Microbial Growth

Intended Learning objectives

At the end of this lecture, student will be able to:

• Explain the process of binary fission

• List the different stages of bacterial growth

• Explain the significance of different phases of a bacterial growth curve

• Differentiate between batch and continuous culture

• Explain methods for continuous culture

Microbial growth

• Growth is defined as an increase in cellular constituents
and may result in an increase in a microorganism’s size, population number, or both

• Techniques can be used to study microbial growth –
following changes in the total cell number, the population of viable microorganisms, or the cell mass

• In the natural environment, growth is often severely
limited by available nutrient supplies and many other environmental factors.

The growth

• Growth may be defined as an increase in cellular constituents. It leads to a rise in cell number when microorganisms reproduce by processes like budding or binary fission.

Batch culture and continuous culture

Batch culture

• When microorganisms are grown in a closed system,
population growth remains exponential for only a few generations and then enters a stationary phase due to factors such as nutrient limitation and waste accumulation.

Continuous culture

• In an open system with continual nutrient addition and
waste removal, the exponential phase can be maintained for long periods

The growth curve

• The growth of microorganisms reproducing by binary fission can be plotted as the logarithm of the number of viable cells versus the incubation time. The resulting curve has four distinct phases

Lag Phase

• When microorganisms are introduced into fresh culture
medium, usually no immediate increase in cell number occurs

• Cell division does not take place right away and there is
no net increase in mass, the cell is synthesizing new components

Bacterial growth curve

Reasons for lag phase

• The cells may be old and depleted of ATP, essential
cofactors, and ribosomes; these must be synthesized before growth can begin

• The medium may be different from the one the microorganism was growing in previously. Here new enzymes would be needed to use different nutrients

• Injured microorganisms require time to recover

• The cells retool, replicate their DNA, begin to increase
in mass, and finally divide

• Duration of lag phase depends on the condition of the
microorganisms and the nature of the medium.

• This phase may be quite long if the inoculum is from an
old culture or one that has been refrigerated.

• Inoculation of a culture into a chemically different
medium also results in a longer lag phase.

• When a young, vigorously growing exponential phase culture is transferred to fresh medium of the same composition, the lag phase will be short or absent.

Exponential Phase

• Microorganisms are growing and dividing at the maximal
rate possible

• Depends on their genetic potential, the nature of the
medium, and the conditions under which they are growing

• Rate of growth is constant

• Each individual divides at a slightly different moment,
the growth curve rises smoothly rather than in discrete jumps

• Uniform in terms of chemical and physiological properties

Stationary Phase

• Population growth ceases and the growth curve becomes
horizontal

• The total number of viable microorganisms remains constant

• Balance between cell division and cell death, or the population may simply cease to divide though remaining metabolically active

Reasons for stationary phase

• Nutrient limitation – if an essential nutrient is severely
depleted, population growth will slow

• Aerobic organisms often are limited by O2 availability

• Accumulation of toxic waste products.

Death Phase

Detrimental
environmental changes

â

Nutrient deprivation
and the buildup of toxic wastes

â

Decline in the number
of viable cells

â

Death phase

The death of a microbial population, is usually logarithmic

• The total cell number remains constant because the cells
simply fail to lyse after dying

• The only way of deciding whether a bacterial cell is
viable is by incubating it in fresh medium; if it does not grow and reproduce, it is assumed to be dead.

• Death is defined to be the irreversible loss of the
ability to reproduce

Phases of bacterial growth curve

The synchronous of microorganisms

• Growth in a cell population in which all cells divide at
the same time

• A population can be synchronised by manipulating the physical or the chemical environment

The Continuous Culture of Microorganisms

• Growing microorganisms in an open system, a system with
constant environmental conditions

• Continual provision of nutrients and removal of wastes

• A microbial population can be maintained in the exponential growth phase and at a constant biomass concentration for extended periods

• Two major types of continuous culture systems commonly are used: (1) Chemostats and (2) Turbidostats

The Chemostat

• Sterile medium is fed into the culture vessel at the same
rate as the media containing microorganisms is removed

• The culture medium for a chemostat possesses an essential nutrient (e.g., an amino acid) is supplied in limiting quantities

• The growth rate is determined by the rate at which new
medium is fed into the growth chamber, and the final cell density depends on the concentration of the limiting nutrient

• The rate of nutrient exchange is expressed as the dilution
rate (D), the rate at which medium flows through the culture vessel relative to the vessel volume, where f is the flow rate (ml/hr) and V is the vessel volume (ml)

D = f/V

• For example, if f is 30 ml/hr and V is 100 ml, the
dilution rate is 0.30 hr-1

• If the dilution rate rises too high, the microorganisms
can actually be washed out of the culture vessel before reproducing because the dilution rate is greater than the maximum growth rate

• At very low dilution rates, an increase in D causes a rise
in both cell density and the growth rate

The Turbidostat

• The turbidostat, has a photocell that measures the
absorbance or turbidity of the culture in the growth vessel

• The flow rate of media through the vessel is automatically
regulated to maintain a predetermined turbidity or cell density

Summary

• Growth may be defined as an increase in cellular
constituents.

It leads to a rise in cell number when microorganisms
reproduce by processes like budding or binary fission.

• Organisms grown in a closed culture with limited nutrients
– batch culture

• Continuous supple of nutrients and removal of waste –
continuous culture

• Phases of bacterail growth curve – lag, log, stationary
and decline

• Continuous culture can be obtained using chemostat or