Bacteriophages

Introduction

Did you know besides animals, plants and humans, even bacteria get infected by a virus? These viruses are termed “bacteriophages”, derived from the Greek word “phagein” which means “to prey upon”. Bacteriophages infect the bacteria and grow inside them, as it is well known that viruses need a host to multiply and cannot increase independently without a host.

Frederick Twort discovered these obligate intracellular parasites in 1915 and Felix d’Herelle in 1917 independently. They are commonly denoted as “phage”. They are the most abundantly found organisms which could be isolated in any source where bacteria are abundantly found. The phages are only specific for preying upon bacteria and no other microbes.

Morphology

The T2, T4, and T6 bacteriophages infecting E.coli are widely studied. The bacteriophage structure is very simple. It includes an icosahedral head comprising double-stranded DNA and a tail. The tail is cylindrical and contains a collar or neck joining the head, a central hollow tube, a tail sheath surrounding the tube, and a hexagonal base plate. The virus attaches itself to the bacterial surface by the tail fibres. The structure varies significantly among the phage.

For instance, the T1, T5, and lambda phage lack base plates, the tail does not have a sheath, and it ends up in essential tail fibres. T3 and T7 have short, non-contractile tails without tail fibres. The function of the tail is to inject the DNA contained in the phage head into the host bacterial cell.

Image 1 − Structure of bacteriophage

Life Cycle of Bacteriophage

All viruses, including bacteriophages, need a living host to replicate and proliferate. The manner by which a bacteriophage infects the bacteria and produces copies of progeny bacteriophages is called the bacteriophage life cycle. There are two types of life cycles; lytic (which involves killing the bacterial host) and lysogenic (the bacterial host is exempted from being killed).

Lytic cycle

The lytic mode of infection is also called the virulent cycle, as it involves multiplication in the host bacterial cell and causing it to lyse. The steps involved in the lytic cycle are as follows −

• Attachment/adsorption − This is a rapid process. The phage binds to the surface of the host bacterial cell by tail fibres to specific receptor sites on the surface of the bacterial cell. These receptor sides could be the components of the bacterial cell wall, such as glycoproteins, lipoproteins, carbohydrates, lipids or proteins. There is an interaction between the amino acid groups of the virus with the surface components of the bacterial cell wall. The phage secretes an enzyme called lysozyme to enter the host cell, which creates a pore on the bacterial cell wall. This enzyme is found in the tail fibres.

• Penetration/Injection − The phage enters the bacterial host cell's cytoplasm. The tail fibres attached to the host cell's surface constrict, allowing the base plate to touch the cell wall. Then, the tail sheath narrows to allow the passage of the central hollow tube inside the bacterial host. The phage DNA molecule is injected into the cytoplasm. Once the DNA is released, the capsid is no longer helpful.

• Biosynthesis of phage components − Once the phage DNA enters the host cell, the cellular metabolism of bacteria is no longer functional, whereas the bacteriophage particles synthesise like the head and tail.

• Assembly − The newly formed phage head and tail now assemble into a new progeny virion.

• Lysis of the host cell − The last step consists of the virus killing the bacterial cell by cell lysis, causing the progeny virions to be released. The process repeats in a different bacterial host, which is how the phage devours the bacteria.

Lysogenic Cycle

This life cycle mode is called the non-infectious or temperate cycle since the bacteria is exempted from being devoured by the bacteriophage. The phage involved in this life cycle is called temperate or lysogenic phage.

The steps involved in the lysogenic cycle are as follows −

• Attachment − Here, the temperate or lysogenic phage identifies the receptor site of the bacterial host and attaches to the host cell surface using the tail fibres. The process is the same as the lytic cycle.

• Penetration/Injection − The phage DNA gets entry into the host cell. The step remains the same as the lytic cycle.

• Integration of genetic material − The phage DNA integrates or joins with the host cell's genetic material to form a prophage. Prophage is a dormant temperate phage, i.e., it retains its DNA and does not produce phage components, unlike in the lytic cycle.

• Replication and cell division − During favourable conditions, the lysogenic bacteria and prophage replicate in synchronisation and pass on to the progeny. The bacterial cell is not harmed. The cell divides to produce new daughter cells.

• Termination − If the lysogenic bacteria are exposed to extreme environmental conditions like heat, UV rays, mutagens, hydrogen peroxide, desiccation etc., the lysogenic cycle is terminated. This process is called induction. The integration process is reversed, and the phage can initiate a lytic cycle.

Eclipse Phase

The period between the entry of phage DNA into the host bacterial cell until the maturation of the virulent phage in the host cell is called the “eclipse phase”. The time could range from 15-30 minutes.

How is bacteriophage useful?

• They serve as a biocontrol agent by eliminating unwanted bacteria in food industries.

• It is widely used as a vector in genetic engineering.

• It helps to indicate the pathogenic bacteria in the environment.

• Like E. coli, the bacteriophage is widely used as a model organism for various studies.

• It is commonly used for the treatment of bacterial infections by phage therapy.

FAQs

Qns 1. What is a burst period?

Ans.A Burst period is also known as the rise period when the host bacterial cell undergoes lysis with the release of infectious phage particles.

Qns 2. What is a virion?

Ans. A matured virus particle consisting of nucleic acid, i.e., DNA or RNA fenced in a protein coat capable of causing an infection, is termed a virion.

Qns 3. What is a capsid?

Ans. The protein coat which guards the viral genetic material, i.e., DNA or RNA, and helps transfer between host cells is called a capsid.

Qns 4. Explain lysogenic conversion.

Ans. When the host bacterial cell is lysogenized by the phage, some phage genes also get expressed, responsible for changing the properties of the bacterial cell. This process is called a lysogenic conversion.

Qns 5. Describe phage assay.

Ans. When an inoculum of phage is spread plated on a culture with a known bacterial strain, clear zones appear after incubation, representing the action of bacteriophages on the bacteria. These clear zones are known as plaques.