bacteriophage microbiology short notes.pptx

Short notes on bacteriophage

1. Bacteriophage

2. Introduction: A virus that infects Replicates within bacterium Derived from “bacteria” and phagein , “to devour-to eat” Single or double stranded Surrounded by protein capsid Genomes may codes as few as four genes and as many as hundred of genes.

3. Bacteriophages Bacteriophages were discovered independently by Twort 1915 in U.K. and Herelle 1917 in France and the term was coined by Herelle . Bacteriophage or Phage is a virus that infects and replicates only within the body of bacteria and no other organism. Highly species-specific towards their host cell. The bacteriophage only infect a single species of bacteria or even specific strains of bacteria within a species.

4. Bacteriophage : Have a potential to destroy bacterial suspension and called Twort d Herelle Phenomenon. Bacteriophage attacking Escherichia coli are called coliphages or T phages. Like all viruses, phages are simple entities (DNA or RNA) that consist of a core of genetic material (nucleic acid) surrounded by protein capsid .

5. The nucleic acid may be either DNA or RNA and may be double stranded or single stranded. The international committee on Taxonomy of viruses had classified the phages based on the nature of their nucleic acid, morphology and physiochemical properties of their virion particles. Currently ICTV has classified the bacteriophages into 9 orders, 48 families with 32 subfamilies and 317 genera.

6. - By morphology phages can be tailed phages, polyhedral (flat sides usually more than 6) or filamentous phages.

7. Structure (T Even Phage) The virion of T even phage is tadpole like structure with a hexagonal (six side and six angle) head connected to a helical tail through a short collar. The head is about 95 nm in length and 65 nm in diameter and composed of about 2000 capsomeres and encloses a tightly packed nucleic acid. The linear dsDNA (50nm long)is coiled,very long and it is genetic material of phage particles and includes the host cell to synthesize more and more phage particle.

8. - The tail has an inner hollow tube called core, surrounded by a contractile sheath which consist of 24 annular rings. The distal end of the tube is connected to a hexagonal basal plate with spike or tail spin at each corner. Six long, flexible tail fibers (150nm) arise from basal plate which helps the bacteriophage to attaches to the bacteria.

9. Lytic Cycle: Depending upon the interaction of phages with the bacterial cells they have been distinguished into two major types of life cycle: a) Lytic cycle or virulent cycle or infective cycle (Virulent phage): Type of lifecycle where the viral DNA remains as a free floating molecule and replicates separately from the bacterial DNA. b) Lysogenic cycle or Temperate Cycle (Non or Avirulent Phage): is defined by the incorporation of the bacteriophage genome into the host genome.

10. a) Lytic Cycle: In the lytic cycle, a bacteriophage infects a bacteria and kills it to release progeny virus. There are five stages in the bacteriophage lytic cycle. i )Attachment: The bacteriophage attaches itself on the surface of bacteria (adsorption). The tips of the tail fibres attach to specific receptors eg : lipopolysaccharides , OmpC protein on host surface (bacterial cell).

11. ii)Penetration: Lysozyme digest the host cell wall and the tail sheath contracts, which act like a hypodemic needle to inject the viral genome (DNA) in the host cell. The phage head and remaining components remain outside the bacteria called Ghosts or Doughnut.

12. iii) Biosynthesis of new viral component: After entering the host cell, the virus synthesizes virus- encoded endonucleases to degrade the bacterial chromosome. It then hijacks the host cell to replicate,transcribe and translate the necessary viral component ( capsomeres,sheath,base plate,tail fibers and viral enzymes) for the assembly of new viruses.

13. iv) Maturation and assembly: On maturation, the head and tail protein of phage DNA assemble and each component of phage DNA is surrounded by a protein coat. Ultimately, the tail structure are added forming a virion . v) Release: The infected bacterial cell is lysed releasing the progeny phages and progeny viruses are liberated into the environment to infect the cells. The phage enzymes weaken the cell wall of bacteria during replication.

14. Lysogenic cycle: - In a lysogenic cycle, the phage genome enters the cell through attachment and penetration. A prime example is the lambda phage(a non-contractile tailed phage, meaning during an infection event it cannot 'force' its DNA through a bacterial cell membrane .) During the lysogenic cycle, instead of killing the host, the phage DNA integrates into bacterial chromosome and becomes part of the host. The integrated phage genome (DNA) is called a prophage . A bacteria carrying a prophage without being lysed is called a lysogen and the process is called lysogeny .

15. As the bacteria replicate its chromosome, it also replicate the phage’s DNA and passes it on to new daughter cells during reproduction. The presence of the phage may alter the phenotype to the bacterium,since it can bring in extra genes. This change in the host phenotype is called lysogen conversion or phage conversion. In the case of V.cholera phage encoded toxin can cause severe diarrhea and in C.botulinum , the toxin can cause paralysis and both are less virulent in absence of the prophage .

16. During lysogeny , when the host cell face harsh condition then the prophage shows induction, which results in the excision of the viral genome from the host chromosome. After induction the temperate phage follows the lytic cycle then again undergoes lysogeny in a newly infected cell. The lytic cycle doesnot allow genetic recombination of the host chromosome whereas the lysogenic cycle allow the genetic recombination of host chromosome.

17. Economic Importance: Advantage Phages are very specific and do not harm the useful bacteria that live in and out of body. They have been proposed as alternative to antibiotics resistance bacterial strains. Have been used in treatment against dysentery, cholrea , plague and many other pathogenic bacterial disease.

18. Useful in the lysis of bacterial present in polluted water. Helps in transduction of genetic material and therefore they have potential uses in biotechnology, research and therapeutics. Phages can be used as bio control agents in agriculture and petroleum industry. Phages can be administered orally, incorporated in drinking water or food to control Salmonella and Campylobacter in poultry. Phages are also excellent as food bio preservation agents.

19. Disadvantages: In comparison to chemical molecules, phages are larger . So , the sites in the body that can be reached by them must be carefully clarified. Phages that are injected into bloodstream are recognized by the human immune system. Some of them are quickly excreted and after a certain period antibodies against the phages are produced by the body. For example: it appear that one type of phage can only be used for intravenous treatment.

20. Diseases caused by Bacteriophages : Bacteriophages only infect bacteria, they do not cause disease in human. Alter the genome of non-virulent bacteria strains, thus, producing more virulent strains. Examples are: Cholera: Most strains of cholera are harmless Responsible for producing harmful cholera strain.

21. b) Scarlet Fever: Commonly affects children Signs and symptoms include sore throat, fever and characteristic red rash Usually spread by inhalation There is no vaccine of this disease. Most of the clinical features are caused by erythrogenic toxin, a substance produced by the bacterium Streptococcus when it is infected by Bacteriophage T12.

22. Classification of Virus