How Cells Grow - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 553

When oxygen and luciferin are in excess, total light emission is proportional to total
ATP present in the sample. Photometers can be used to detect emitted light. Small concentra-
tions of biomass can be measured by this method, since very low concentrations of ATP
(10 12 g-ATP/L) can be measured by photometers or scintillation counters. The ATP content
of a typical bacterial cell is 1 mg-ATP/g-cells, approximately.
Sometimes, nutrients used for cellular mass production can be measured to followmicrobial
growth. Nutrients used for product formation are not suitable for this purpose. Nitrate, phos-
phate, or sulfatemeasurements can be used. The utilization of a carbon source or oxygen uptake
rate (OUR) can be measured to monitor cellular growth when cell mass is the major product.
The products of cell metabolism can be used to monitor and quantify cellular growth.
Certain products produced under anaerobic conditions, such as ethanol and lactic acid,
can be related nearly stoichiometrically to microbial growth. Products must be either growth
associated (ethanol) or mixed growth associated (lactic acid) to be correlated with microbial
growth. For aerobic fermentations, CO 2 is a common product and can be related to microbial
growth. In some cases, changes in the pH or acid e base addition to control pH can be used to
monitor nutrient uptake and microbial growth. For example, the utilization of ammonium
results in the release of hydrogen ions (H þ ) and therefore a drop in pH. The amount of
base added to neutralize the H þ released is proportional to ammonium uptake and growth.
Similarly, when nitrate is used as the nitrogen source, hydrogen ions are removed from the
medium, resulting in an increase in pH. In this case, the amount of acid added is proportional
to nitrate uptake and therefore to microbial growth.
In some fermentation processes, as a result of mycelial growth or extracellular polysaccha-
ride formation, the viscosity of the fermentation broth increases during the course of fermen-
tation. If the substrate is a biodegradable polymer, such as starch or cellulose, then the
viscosity of the broth decreases with time as biohydrolysis continues. Changes in the
viscosity of the fermentation broth can be correlated with the extent of microbial growth.
Although polymeric broths are usually non-Newtonian, the apparent viscosity measured
at a fixed rate can be used to estimate cell or product concentration.
11.2. BATCH GROWTH PATTERNS
When a liquid nutrient medium is inoculated with a seed culture (inoculums), the organ-
isms selectively take up dissolved nutrients from the medium and convert them into
biomass. A typical batch growth curve includes the following phases: (1) lag phase, (2) log-
arithmic or exponential growth phase, (3) deceleration phase, (4) stationary phase, and (5)
death phase. Figure 11.1 describes a batch microbial growth cycle. The semi-log plot is
employed to idealize the growth regimes with the aid of the straight lines drawn.
The lag phase occurs immediately after inoculation and is a period of adaptation of cells to
a new environment. Microorganisms reorganize their molecular constituents when they are
transferred to a new medium. Depending on the composition of nutrients, new enzymes are
synthesized, the synthesis of some other enzymes is repressed, and the internal machinery of
cells is adapted to the new environmental conditions. These changes reflect the intracellular
mechanisms for the regulation of the metabolic processes discussed in Chapter 10. During
this phase, cell mass may increase a little, without an increase in cell number density.