Exponential phase, microbial growth

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 ... 

However, whilst equation 5.61 is not based on any theory which relates to biological observation other than that the growth curve is sigmoidal, it does serve to present data in a compact form. It can be used to describe the lag, exponential and stationary phases of microbial growth and the constants involved can be related... 

The growth of microbial populations is normally limited either by the exhaustion of available nutrients or by the accumulation of toxic products of metabolism. As a consequence, the rate of growth declines and growth eventually stops. At this point a culture is said to be in the stationary phase. The transition between the exponential phase and the stationary phase involves a period of unbalanced growth during which the various cellular components are synthesized at unequal rates. Consequently, cells in the stationary phase have a chemical composition different from that of cells in the exponential phase. 

The maximum biosurfactant production was verified at pH 7.0 and 8.0. The addition of EDTA and microsalts favored microbial synthesis of surface-active compounds. On the other hand, the addition of yeast extract stimulated cell growth to the detriment of biosurfactant production. The most suitable concentration of commercial sucrose for biosurfactant synthesis was 10 g/L. Biosurfactant production occurred in the late-exponential phase, achieving its maximum value at the early stationary phase of growth. The values of surface tension that we obtained compare favorably with those obtained with commercial synthetic surfactants. 

Microbial growth is generally described in terms of cell numbers, although an increase in the mass of the cell population also usually occurs. In laboratory culture, bacteria exhibit a growth curve that can be divided into four main phases lag exponential stationary and death (Fig. 5.45). 

When one looks at real processes, one can see that they actually show several different types of kinetic behavior. As shown in Fig. 6.31, reaction rates (or conversion) vary as a function of concentration There are normal catalytic processes (curve a) and autocatalytic processes (curve b) that are dominant in microbial growth (cf. Equ. 2.7) (Levenspiel, 1972). Biotechnological processes are a combination of both (curve c). At the beginning they are autocatalytic later, after the exponential growth phase, they shift to ordinary kinetics. 

The model does not include the death phase, but industrially the amount of nutrient is chosen to terminate the microbial growth before the onset of the death phase, anyway. The cell growth is initiated by inoculation with cells from a stationary nutrient exhausted culture, i.e., when Ca = Cr = G and M = D. Numerical integration of (15.2-5 to 15.2-8) leads to results for a batch culture given in Fig. 1.5.2-1. The model shows a lag phase, an exponential growth phase, a change in the cell composition and a stationary phase with a relatively small number of cells. 

The culture passes through a series of phases characterised by the rate of growth, starting with a lag phase during which little or no increase in the microbial density occurs. This gives way during the acceleration phase to a period of exponential... 

When a more detailed analysis of microbial systems is undertaken, the limitations of unstructured models become increasingly apparent. The most common area of failure is that where the growth is not exponential as, for example, during the so-called lag phase of a batch culture. Mathematically, the analysis is similar to that of the interaction of predator and prey, involving a material balance for each component being considered. 

Malolactic fermentation (MLF) in wine is by definition the enzymatic conversion of L-malic acid to L-lactic acid, a secondary process which usually follows primary (alcoholic) fermentation of wine but may also occur concurrently. This reduction of malic acid to lactic acid is not a true fermentation, but rather an enzymatic reaction performed by lactic acid bacteria (LAB) after their exponential growth phase. MLF is mainly performed by Oenococcus oeni, a species that can withstand the low pFi (<3.5), high ethanol (>10 vol.%) and high SO2 levels (50 mg/L) found in wine. More resistant strains of Lactobacillus, Leuconostoc and Pediococcus can also grow in wine and contribute to MLF especially if the wine pH exceeds 3.5 (Davis et al. 1986 Wibowo et al. 1985). The most important benefits of MLF are the deacidification of high acid wines mainly produced in cool climates, LAB contribute to wine flavour and aroma complexify and improve microbial sfabilify (Lonvaud-Funel 1999 Moreno-Arribas and Polo 2005). 

Based on experimental observations by Monroy-Fernmdez et al. (30), the mechanism of microbial attack of arsenopyrite can be explained as follows. Two surface reactions take place initially that are promoted by attached bacteria (80%) and planktonic bacteria (20%) in the exponential growth phase ... 

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.

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