Viruses
Definition
Viruses : are biological particles (Latin virus meaning "poison" ) obligate intracellular parasitic microorganisms which are smaller than bacteria and so could passes through bacterial filters, they have not contain nucleus , cytoplasm, cellular structure and no cell membrane , but have genetic material and protein .Viruses are known to be responsible for several disease of man ,animal, plant etc.
General features
1-They contain one kind of nucleic acid either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
2-Obligate intracellular parasites because viruses do not contain within their coats the machinery for replication ,for this the nucleic acid of an infecting virus takes over the control of an infected cell to produced new viral nucleic acids and proteins .
4- No ribosome : viruses can not be synthesize their own proteins ,for this they utilize host cell ribosome s during translation.
5-Virus size:Viruses are only visualized by electron microscopes . They are between (20 -300) nm in length (1nm=1/10mm),viruses can pass through bacterial filters.
6- Characteristic shapes : viruses have several shapes like spherical (complex) , helical , rod or polyhedral , sometimes with tails or envelopes, most common polyhedron is the icosahedrons which as triangular faces .
7- Viruses can be classified according to their :-
a- Morphology (size, shape, types of symmetry, presence of tail or envelope).
b- Biological properties such as pathoginicity , host range which refers to the type of cell in which the viruses are able to replicate in many hostes like animals , plant, insects and bacterial viruses called bacteriophage or bacteria eaters .
c-Type of nucleic acid , deoxyribonucleic acid, ribonucleic acid
single-stranded, double-stranded, linear, circular ,segmented etc.
Viruses structure
The virion is the complete infective virus particles , each virus particle contains its own genetic material surrounded by a protein coat , the genetic material can be a molecule of either DNA or RNA, double or single stranded, it lies in center or core of a virus particle , surrounding the core by a protein coated called a capsid made up of protein subunits called capsomers that assembled like bricks in a wall , the organization of these subunits determines the shape of the virus .
Function of capsid
1-It protects the nucleic acid genome from the damage or destruction by external agents.
2-It mediates the attachment of the virus to specific receptors on the host cell surface.
Virus envelope
Some viruses may have an ultimate exterior cover of lipoprotein, the envelope ,which is largely derived from the outer membrane of the host cell cytoplasm membrane .The virion may or may not have an envelope outside the capsid. Enveloped viruses are more sensitive to heat ,detergents and lipid solvents such as alcohol and ether than non- enveloped virus Herps virus.
Lytic cycle of T4- phage
Viruses can multiply by using a host energy and chemical supply to make its own nucleic acid and proteins , these substance are assembled into many viral particles. A virus undergoes a lytic cycle when it invades a host cell multiplies and causes cell lyses.
There are five steps composed of lytic cycle as fallow :
1- Attachment
The first step in lytic cycle is the attachment or adsorption of phage -T4 is begin when the tail fibers recognize and attaches to specific region on the cell wall of E. coli called receptors site.
2- Entry
In the beginning an enzyme in the tail breaks down the bacterial cell wall in the receptor site, then the sheath of tail contracts and drives the DNA core through the cell wall of E. coli and empty protein coat remains outside has no role in the cycle.
3-Replication
After infection, viral DNA takes over the biosynthetic machinery of the host cell and uses it to produce the parts needed for production of new virus particles. Viral DNA replaces the host cell DNA as a template for both replication to produce more viral DNA and transcription to produce viral m RNA, viral mRNA are then translated, using host cell ribosome, into viral proteins such as the coat proteins.
4- Assembly
The cell becomes a factory for making viruses , the phage DNA and protein are assembled into 200-300 new phage.
5- Release
Lastly, the cell wall membrane breaks down and new phages are released, those newly viruses can infect other cells and begins the cycle over again .
Rickettesia
Rickettesiae are intermediate forms between bacteria and viruses , they resemble some of bacteria in morphology but differing only in being of much smaller size but they are larger than viruses , rod – shaped forms have a diameter ranging from 0.2-0.5µ and a length reaching 2 µ , others are spherical and cocco-bacillus . R. is like viruses intracellular parasites in arthropods carry such as fleas ,mites and ticks , but are similar to the bacteria in that they have a rigid cell wall and contain both genetic material DNA , RNA. The cellular structure , the staining preparation and method of reproduction by binary fission separated them sharply from the viruses . Contrary to bacteria R. are non motile and do not spore former . Rickettesia causes typhus fever , the disease is transmitted by lice , fleas, ticks and mites , when an insect carrying the R. organism bite a man , it leaves some of the parasite cells in place of bite , scratching the affected part helps entrance of pathogen through a wound ,the pathogen will quickly find its way to the blood circulation . Infection might also take place through polluted food with feces of the insect vector .
In a healthy animal, the internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms. However, the surface tissues, ex. skin and mucous membranes in mouth , nose , lower intestinal tract etc., are constantly in contact with environmental organisms and become readily colonized by various microbial species. The mixture of organisms regularly found at any anatomical site is referred to as the normal flora, or called "indigenous microbiota". The normal flora of humans consists of a few eukaryotic fungi and protists, but bacteria are the most numerous and obvious microbial components of the normal flora , as shown in below table :
BACTERIUM Skin Nose Pharynx Mouth GIT
Staphylococcus epidermidis ++ ++ ++ ++ +
Staphylococcus aureus* + + + + ++
Streptococcus mitis + ++ +/-
Streptococcus salivarius
++ ++
Streptococcus mutans* + ++
Enterococcus faecalis*) +/- + ++
Neisseria sp. + ++ +
Enterobacteriaceae*(Escherichia coli) +/- +/- + ++
Haemophilus influenzae* + + +
In biology, symbiosis is defined as "life together "that two organisms live in an association with one another ,thus, there are at least three types of relationships based on the quality of the relationship for each member of the symbiotic association :
1-Mutualism : When two organisms live in association both of them benefit from their presence together or near each other, their mode of living is known as mutualism, for example genus Rhizobium which form nodules on roots of Leguminous plants . The Rhizobium fixes atmospheric nitrogen , transform it to nitrate which are absorbed by the Leguminous plants and the latter provides the nitrogen fixing bacteria with organic food material and growth materials .
2- Commensalisms : A type of symbiosis in which one of the members of this association benefit and the other is not harmed for example the relationship with Staphylococcus epidermidis, a consistent inhabitant of the skin of human, by produces lactic acid that protects the skin from colonization by harmful microbes that are less acid tolerant.
3- Parasitisms
In biology , the term parasite refers to an organism that grows, feeds and is sheltered on or in a different organism . The parasite is termed obligate when it can live only in a host for example viruses and Rickettesia ; it is classified as facultative when it can live both in a host as well as in free form as saprophytic life for e.g. Bacillus anthraces causing anthrax. Parasites that live inside the body are termed endoparasites whereas those that exist on the body surface are called ecto-parasites.