A Bacteriophage (from "bacteria" and Greek "to eat") is any one of a number of viruses that infect bacteria. The term is commonly used in its shortened form, phage. Bacteriophages were jointly discovered by Frederick Twort (1915) in England and by Felix d'Herelle (1917) at the Pasteur Institute in France (Sridhar et.al, 2006). Felix d'Herelle coined the term "Bacteriophage". Bacteriophage means to eat bacteria, and are called so because virulent bacteriophage can cause the compete lysis of a susceptible bacterial culture. Bacteriophages are among the most common biological entities on Earth (Collman, J. P. 2001). They are much smaller than bacteria they destroy, and usually measures between 20 and 200 nm in size.
There are at least 12 distinct groups of bacteriophages, which are very diverse structurally and genetically.
Examples of phages:
T-even phages such as T2, T4 and T6 that infect E.coli
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Temperate phages such as lambda and mu
Spherical phages with single stranded DNA such as PhiX174
Filamentous phages with single stranded DNA such as M13
RNA phages such as Qbeta
Typically, bacteriophages consist of an outer protein hull enclosing genetic material. The genetic material can be single stranded RNA, double stranded RNA, single stranded DNA and double stranded DNA between 5 and 500 kbp long with either circular or linear arrangement (Fields of Virology). Most phages range in size from 24-200 nm in length. T4 is among the largest phages; it is approximately 200 nm long and 80-100 nm wide. All phages contain a head structure, which can vary in size and shape. Some are icosahedral (20 sides) others are filamentous (Sridhar et.al,2006). The head encloses nucleic acid and acts as the protective covering. Some phages have tails attached to the phage head. The tail is a hollow tube through which the nucleic acid passes during infection. T4 tail is surrounded by a contractile sheath, which contracts during infection of the bacterium. At the end of the tail, phages like T4 have a base plate and one or more tail fibers attached to it. The base plate and tail fibers are involved in the binding of the phage to the bacterial cell. Not all phages have base plates and tail fibers (Sridhar et.al, 2006). Include Cylindrical phages also
The first step in the infection process is the adsorption of the phage to the bacterial cell. This step is mediated by the tail fibers or by some analogous structure on those phages that lack tail fibers. Phages attach to specific receptors on the bacterial cell such as proteins on the outer surface of the bacterium, LPS, pili, and lipoprotein. This process is reversible. One or more of the components of the base plate mediates irreversible binding of phage to a bacterium. (Sridhar et al, 2006). For example, the specificity of adsorption of lactococcal phages and the location of phage receptor substances has been reviewed by Lawrence et. al., 1976. Lactococci have been shown to have different receptor sites that may reside in both the cell wall and plasma membrane. To enter a host cell, bacteriophages attach to specific receptors on the surface of bacteria, includingÂ lipopolysaccharide, teichoic acid,proteins or even flagella. This specificity means that a bacteriophage can only infect certain bacteria bearing receptors that they can bind to, which in turn determines the phage's host range. (Prescott, L. (1993). Microbiology).
Once attached completely, the tail contracts, possibly with the help ofÂ ATPÂ present in the tail (Prescott, L. 1993) injecting genetic material through the bacterial membrane.
Just within few minutes, bacterialÂ ribosomesÂ start translating viral mRNA into protein. For RNA-based phages,Â RNA replicase is synthesized early in the process. Proteins modify the bacterialÂ RNA polymerase so that it preferentially transcribes viral mRNA. The host's normal synthesis of proteins and nucleic acids is disrupted, and it is forced to manufacture viral products instead. These products go on to become part of new virions within the cell, helper proteins which help assemble the new virions, or proteins involved in cellÂ lysis.
In the case of theÂ T4 phage, the construction of new virus particles involves the assistance of helper proteins. The base plates are assembled first, with the tails being built upon them afterwards. The head capsids, constructed separately, will spontaneously assemble with the tails. The DNA is packed efficiently within the heads. The whole process takes about 15 Phages may be released via cell lysis, by extrusion, or by budding. (Prescott, L.1993. Microbiology) Released virions are described as free, and, unless defective, are capable of infecting a new bacterium. In contrast to virion release, phages displaying aÂ lysogenicÂ cycle do not kill the host but, rather, become long-term residents asÂ prophage. (Prescott, L. 1993).
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In a Lytic or virulent phages are phages, which multiply in bacteria and kill the cell by lysis at the end of the life cycle. Soon after the nucleic acid is injected, the phage cycle is said to be in eclipse period. During the eclipse phase, no infectious phage particles can be found either inside or outside the bacterial cell. Eclipse phase represents the interval between the entry of phage nucleic acid into bacterial cell and release of mature phage from the infected cell. This phase is devoted to synthesis of phage components and their assembly into mature phage particles. The phage nucleic acid takes over the host biosynthetic machinery and phage specified m-RNA's and proteins are made. In some cases the early phage proteins actually degrade the host chromosome. Structural proteins (head, tail) that comprise the phage as well as the proteins needed for lysis of the bacterial cell are separately synthesized. Nucleic acid is then packaged inside the head and then tail is added to the head. The assembly of phage components into mature infective phage particle is known as maturation. In Lysis and Release Phase the bacteria begin to lyse due to the accumulation of the phage lysis protein and intracellular phage are released into the medium. It is believed that phage enzymes weaken the cell wall of bacteria. The number of particles released per infected bacteria may be as high as 1000. The average yield of phages per infected bacterial cell is known as burst size.
Whereas, Lysogenic or temperate phages are those that can either multiply via the lytic cycle or enter a dormant state in the cell. In most cases the phage DNA actually integrates into the host chromosome and is replicated along with the host chromosome and passed on to the daughter cells. This integrated state of phage DNA is termed prophage. This process is known as lysogeny and the bacteria harboring prophage are called lysogenic bacteria.
When a cell becomes lysogenized, occasionally extra genes carried by the phage get expressed in the cell. These genes can change the properties of the bacterial cell. This process is known as lysogenic conversion or phage conversion.
Phages are estimated to be the most widely distributed and diverse entities in the biosphere (Mc Grath S et al ,2007). They occur widely in nature and can readily be isolated from feces and sewage. One of the densest natural source of phages and other viruses is sea water, where up to 9Ã-108 virions per milliliter have been found in microbial mats at the surface, and the 70% of marine bacteria may be infected by phages.
They have been used for over 60 years as an alternative to antibiotics in the former and Eastern Europe. (BBC Horizon ,1997). They are seen as a possible therapy against multi drug resistant strains of mtany bacteria. The double stranded DNA tailed phages, or Caudovirales account for 95% of all the phages reported in the scientific literature, and possibly make up the majority of the phages on the planet.