As known viruses do not fall strictly into the category of unicellular microorganisms because they do not have a cellular organization.
Simplest of microorganisms are cells enclosed within a cell wall, and contain both types of nuclei acid (DNA and RNA), synthesizing their own macromolecular constituents and multiplying by binary fission.
Virus lack the enzyme system to synthesize their proteins, and depend on host for the synthesis of their proteins, for replication. They do not replicate by binary fission, but use a complex method to replicate within the host cell. They are unaffected by antibacterial antibiotics.
In spite of these basic differences, viruses are generally considered microorganisms in medical microbiology.
The medical importance of virus lies in their ability to cause a very large number of human diseases. In human, viral disease range from common cold to terrifying such as rabies or AIDS. They may be sporadic like mumps, endemic like infectious hepatitis, epidemic like dengue fever or pandemic like influenza.
Viruses can cause cancer in animal and birds, as well as in humans.
Morphology Of Viruses
The extra cellular infectious virus particle is called the virion.
Viruses vary widely in size. The largest among them measuring about 300 nm, are as large as the smallest bacteria (mycoplasma). The smallest virus (for example parvovirus) measuring about 20 nm are nearly as small as the largest protein molecules such as hemocyanin.
The earliest method of estimating the size of virus particles was by passing them through collodion membrane filters of graded porosity. The average pore diameter of the finest filter that permitted passage of the vision gave an estimate of its size. With the development of ultracentrifuge, a second method become available. The third and most direct method of measuring virus size is electron microscopy. By this method, both shape and size of visions can be studied.
The virion consists essentially of a nucleic acid surrounded by a protein coat, the capsid. The capsid with the enclosed nucleic acid is known as the nucleocapsid. The function of the capsid is to protect the nucleic acid from inactivation by nucleuses and other deleterious agents in the environment. One of the major function of capsid is to introduce viral genome into the host cells by adsorbing readily to cell surfaces.
Chemical Properties Of Virus
Viruses contain only one type of nucleic acid, either single or double stranded DNA or RNA. In this respect viruses are unique, as nowhere else in nature is genetic information solely carried by RNA. Viral nucleic acids may be extracted by treatment with detergents or phenol and in the case of some viruses (for example pi coronavirus, papovavirus), the extracted nucleic acid is capable of initiating infection when introduced into host cells.
Capsid of viruses is makes up by proteins. Viral protein, beside protecting the nucleic acid, also determines the antigenic specificity of the virus. Enveloped viruses contain lipids derived from the host cell membrane. Some virus also contain the small amount of carbohydrate. Most viruses do not contain any enzyme for the synthesis of viral components or for energy production but some have other enzyme, foe example, the neuraminidase in the influenza virus. Retroviruses have a unique enzyme, RNA-dependent-DNA polymerase of ‘transcriptase’ which can transcribe RNA into DNA.
Viruses are heat labile, with few exceptions. There are individual variations but in general, they are inactivated within seconds at 56 C, minutes at 37 C and days at 4 C. they are stable at low temperature. For long term storage of virus, they are kept frozen at – 70 C. A better method for prolonged storage is lyophilisation or freeze drying (drying the frozen virus under vacuum). Lyophilised viruses can be stored for years and reconstituted when required by adding water.
Viruses vary greatly in their resistance to acidity. For example, enteroviruses are very resistant to acid pH while rhinoviruses are very susceptible. All viruses are disrupted under alkaline conditions.
Viruses are inactivated by sunlight, UV rays and ionizing radiations. They are, in general, more resistant than bacteria to chemical disinfectants, probably because they lack enzymes. Phenolic disinfectants are only weakly virucidal.
The most active antiviral disinfectants are oxidizing agents such as hydrogen peroxide, potassium permanganante and hypochlorites. Organic iodine compounds are actively virucidal. Chlorination of drinking water kills most viruses but its efficacy is greatly influenced by the presence of organic matter. Some viruses (such as hepatitis virus, polioviruses) are relatively resistant to chlorination.