Microbial Growth

What are Microbes?

The microscopic organisms that are difficult to see clearly with unaided eyes and require a microscope to visualize them.

Figure 1: Imaging of microbes under microscope.

They include Bacteria, Archaea, Fungi, Protozoa, Algae and Viruses

What is microbial growth?

Microbial growth refers to an orderly increase in number of cells or population size that occur by cell division.

When Microbes are provided with nutrients and required environmental factors, they become metabolically active and grow.

Figure 2: Batch culture of different bacterial colonies.

Methods of Bacterial Growth

Binary fission

Budding

Fragmentation

Sporulation

Binary fission

Binary fission refers to an asexual reproduction, where a single cell divides by splitting into two genetically identical daughter cells.

It is most common in microbes.

E.g.- Escherichia coli

Figure 3: Binary fission in bacteria.

Budding

Another mode of asexual reproduction, where the new organism develops as an outgrowth (bud) on the parent’s body.

The bud grows by cell division, eventually detaches and becomes individual organism.

E.g.- Hyphomicrobium vulgare

Figure 4: Buddying in bacteria.

Fragmentation

It is also an asexual reproduction, where the parent’s body breaks into two or more fragments, each grows into a complete independent organism.

E.g.- Nostoc commune

Figure 5: Fragmentation in bacteria.

Sporulation

Sporulation is the process of spore formation in certain bacteria as a survival strategy under unfavorable environmental conditions.

E.g.- Bacillus subtilis

Figure 6: Sporulation in bacteria.

Bacterial Cell Cycle

The bacterial cell cycle consists of three phases:

Growth Phase

Chromosome Replication and Partitioning

Cytokinesis

1. Growth phase

It is similar to the G1 phase of the eukaryotic cell cycle.

In this phase, the cell increases in size, mass and metabolic activity.

2. Chromosome Replication and Partitioning

Both of the processes occur simultaneously.

Chromosome Replication is similar to the S phase whereas Partitioning is similar to the M phase of the eukaryotic cell cycle.

Replication

The circular DNA of the bacteria undergoes replication to divide the genetic materials into the daughter cells by starting from a site called origin of replication (Ori C).

Partitioning

The partitioning system for bacterial chromosomes has three components: ParA and ParB proteins and a parS region on the chromosome.

The C. crescentus chromosome has two parS sites close together and near the origin of replication, and this region functions like a eukaryotic centromere.

Following replication initiation, ParB binds to the parS sites on each chromosome.

Additional molecules of ParB bind nearby and ultimately cover a large chromosomal region. This protein-DNA assembly is termed the partitioning complex.

One partitioning complex remains at the stalk pole of the dividing cell, while the other is guided by ParA to the opposite pole that uses ATP as energy source.

Figure 7: ParAB/parS Partitioning system of Caulobacter crescentus.

Cytokinesis

The cytokinesis of bacteria is different from that of eukaryotes.

A set of proteins called, FtsZ assemblise to form a ring called Z-ring at the centre of the dividing cell and cause the separation.

The correct placement of the FtsZ is done by another protein called, MinCDE proteins.

The MinCDE proteins are more concentrated towards the poles preventing the FtsZ protein formation.

Figure 8: The MinCDE complex.

Bacterial Cell Cycle

Figure 9: Bacterial cell cycle.

Growth 

1. Balanced Growth

It is defined by Campbell.

It is defined by the doubling of every biochemical unit of the cell within the time period of a single division without any change in the rate of growth.

Theoretically, cultures in balanced growth have a constant chemical composition.

In practice, it is difficult to achieve.

2. Monoauxic Growth: Growth of Batch Culture

Growth of microorganisms in a limited volume of liquid medium having only one type of carbon/energy source is generally termed as Monoauxic Growth: Batch Cultute.

Here the growth curve is sigmoid with 4 phases- Lag, Log, Stationery and Death.

Growth Curve

It is the graphical representation of how microbes grow in a medium.

When microorganisms are cultivated in broth, they usually are grown in a batch culture.

as nutrients are consumed, their population decline, and wastes accumulate.

Population growth of microbes in a batch culture can be plotted as the logarithm of the number of viable cells Vs the incubation time. The resulting curve has five distinct phases.

Figure 10: Bacterial growth curve in monoauxic growth.

Lag Phase

In this phase the cells may be synthesizing various metabolically important componets like ATP, ribosomes, and some essential cofactors.

This is the phase of acclimatization.

Exponential Phase

During this phase, microorganisms grow and divide at the maximal rate possible given their genetic potential, the nature of the medium, and the environmental conditions.

Their rate of growth is constant during this phase; that is, they are completing the cell cycle and doubling in number at regular intervals.

The population is most uniform in terms of chemical and physiological properties during this phase; therefore exponential phase cultures are usually used in biochemical and physiological studies.

Stationery Phase

The growth curve becomes horizontal.

In stationary phase, the total number of viable microorganisms remains constant.

This may result from a balance between cell division and cell death, or the population may simply cease to divide but remain metabolically active.

Final population size depends on nutrient availability and other factors, as well as the type of microorganism.

Death Phase

Cells growing in batch culture cannot remain in stationary phase indefinitely. Eventually, they enter into the death phase.

During this phase, the number of viable cells declines exponentially, with cells dying at a constant rate.

This is due to detrimental environmental changes like unavailability of nutrients and build-up of toxic wastes in the culture medium.

Long Term Stationery Phase

Here, some microbe’s population size remains more or less constant.

During this time, the bacterial population continually evolves so that actively reproducing cells are those best able to use the nutrients released by their dying brethren and best able to tolerate the accumulated toxins.

This dynamic process is marked by successive waves of genetically distinct variants.

Thus natural selection can be witnessed within a single culture vessel.

3. Diauxic Growth

Diauxic growth is a diphasic growth represented by two growth curves intervened by a short lag phase produced by an organism utilizing two different substrates.

J. Monod first observed this phenomenon when he grew E. coli in a medium containing glucose and lactose.

Under these conditions, glucose is first utilized, then lactose is utilized after exhaustion of glucose.

Figure 11: Bacterial growth curve in diauxic growth.