Lag-phase

Lag phase A period of time when the change of cell number is zero. 

Accelerated growth phase The cell number starts to increase and the division rate increases to reach a maximum. 

Decelerated growth phase After the growth rate reaches a maximum, it is followed by the deceleration of both growth rate and the division rate. 

Stationary phase The cell population will reach a maximum value and will not increase any further. 

Death phase After nutrients available for the cells are depleted, cells will start to die and the number of viable cells will decrease. 

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Fig. 2. Assay reaction curves for (a) substrates, (b) enzymes, and (c) enzymes exhibiting a lag phase or reduced reaction rate where t is measurement time.

The death rate coefficient is usually relatively small unless inhibitoiy substances accumulate, so Eq. (24-10) shows an exponential rise until S becomes depleted to reduce [L. This explains the usual growth curve (Fig. 24-21) with its lag phase, logarithmic phase, resting phase, and declining phase as the effect of takes over. 

Starting from an inoeulum, at t = 0, and an initial quantity of limiting substrate at t = 0, the biomass will grow after a short lag phase and will eonsume substrate. The growth rate slows as the substrate eoneentration deereases, and beeomes zero when all the substrate has been eonsumed. Simultaneously, the biomass eoneentration initially inereases slowly, then faster until it levels off when the substrate beeomes depleted. Figure 11-21 shows a sketeh of a bateh fermenter. 

Typical units for productivity are kg m 3 h 1. Factors that influence productivity include the production time of the fermentation, the time required to dean and set up the reactor, the sterilisation time and the length of the lag phase of growth. Figure 2.2 shows how total productivity and maximal productivity can be calculated for a batch fermentation. The dedsion as to when the fermentation is terminated (maximum or total productivity) depends on the operating costs, which include the capacity of the fermentation vessel, energy costs and labour costs. 

In continuous fermentation, maximum productivity equals total productivity since the preparation time and the time in lag phase of growth are small relative to the total fermentation time. 

Batch fermentation is the most widely used method of amino add production. Here the fermentation is a dosed culture system which contains an initial, limited amount of nutrient. After the seed inoculum has been introduced the cells start to grow at the expense of the nutrients that are available. A short adaptation time is usually necessary (lag phase) before cells enter the logarithmic growth phase (exponential phase). Nutrients soon become limited and they enter the stationary phase in which growth has (almost) ceased. In amino add fermentations, production of the amino add normally starts in the early logarithmic phase and continues through the stationary phase. 

For economical reasons the fermentation time should be as short as possible with a high yield of the amino acid at the end. A second reason not to continue the fermentation in the late stationary phase is the appearance of contaminant-products, which are often difficult to get rid off during the recovery stage. In general, a relatively short lag phase helps to achieve this. The lag phase can be shortened by using a higher concentration of seed inoculum. The seed is produced by growing the production strain in flasks and smaller fermenters. The volume of the seed inoculum is limited, as a rule of tumb normally 10% of the fermentation volume, to prevent dilution problems. 

The lag phase (circa 2 hours) during which the cells are not capable of multiplying ... 

The logistic equation leads to a lag phase, an exponential initial growth rate and a stationary population of concentration (xm). In a population, it is often the case that the birth rate decreases as the population itself increases. The reasons may vary from increased scientific or cultural sophistication to a limited food supply. 

Figure 3.7 shows the growth of R. rubrum in a batch fermentation process using a gaseous carbon source (CO). The data shown follow the logistic model as fitted by (3.14.2.11) with the solid lines, which also represent an unstructured rate model without any lag phase. The software Sigma Plot was used to fit model (3.14.2.11) to the experimental data. An increase in concentration of acetate in the prepared culture media did not improve the cell dry weight at values of 2.5 and 3 gT-1 acetate, as shown in Figure 3.7. However, the exponential growth rates were clearly observed with acetate concentrations of 0.5-2 g-F1 hi the culture media.

The objective of a good process design is to minimise the lag phase period and maximise the length of exponential growth phase. 

Models for batch culture can be constructed by assuming mechanisms for each phase of the cycle. These mechanisms must be reasonably comph-cated to account for a lag phase and for a prolonged stationary phase. Unstructured models treat the cells as a chemical entity that reacts with its environment. Structured models include some representation of the internal cell chemistry. Metabolic models focus on the energy-producing mechanisms within the cells. 

A simple way to model the lag phase is to suppose that the maximum growth rate fimax evolves to its final value by a first-order rate process jUmax = Moo[l — exp(—af)]. Repeat Example 12.7 using a=lh. Compare your results for X, S, and p with those of Example 12.7. Make the comparison at the end of the exponential phase. 

Equation (12.17) postulates that spontaneous deaths occur throughout the batch cycle. This means that dXjdt is initially negative. Is it possible to lose the inoculum completely if the induction period is too long Long induction periods correspond to small values of ot in the lag phase model of Problem 12.6. Find the critical value for ot at which the inoculum is lost. 

In contrast to E. coli, B. subtilis cells do not enter a 4-h lag phase. Instead, they continue growth with a generation time of more than 24 h and induce the synthesis of 36 CSPs which function at various levels of cellular physiology, such as protein synthesis, carbohydrate uptake and chemotaxis [120]. 

Visual proof of linear kinetics, making obvious the occurrence of undesirable conditions such as substrate depletion or lag phase non-linearity. Visual display of changes in the reaction rate. Maximum accuracy as the measurement can be made in the region of maximum linear velocity. 

Non-linearity during progession of enzyme reaction or an initial lag phase which is undetected and may result in significant error. 

The first phase, A, is called the lag phase. It will be short if the culture medium is adequate, i.e. not necessarily minimal, and is at the optimum temperature for growth. It may be longer if the medium is minimal or has to warm up to the optimum growth temperature, and prolonged if toxic substances are present other things being equal, there is a relationship between the duration of the lag phase and the amount of the toxic inhibitor. 

Fig. 1.12 Typical bacterial growth curve A, lag phase B, log phase C, stationary phase D, phase of decline.

In the water-like solvent tert-butyl alcohol, a-tocopherol was found to prevent lipid oxidation, showing a distinct lag-phase for oxygen consumption. This was in contrast to quercetin or epicatechin, which were only weak retarders of lipid oxidation without any clear antioxidative effect. Quercetin or epicatechin, when combined with a-tocopherol, increased the lag-phase for oxygen consumption as seen for a-tocopherol alone. The stoichiometric factor for a-tocopherol, a-TOH, as chain-breaking antioxidant has the value n = 2 according to the well-established mechanism ... 

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