How Cells Grow - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 550

In biosynthesis, the cells, also referred to as the biomass, consume nutrients to grow and
produce more cells and important products. Growth is a result of both replication and change
in cell size. Microorganisms can grow under a variety of physical, chemical, and nutritional
conditions. In a suitable nutrient medium, organisms extract nutrients from the medium and
convert them into biological compounds. This transformation of nutrients to energy and bio-
products is accomplished through a cell's use of a number of different enzymes in a series of
reactions to produce metabolic products as outlined in Chapter 10. These products can either
remain in the cell (intracellular) or be secreted from the cells (extracellular). In the former
case, the cells must be lysed (ruptured) and then the product can be purified from the whole
broth (reaction mixture).
Theoretically, cell growth is very complicated due to the complicated metabolic pathways
as illustrated in Chapter 10. While one may be able to guess how cells grow mathematically
by extrapolation from what we have learned in Chapters 8 and 10, we shall return to basics
and exam cell growth from experimental observations following the progression of natural or
physical observatory development. In this chapter, we will learn how cells grow quantita-
tively based on experimental observations, how the complicated kinetics may be simplified
by approximation, and how cell culture and fermentation are modeled or quantitatively
predicted.
11.1. QUANTIFYING BIOMASS
Quantification of cell biomass in a culture medium is essential for the determination of the
kinetics of microbial growth. The methods used in the quantification of cell concentration can
be direct and indirect. In many cases, the direct methods are not feasible due to the presence
of suspended solids or other interfering compounds (for example, color) in the medium.
Either cell number or cell mass can be quantified depending on the type of information
needed and the properties of the system. Cell mass concentration is often preferred to the
measurement of cell number density when only one is measured, but the combination of
the two measurements is often desirable.
11.1.1. Cell Number Density
A Petroff e Hausser slide or a hemocytometer is often used for direct cell counting. In this
method, a calibrated grid is placed over the culture chamber, and the number of cells per
grid square is counted using a microscope. To be statistically reliable, at least 20 grid squares
must be counted and averaged. The culture medium should be clear and free of particles that
could hide cells or be confused with cells. Stains can be used to distinguish between dead and
live cells. This method is suitable for nonaggregated cultures. It is difficult to count molds
under the microscope because of their mycelial nature.
Plates containing appropriate growth medium gelled with agar (Petri dishes) are used for
counting viable or live cells. (The word viable used in this context means capable of reproduc-
tion.) Culture samples are diluted and spread on the agar surface, and the plates are incu-
bated. Colonies are counted on the agar surface following the incubation period. The
results are expressed in terms of colony-forming units. If cells form aggregates, then a single