Bacteriophages: Definition & History • Bacteriophages are viruses that can infect and destroy bacteria. • They have been referred to as bacterial parasites, with each phage type depending on a single strain of bacteria to act as host.
Bacteriophages: Classification • Based on two major criteria: • phage morphology (electron microscopy) • nucleic acid properties
Bacteriophages: Classification • At present, over 5000 bacteriophages have been studied by electron microscopy and can be divided into 13 virus families.
13 Bacteriophage families Double stranded DNA, Non-enveloped Double stranded DNA, Enveloped SIRV 1, 2 P2 Rudiviridae Myoviridae Plasmaviridae T2 Fuselloviridae SSV1 TTV1 Tectiviridae λ PRD1 Siphoviridae Lipothrixviridae PM2 P22 Corticoviridae Podoviridae Single stranded RNA Double stranded RNA Single-stranded DNA M13 & fd Inoviridae MS2 phi666 Leviviridae ΦX174 Microviridae Cystoviridae
Bacteriophages: Virulence Factors Carried On Phage • Temperate phage can go through one of two life cycles upon entering a host cell. • Lytic: Is when growth results in lysis of the host and release of progeny phage. • Lysogenic: Is when growth results in integration of the phage DNA into the host chromosome or stable replication as a plasmid. Most of the gene products of the lysogenic phage remains dormant until it is induced to enter the lytic cycle.
Bacteriophages: Lysogenic Conversion • Some lysogenic phage carry genes that can enhance the virulence of the bacterial host. • For example, some phage carry genes that encode toxins. • These genes, once integrated into the bacterial chromosome, can cause the once harmless bacteria to release potent toxins that can cause disease.
Bacteriophages: Lysogenic Conversion Examples of Virulence Factors Carried by Phage
Bacteriophages: Cholera • The effect of lysogenic conversion can be seen clearly in the disease cholera. • Cholera is caused by a Gram negative, curved rod called Vibrio cholerae. • The bacterium is transmitted through contaminated water and results in severe diarrhea and rapid dehydration of the infected person. • The most effective treatment involves intravenous or oral liquid replacement therapy.
Bacteriophages: Cholera • Vibrio cholerae did not always cause disease. • Infection with the CTX phage gives the bacterium its toxinogenicity. (cholerae toxin) • The phage recognizes a pilus on the surface of the bacterium and uses it to enter the cell. • Once inside the cell, the CTX phage integrates into the chromosome and the lysogen expresses cholera toxin.
Bacteriophages: CTX Phage • The CTX phage has received special attention because it is the first filamentous phage found to transfer toxin genes to its host. • The ability to transfer virulence genes by phage has important implications on the development of vaccines against bacteria. • For example, some of the first vaccines tested against V. cholera had a chromosomal deletion of the gene encoding cholera toxin. • This resulted in a bacterium that was nonvirulent, and thus was useful for human vaccines. • However, the vaccine strain could readily acquire a functional copy of the cholera toxin gene by infection with CTX phage, turning an innocent vaccine strain into a fully virulent strain.
Bacteriophages: Cholera Vibrio cholerae and other pathogens (diarrheal diseases). Caused 2.5 million deaths from cholera and other diarrheal diseases in 1997. Contaminated water and foods are primary vectors. Most victims are under five years of age and live in developing countries. Death preventable with oral rehydration therapy, proper nutrition, and antibiotics. Followed by AIDS with 2.3 million deaths in 1997.
Bacteriophage:The Flesh-Eating Bacteria • Necrotizing fasciitis, the flesh-eating bacteria, is really just a Group A Streptococcal infection. • This bacteria is the same as the one that causes “strep throat.” • Some strains have acquired new virulence factors and code for exotoxins and hemolysins. • There are two new exotoxins: • A protease that degrades host cell proteins. • A "superantigen" that so excites the immune systems that it causes healthy cells to commit suicide (cytokines, programmed cell death, apoptosis). • It is contacted from aerosols released by a sneeze or cough of a Strep A infected individual. • If it enters the body through a cut or abrasion on the skin, then it may infect the fascial tissue between the skin and the muscles.
Bacteriophage:The Flesh-Eating Bacteria • Then it rapidly kills tissues causing gangrene conditions. • If treat early with antibiotics and removal of infected tissue then amputation and death can be averted. • There are between 500-1500 case in the U.S.A. each year • Flesh-eating bacteria has a death rate of 20-50%.
Bacteriophage:Relatives of Flesh-Eating Bacteria Other Group A Streptococci which have acquired virulence factors: • Scarlet Fever Toxin • Streptococcal Toxic Shock Syndrome
Bacteriophage:Therapeutic Uses • Bacteriophage has also been used to fight many bacterial infections. • Some examples of diseases treated with phage therapy: • staphylococcal skin disease • skin infections caused by Pseudomonas • Klebsiella • Proteus • E. coli • P. aeruginosa infections in cystic fibrosis patients • neonatal sepsis • surgical wound infections • Likewise, bacteriophage has also been used to treat animal disease.
References • Brock, T. 1997. Biology of Microorganisms, Prentice Hall, NJ. • Calendar, R. 1988. The Bacteriophages, Volume 2, Plenum Press, NY, pp.683-715. • Salyers, A., and D. Whitt. 1994. Bacterial Pathogenesis: A Molecular Approach, ASM Press, Washington D.C. pp.141-155,169-181. • Waldor, M. 1998. Bacteriophage biology and bacterial virulence. Trends Microbiol. 6:295-296 • Waldor, M., and J. Mekalanos. 1996. Lysogenic conversion by a filamentous phage encoding cholera toxin. Science 272:1910-1914 http://www.evergreen.edu/phage/phagetherapy/phagetherapy.html http://www.flesheatingbacteria.net/ http://justice.loyola.edu/~klc/BL472/GAS/ http://www.med.sc.edu:85/mayer/phage.htm