Microbial stationary phase cells

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. 

Pariza and Yang (123) have recently described the microbial production of 9,1 l-c,t-CLA from linoleic acid using cultures of Lactobacillus sp. In their patented method, early stationary phase Lactobacillus cultures were incubated with linoleic acid dissolved in propylene glycol. A total CLA level of 7 mg/g cells was produced, which was over 96% 9,ll-c,t-CLA. This type of conversion may lead to improved CLA products in the future. 

Microbial monitoring based on ATP must be used with caution because different microbes possess different amounts of ATP and also metabolically active cells might harbor more ATP. We noticed that highest ATP was synthesized in early logarithmic phase (2 to 6 h) and the ATP pool decreased during early stationary phase (10 to 16-h) for boto Gram-positive and Gram-... 

There are a number of factors that influence heat resistance, for example, history of microorganisms, composition of foods, pH, salt, and growth phase of microorganisms (Hansen and Riemann, 1963). It is known that cells from cultures in the logarithmic phase are less heat resistant than cells in the lag or stationary phase (Lemcke and White, 1959). Substrate or food composition has a very important effect on heat resistance. Decrease in moisture content can increase the heat resistance of microbial cells. Moist heat is much more effective in terms of microbial inactivation than dry heat since proteins, which may be destroyed during thermal processing, are more stable in a dry state (Hansen and Riemann, 1963). 

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. 

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). 

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