Microbiologia

The lesson covers the following objectives in detail:

I. Bacterial Growth Kinetics
A. Modeling the Microbial growth Kinetics

II. Batch Growth
a. Application of Batch fermentations
b. Optimization of Batch fermentations

III. Continuous Growth Kinetics
Introduction
Microbial growth can be defined as an orderly increase in cellular
components, resulting in cell enlargement andeventually
leading to cell division. This definition is not strictly accurate as
it implies that a consequence of growth is always an increase in
cell numbers. However, under certain conditions growth can
occur without cell division, for example, when cells are synthesizing storage compounds, e.g. glycogen or
poly-b-hydroxybutyrate. In this situation the cell numbers
remain constant, but theconcentration of biomass continues to
increase. This is also true for coenocytic organisms, such as
some fungi, that are not divided into separate cells. Their
growth results only in increased Size.

I. bacterial Growth Kinetics
Growth kinetics of homogeneous unicellular suspension.
cultures can be modeled using differential equations in a
continuum model. However, filamentous growth and growthin heterogeneous cell aggregates and assemblage, particularly
biofilms, colonies, flocs, mats and pellicles, is much more
complex. In fact, heterogeneous systems require a very different
approach using cellular automaton and Swarm system models,
e.g. BacSim. The growth kinetics of filamentous organisms and
heterogeneous systems will not be discussed here.

Microbial fermentations in liquidmedia can be carried out under
different operating conditions, i.e. batch growth, fed-batch
growth or continuous growth. Batch growth involves a closed
system where all nutrients are present at the start of the
fermentation within a fixed volume. The only further additions
may be acids or bases for pH control, or gases (e.g. aeration, if
required). In fed-batch systems fresh medium ormedium
components are fed continuously, intermittently or are added as
a single supplement and the volume of the batch increases with
time. Continuous fermentations are open systems where fresh
medium is continuously fed into the fermentation vessel, but
the volume remains constant as spent medium and cells are
removed at the same rate.

II. Batch Growth
During batch fermentations thepopulation of microorganisms
goes through several distinct growth phases: lag, acceleration,
exponential growth, deceleration, stationary and death (Fig.
5.1). In the lag phase virtually no growth occurs and the
microbial population remains relatively constant. Nevertheless,
it is a peripd of intense metabolic activity as the microbial
inoculum adapts to the new environment. When cells areinoculated into fresh medium they may be deficient in essential
enzymes, vitamins or cofactors, etc., that must be synthesized in
order to utilize available nutrients, prior to cell division taking
place.

Modeling the Microbial Growth Kinetics
The growth model that will be examined is bacterial binary
fission in homogeneous suspension cultures, where cell
division produces identical daughtercells. Each time a cell
divides is called a generation and the time taken for the cell to
divide is referred to as the generation time. Therefore, the
generation time or doubling time (td) is the time required for a
microbial population to double. Theoretically, after one
generation, both the microbial cell population and biomass
concentration have doubled. However, as previously stated,
undercertain conditions growth can be associated with an
increase in biomass and not cell numbers. Also, the generation
time recorded during microbial growth is in reality an average
value, as the cells will not be dividing at exactly the same rate. At
anyone time there are cells at different stages of their cell cycle.
This is termed asynchronous growth. However, under certain
conditions...