], is helpful, if the timeline allows and if there is a sufficient amount of the substrate to support the screening. 

There are numerous ways to optimize the microbial production of 1,3-PD from glycerol, and remarkable progress has already been achieved. Main concerns for optimization are (1) preventing undesired by-product formation to achieve... 

Fig. 23.5 Aqueous-organic two-liquid-phase system for microbial production of flavour compounds. Here the formation of 2-phenylethanol from L-phenylalanine is exemplarily shown [120]. The organic solvent used for in situ extraction has to be carefully selected on the basis of multiple criteria, such as biocompatibility, non-flammability and legislative regulations. For a more detailed description of flavour production in two-phase systems, see Chap. 24 by Larroche et al.

Luedeking and Piret<56) proposed that the formation of microbial products could be described by a two parameter model. This was based on their studies of lactic acid fermentation using Lactobacillus delbruekii in which the accumulation of the acid is not directly linked to the growth rate. The function used was ... 

Namkung, E., and B. E. Rittmann. 1986. Soluble microbial products (SMP) formation kinetics by biofilms. Water Resources 20 795-806. 

From this screening work, it is expected that novel microbial products which affect the ACAT-1, TG synthetic pathway or unknown sites responsible for macrophage-derived foam cell formation will be discovered, leading to a new type of anti-atherosclerotic agent and providing a novel target for pharmaceutical intervention. 

Takatori, T., Gotouda, H., Terazawa, K., Mizukami, K., and Nagao, M. (1987). The mechanism of experimental adipocere formation substrate specificity of microbial production of hydroxy and oxo fatty acids. Forensic Sci. Int. 35, 277-281. 

From kinetics studies of unicellular organisms, a set of mathematical expressions have been established to represent the most frequent phenomena in bioprocesses. These phenomena involve a limitation or inhibition of growth and product formation, caused by the presence of substrates, products, or byproducts in culture media. Many of these expressions do not derive from known kinetic mechanisms. In fact, they are simply mathematical expressions with fitted parameters that are able to reproduce experimentally observed kinetic profiles. These equations have been derived and used in many unstructured microbial or cell models. 

Zeng AP (1995), A kinetic model for product formation of microbial and mammalian cells, Biotechnol. Bioeng, 46 314-324. 

Glycerol could offer a significant advantage in several microbial fermentations as compared to glucose, because in certain cases it could lead to higher production yields and less by-product formation (Lee et al., 2001 Bories et al., 2004 Dharmadi et al., 2006). However, intensive research is still required in order to develop bioprocessing schemes for viable chemical production from glycerol. 

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