General Characteristics, Classification, and Structure of Viruses. Collection, Transportation, and Storage of Virological Samples.

General Characteristics, Classification, and Structure of Viruses

General Characteristics of viruses

Viruses are ultramicroscopic structure of consists of DNA or RNA enclosed in a protein coat called a capsid. They are obligate intracellular parasites. Obligate intracellular organisms multiply inside host cells and not outside the host body. Obligates utilize host cell energy to survive. The following are the general characteristics of the viruses.

• 1. Size: The size of viruses varies from 20 to 300 nanometers. They are smaller than bacteria and not visible under a light microscope. They are Ultramicroscopic parasites. They are filterable due to their size smaller than bacteria. Thus they can pass through bacteria-proof filter membranes.

• 2. They are living organisms inside host cells. They are non-living particles outside host cells.

• 3. They are called viruses, if present in the host cells. They are called Virion, if present outside host cells.

• 4. Structure: Viruses have either RNA or DNA surrounded by a protective protein coat called caspid. Some viruses have also an envelope over capsid derived from the host cells. This Envelope is a lipoprotein membrane. They do not have any cellular organelles.

• 5. Genetic materials: A virus has genetic material DNA or RNA that replicates to produce new viral particles.

• 6. Metabolism: The virus does not have metabolic activity. They utilize the metabolic activity of host cells to synthesize virus protein and replicate to produce new virus particles. They are non-living outside the host body and living inside the host body.

• 7. Replication: Virus replication is not by binary fission or mitosis.

• 8. Cultivation: The virus does not grow in culture media. Their number increases only inside host cells.

• 9. Specificity: Virus growth occurs only in specific hosts.

• 10. Pathogenicity: The virus can produce mild to severe illness. An example of mild illness due to the virus is common cold. Example of severe disease due to virus is HIV, Ebola, etc.

• 11. Evolution: The virus mutates very rapidly to produce a new strain of virus. Thus it is difficult to develop a vaccine against virus infection.

Classification of virus

Viruses are classified on the basis of their structure, genome type, mode of replication, and host range. The current system of classification is based on the International Committee Taxonomy of Viruses (ICTV).

• · There are two realms of viruses. These are Riboviria and Duplodnaviria.

• · Viruses are classified into two groups on the basis of their shape. These are icosahedral virus and helical virus,

• · Viruses are classified into two groups on the basis of covering present on the viruses. These are Enveloped viruses and non-enveloped viruses.

• · Viruses are classified into two classes on the basis of nucleic acid present. These are DNA viruses and RNA viruses.

• · Baltimore classification classifies viruses into seven classes. These are from Class-I to Class-VII.

Baltimore classification of the viruses

• Class-I: Double-stranded DNA viruses: These viruses have double-stranded DNA. They replicate their genetic material with the host cell nucleus. Examples: Herpes viruses, poxviruses, and adenoviruses.

• Class-II: Single-Stranded DNA viruses: These viruses have single-stranded DNA. It is converted to a double-stranded form for replication. Example: parovirus.

• Class-III: Double-stranded RNA viruses: These viruses have a double-stranded RNA. They replicate their genetic materials in the cytoplasm of the host cell. Example: Resovirus.

• Class-IV: (Positive-sense single-stranded RNA viruses): These viruses have single-stranded RNA. It is directly translated into protein by the host cell. Example: coronavirus, picornavirus, Zika virus.

• Class-V: (Negative-sense single-stranded RNA viruses): These viruses have single-stranded RNA. They require viral RNA polymerase. Examples: influenza viruses and rhabdoviruses.

• Class-VI: (Single-stranded RNA reverse-transcribing viruses): These viruses have single-stranded RNA. Virus enzyme reverse transcriptase changes RNA to DNA, Example: Retrovirus (HIV).

• Class-VII: (Double-stranded DNA reverse transcribing viruses): These viruses have double-stranded DNA. Virus enzyme reverse transcriptase changes DNA to RNA.Example Hepatitis B virus.

Structure of virus

Viruses have characteristics of living organisms. They replicate and evolve. They are also considered as non-living particles. They do not have a cellular structure. They do not have metabolic activity. The following are the details of the virus structure.

• 1. Shape and size: The size of the virus is smaller than the smallest bacteria. The size of the virus is larger than macromolecules in the cells.

• 2. Genetic materials: The virus contains either RNA or DNA as genetic materials. These genetic materials may be single-stranded or double-stranded, linear or circular, segmented or non-segmented.

• 3. Caspid: A protective protein coat surrounds a virus called caspid. The arrangement and shape of protein in capsid is called capsomer. Capsomer decides the shape of the virus. This shape is either icosahedral or helical. The caspid provides stability to the viral genetic materials. It also stops genetic material degradation. Capsid also develops antigenic properties in virus cells. It neutralizes the antibodies of the host

• 4. Envelope: In some viruses, the caspid is surrounded by a lipid bilayer called an envelope. Viruses derive lipid bilayer from the host cell membrane. Chemically, it is a glycoprotein. It helps the virus to attach to host cells and entry into host cells. A virus without an envelope is called a naked virus or non-enveloped virus. A virus with an envelope is called an enveloped virus.

• 5. Spike protein (Peplomers or surface protein): Enveloped viruses have surface proteins. These proteins extend from the envelope. It helps to recognise the receptor on the host cells and also helps in the entry of virus cells into host cells.

• 6. Matrix proteins: Enveloped viruses may have matrix proteins on the caspid and the envelope. Matrix proteins provide structural support and provide the definite shape of virus cells

Collection, Transportation and Storage of Virological Samples

Collection of virological sample

Virological samples are collected to study and analyse viruses. The following steps are followed to collect virological samples.

• 1. Sample Selection: The selection of a sample depends upon the objectives of sample collection. Common examples of virological samples are blood, respiratory aspirants, stool, urine, tissue samples, saliva, post-mortem specimens, vesicle fluid, conjunctiva swabs, mucus membrane swabs, nasopharyngeal swabs, nasal washing, etc.

• 2. Personal Protective Equipment (PPE Kit): PPE minimize the risk of contamination and protect laboratory personnel from viral infection.

• 3. Biosafety measures: Follow biosafety measures throughout the sample collection procedure. Dispose of used PPE and contaminated materials properly after collection of virological sample.

• 4. Sample collection tube: Needles, swabs, specimen containers, etc are sample collection tools to collect virological samples.

• 5. Sample collection technique: There are several sample collection techniques such as swabbing at the back of the throat, swabbing at the back of the nostril, blood withdrawal from the vein, stool collection or any specialised technique. The selection of the technique depends upon the objectives of blood collection.

• 6. Sample handling: Proper handling of collected samples is a must to maintain the integrity of the virological sample. Sterile containers, an aseptic technique, and proper transportation media are required for virological sample handling. Sample dryness should not be allowed. The virus will be inactive in dry conditions and behave like a chemical. It can be identified or analysed. Virological samples are placed into virus transport media (VTM) to prevent sample dryness during their handling.

Transportation of Virological Samples

The virological sample requires careful handling to ensure the safety of personnel, the general public and the sample. The following steps are followed to transport virological samples.

• 1. Packaging: Leakproof and puncture-resistant containers should be used to store virological samples. This container can withstand transportation conditions. This is called a primary container. A secondary container should also be used. The secondary container holds the primary container and prevents leakage if the primary container is damaged.

• 2. Labelling: Both primary and secondary containers are labelled with biohazard symbols with other information such as name, date of collection, type of sample, etc.

• 3. Shipping container: Place the container having the sample into a shipping container. with padding or absorbent material. It protects the sample inside the sample container. Shipping containers should also be labelled with the biohazard symbol.

• 4. Temperature control: A virological sample requires a specific temperature to protect the sample during transportation. Refrigeration, proper insulation, dry ice, etc may be required to maintain proper temperature during transportation,

• 5. Compliance with regulation: Local and international regulations must be observed during the transportation of virological samples. These regulations specify packaging, documentation, labelling, and transportation requirements.

• 6. Transport logistics: A transporter or courier with experience in handling biological samples should be selected to transport the virological samples.

• 7. Emergency numbers: Emergency contact information such as the contact number of the relevant person or institution should be mentioned in shipping documentation. They may be contacted in case of spills, accidents, or any other emergency conditions.

Storage of Virological Samples

Proper storage conditions are required to maintain the integrity of the virological sample during the storage period. The following guidelines should be followed regarding the storage of virological samples

• 1. Temperature: The optimal temperature to store virological samples depends upon the specific virus. Commonly temperatures between -80 degrees C and -20 degrees C are used to store virological samples. -80 degree C temperature is used for long-term storage of virological samples. -20 degrees C temperature is used for short-term storage of virological samples.

• 2. Cryopreservation: Liquid nitrogen or a specialized cryogenic freezer is used for the long-term preservation of virological samples.

• 3. Sample containers: Sample containers to store virological samples should be resistant to low temperatures, sterile, leakproof and prevent cross-contamination. Cryotubes or cryovials are used to store virological samples. They are made of polypropylene.

• 4. Labelling and documentation: Each sample should be properly labelled with all required information such as sample name, date of collection, patient details, tests to be performed, storage conditions etc.

• 5. Restriction: Only authorised persons shall be allowed to access the sample. This prevent accidental and intentional accident, contamination, loss of sample etc

.Dr Pramila Singh