Nutritional Requirement & Genetics of Bacteria

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:

1. 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.

2. 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.

3. 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.

4. 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.

5. 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

1. 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.

2. 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

1. 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.

3. 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

1. 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