CHAPTER:
3
Nutritional Requirement & Genetics
of Bacteria
Nutritional Types of
Bacteria:
• Bacteria,
like all living cells, require energy and nutrients to build proteins and
structural membranes and drive biochemical processes.
• Bacteria
require sources of carbon, nitrogen, phosphorous, iron and a large number of
other minerals. Carbon, nitrogen and water are used in highest quantities.
• The
nutritional requirements for bacteria can be grouped according to the carbon
source and the energy source:
- Autotrophs:
• Autotrophs
are bacteria which obtain their nutrition from inorganic compounds.
• Carbon
dioxide is typically the sole source of cellular carbon.
• Autotrophs
will use hydrogen sulfide, ammonia or hydrogen gas to reduce carbon into
necessary sugars.
• Nitrifying bacteria, which oxidize ammonia to
create nitrites and nitrates, are an example of bacteria which use autotrophic
nutrition.
- Heterotrophs:
• Bacteria
that require organic sources of carbon such as sugars, fats and amino acids are
termed heterotrophs.
• Saprophytic
bacteria are an example. They attain their nutrition from dead organic matter.
• Using
enzymes, these bacteria will break down complex compounds and use the nutrients
to release energy.
• Saprophytic
bacteria are essentially decomposers and play an important role in ecosystem by
releasing simpler products which plants and animals can use.
- Phototrophs:
• Phototrophic
bacteria absorb light energy, then utilize this in photosynthesis to create
cellular energy.
• There
are two types of phototrophs; those which do not produce oxygen as a byproduct
are termed anaerobic phototrophs, while those which do produce oxygen are
termed aerobic phototrophs.
• Both
autotrophs and heterotrophs can be phototrophs. Cyanobacteria are an example of
bacteria which execute photoautotrophic nutrition.
- Chemotrophs:
• These
bacteria obtain chemical energy from their surroundings and convert it into
adenosine triphosphate (ATP) for cellular use.
• Chemotrophs
attain energy from oxidation-reduction reactions of inorganic compounds such
ammonia, hydrogen sulfide and iron.
• For
instance, sulfur bacteria is a chemoautotroph which produces energy by
oxidizing hydrogen sulfide into sulfur and water.
- Lithotrophs:
• Lithotrophs
are bacteria which use reduced inorganic compounds as the electron donor
(H-donor) in anaerobic or aerobic respiration.
Nutritional Requirement
of Bacteria:
• There
are basically three groups of nutrients required for normal bacterial growth:
1.
Mineral Nutrient
2.
organic Nutrient
3.
Growth Factors
- Mineral Nutrients:
• Mineral
nutrients are also known as inorganic nutrients.
• On
the requirements of mineral nutrients, these are further grouped into:
a.
Major Nutrients: C, H, O, N, P, K, S & Mg
b.
Minor Nutrients: Ca, Mn, Zn, Cu, Fe, Mo.
Ø Water
is also major requirements which constituents important part of the cell.
Ø Nitrates,
Nitrites and organic nitrogenous compound like amino acids, nucleic acids,
peptides, peptones, proteins etc provides source of N to bacteria.
Ø P
& S are also required for bacteria.
- Organic Nutrients:
• Dead
and decayed bodies of living organisms provides organic substances to the bacteria.
• Carbohydrates,
proteins, fats, nucleic acids, are common organic nutrients
• Bacteria
and other microorganisms derived their energy from degradation products of Carbohydrates,
proteins, fats, nucleic acids.
3. Growth Factors:
• Most
bacteria are able to produce growth factors by themselves.
• Those
which do not synthesize important growth factors , requires, additional
supplements of these growth factors.
• The
important growth factors are:
Vitamins
(Thiamine. Niacin, Pyridoxin, Riboflavin, ` Nicotinic
acids , biotin etc)
p-Amino
benzoic acid
Inositol,
Folic acids, Lipoic acids
Genetics
of Bacteria (Recombination Mechanism/ Gene transfer technique):
• Bacteria
reproduce by the process of binary fission.
• In
this process, the chromosome in the mother cell is replicated and a copy is
allocated to each of the daughter cells.
• As
a result, the two daughter cells are genetically identical.
• Several
events occur that change the bacterial chromosome and then these changes are
passed on to future generations by binary fission.
Recombination:
• Genetic
recombination refers to the exchange between two DNA molecules.
– It
results in new combinations of genes on the chromosome.
– In
crossing over, two homologous chromosomes (chromosomes that contain the same
sequence of genes but can have different alleles) break at corresponding
points, switch fragments and rejoin.
– The
result is two recombinant chromosomes.
• In
bacteria, crossing over involves a chromosome segment entering the cell and
aligning with its homologous segment on the bacterial chromosome.
• The
two break at corresponding point, switch fragments and rejoin.
• The
result, as before, is two recombinant chromosomes and the bacteria can be
called a recombinant cell
• But
one question still remains...how did the chromosome segment get in to the cell?
• The
answer is Genetic Transfer!
Genetic Transfer:
• Genetic
transfer is the mechanism by which DNA is transferred from a donor to a
recipient.
• Once
donor DNA is inside the recipient, crossing over can occur.
– The
result is a recombinant cell that has a genome different from either the donor
or the recipient.
• In
bacteria genetic transfer can happen by
three ways:
– Transformation
– Transduction
– Conjugation
- Transformation:
• It
is a process in which free DNA is taken up by a cell, resulting in a genotypic
change in the recipient.
• After
death or cell lyses, some bacteria release their DNA into the environment.
• Other
bacteria, generally of the same species, can come into contact with these
fragments, take them up and incorporate them into their DNA by recombination.
– This
method of transfer is the process of transformation.
• Any
DNA that is not integrated into the chromosome will be degraded.
• The
genetically transformed cell is called a recombinant cell because it has a
different genetic makeup than the donor and the recipient.
– All
of the descendants of the recombinant cell will be identical to it.
– In
this way, recombination can give rise to genetic diversity in the population.
Griffith's Experiment:
• The
transformation process was first demonstrated in 1928 by Frederick Griffith.
• Griffith
experimented on Streptococcus pneumoniae, a bacteria that causes
pneumonia in mammals.
• When
he examined colonies of the bacteria on petri plates, he could tell that there
were two different strains.
– The
colonies of one strain appeared smooth.
• Later
analysis revealed that this strain has a polysaccharide capsule and is
virulent, that it, it causes pneumonia.
– The
colonies of the other strain appeared rough.
• This
strain has no capsules and is avirulent.
When Griffith injected
living encapsulated cells into a mouse, the mouse died of pneumonia and the
colonies of encapsulated cells were isolated from the blood of the mouse.
• When
living nonencapsulated cells were injected into a mouse, the mouse remained
healthy and the colonies of nonencapsulated cells were isolated from the blood
of the mouse.
• Griffith
then heat killed the encapsulated cells and injected them into a mouse.
• The
mouse remained healthy and no colonies were isolated.
• The
encapsulated cells lost the ability to cause the disease.
• However,
a combination of heat-killed encapsulated cells and living nonencapsulated
cells did cause pneumonia and the mouse died and colonies of living
encapsulated cells were isolated from the mouse.
• How
can a combination of these two strains cause pneumonia when either strand alone
does not cause the disease?
• If
you guessed the process of transformation you are right!
• The
living nonencapsulated cells came into contact with DNA fragments of the dead
capsulated cells.
• The
genes that code for the capsule entered some of the living cells and a crossing
over event occurred.
• The
recombinant cell now has the ability to form a capsule and cause pneumonia.
• All
of the recombinant's offspring have the same ability.
• That
is why the mouse developed pneumonia and died.
• Bacterial
transformation has been demonstrated in several bacteria. For e.g.: Bacillus
subtilis, Haemophilus influenzae, Rhizobium, E. coli, Streptococcus and
Streptomyces.
• The
process of transformation in all these organisms has certain common features:
• The
purified donor DNA is first transported
across the cell membrane into the recipient “Component cells” (Cell that can
take of DNA)
• The
DNA undergoes recombination with the recipient DNA and is then expressed.
 |
| Griffth's Experiment |
2.
Conjugation:
• A
second mechanism by which genetic transfer takes place is conjugation.
• Conjugation
in bacteria is a mechanism for gene transfer that requires cell-to-cell
contact.
• This
mechanism requires the presence of a special plasmid called the F plasmid.
• Therefore,
we will briefly review plamid structure before continuing.
– Plasmids
are small, circular pieces of DNA that are separate and replicate indepentently
from the bacterial chromosome.
– Plasmids
contain only a few genes that are usually not needed for growth and
reproduction of the cell.
– However,
in stressful situations, plasmids can be crucial for survial.
– The
F plasmid, for example, facilitates conjugation.
• This
can give a bacterium new genes that may help it survive in a changing
environment.
– Some
plasmids can integrate reversibly into the bacterial chromosome.
• Such plasmid
which has capacity of integrationis called an episome.
• Bacteria
that have a F plasmid are referred to as as F+ or male.
– Those
that do not have an F plasmid are F- of female.
• The
F plasmid consists of 25 genes that mostly code for production of sex pilli.
• A
conjugation event occurs when the male cell extends his sex pili and one
attaches to the female.
– This
attached pilus is a temporary cytoplasmic bridge through which a replicating F
plasmid is transferred from the male to the female.
– When
transfer is complete, the result is two male cells.
• This
F plasmid can behave as an episome.
– When
the F+ plasmid is integrated within the bacterial chromosome, the cell is
called an Hfr cell (high frequency of recombination cell).
– The
F plasmid always inserts at the same spot for a bacterial species.
• The
Hfr cell still behaves as a F+ cell, transferring F genes to a F-cell, but now
it can take some of the bacterial chromosome with it.
• Replication
of the Hfr chromosome begins at a fixed point within the F episome and the
chromosome is transferred to the female as it replicates.
• Movement
of the bacteria usually disrupts conjugation before the entire chromosome,
including the tail of the F episome can be transferred.
– Therefore,
the recipient remains F- because the F plasmid is not entirely transferred.
• A
cross over event can occur between homologous genes on the Hfr fragment and the
F- DNA.
• Pieces
of DNA not recombined will be degraded or lost in cell division.
• Now
the recombinant genome can be passed on to future generations.
- Transduction:
• Transduction is a process in which
bacterial DNA is transferred from one cell to another with the help of a virus.
• This method involves the transfer
of DNA from one bacterium to another with the use of a bacteriophage (phage).
– A phage is a virus that infects
bacteria.
– The phage T4 and the phage lambda,
for example, both infect E. coli.
• Phages are obligatory intracellular
parasites and must invade a host cell in order to reproduce.
– T4 multiplies by the lytic cycle
which kills the host and lamba multiplies by the lysogenic cycle which does not
cause the death of the host cell.
– In lysogeny, the phage DNA remains
latent in the host until it breaks out in a lytic cycle.
• Bacterial Transduction is of two
types:
1.
Generalized Transduction 2.
Specialized Transduction
- Generalized
Transduction:
• In generalized transduction, any
bacterial genes can be transferred because the host's chromosome is broken down
into fragments.
– Whatever piece of bacterial DNA
available to get packaged within the phage is the genetic material that will be
transferred between cells.
2.
Specialized Transduction:
• In specialized transduction, on the
other hand, only certain bacterial genes can be transferred.
– These genes, as you will see, must
exist on either side of the prophage.
– Specialized transduction requires a
phage that uses the lysogenic cycle for reproduction.
Steps
Of General Transduction:
– A phage attaches to cell wall of
bacterium and injects DNA.
– The bacterial chromosome is broken
down and biosynthesis of phage DNA and protein occurs.
– Sometimes bacterial DNA can be
packaged into the virus instead of phage DNA.
– The cell lyses, releasing viruses.
– The phage carrying bacterial DNA
infects another cell.
– Crossing over between donor and
recipient DNA can occur producing a recombinat cell.
Steps
In Specialized Transduction:
• In the lysogenic cycle, phage DNA
can exist as a prophage integrated in the bacterial chromosome.
• Occasionally when the prophage
exits it can take only adjacent
bacterial genes with it.
• The phage DNA directs synthesis of
new phages.
• The phage particles carry phage DNA
and bacterial DNA.
• The cell lyses, releasing the
phages.
• A phage carrying bacterial DNA
infects another cell.
• The joined phage and bacterial DNA
circularize.
• Along with the prophage, bacterial
DNA integrates with the recipient chromosome by a cross over event.
This forms a recombinant cell
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