Microbiology, Parasitology & Entomology – By the end of this session, students are expected to be able to:
Read the complete lesson in an organized slide-by-slide format. This topic contains 17 learning sections from the source presentation.
LESSON CONTENTS — 17 SECTIONS
Learning Objectives
By the end of this session, students are expected to be able to:
- Define different terms used in virology
- Describe characteristic of viruses
- Describe structure of the virus
- Compare virus and bacteria cell structures
Definitions of Terms in used in Virology
Viruses: Are smallest infectious agents (ranging from about 20 nm to about 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as their genome
Virology: Is study of viruses
Capsid: The protein shell, or coat that encloses the nucleic acid genome
Nucleocapsid: The protein-nucleic acid complex plus a capsid. Sometimes it is a complete virion
Capsomeres: Morphologic units of a capsid
Cont…
Virion: The complete virus particle. In some instances the virion is identical with the
nucleocapsid. In more complex virion, it includes the nucleocapsid plus a surrounding envelope
Envelope: A lipid-containing membrane that surrounds some virus particles
Structural units: The basic protein building blocks of the coat. They are usually a collection of more than one identical protein subunit. The structural unit is often referred to as a protomer
Subunit: A single folded viral polypeptide chain
Defective virus: A virus particle that is functionally deficient in some aspect of replication
Characteristics of Viruses
They are small in size ranging from about 20 nm to about 300 nm in diameter.
They are akaryotic particles (neither eukaryotes nor prokaryotes)
They contain either DNA or RNA and not both as their genome
They exhibit living properties when inside the living cells (i.e. they are incapable of independent reproduction unless they are in the living cell)
They are non motile
They can be grown in cell cultures
Cont….
Viruses are known to infect unicellular organisms such as mycoplasmas, bacteria, and algae and all higher plants and animals
Structure of Viruses
Generally the virus structure is made of three basic units
Envelop made of glycoprotein and lipids
Capsid
Viral core (RNA or DNA)
Basic Structure of Virus
Slide 9
Parts of the Viral Structure and its Function
Envelop is the outermost coat of the virus made of lipid and proteins
Its function is to support glycoproteins or spikes used for attachment to the host cell
Gives stability to the virus
Protects the inner parts of the virus
It is usually derived from the host cell during release (budding)
Virus with an envelop are less stable than non enveloped virus (they can be destroyed by disinfectants or adverse condition of the environment)
Cont…
Capsid is a protein coat which covers the viral genome, it is made up of repeating subunits called capsomers
The repeating subunits of the capsid give the virus a symmetric appearance that is useful for classification purposes
Some viral nucleocapsids have spherical (icosahedral) symmetry where as others have helical symmetry
All human viruses that have a helical nucleocapsid are enveloped (there are not naked helical viruses that infect humans)
Viruses that have an icosahedral nucleocapsid can be either enveloped or naked
Cont…
Viral Core contains genetic materials necessary for replication. They include either DNA or RNA, never both
Deoxyribonucleic acid (DNA) containing viruses are double stranded except parvoviruses which have single strand DNA
Ribonucleic Acid (RNA) containing viruses are single stranded. Example human immunodeficiency virus (HIV)
Nucleic acid can either be linear or circular, continuous or seg
Comparison between Viral and Bacterial Cell Structures
Key Points
A virus is the smallest infectious agent (ranging from about 20 nm to about 300 nm in diameter) and contains only one kind of nucleic acid (RNA or DNA) as its genome
Most viruses are pathogenic to human
Viruses are obligate intracellular i.e replicate only when they are inside the living cells
Viruses contain either DNA or RNA but not both
Evaluation
List basic structures of a virus.
What are characteristics of a virus?
References
- Becker, F.J. & Silverton, R.E. (1985). Introduction to Medical Laboratory Technology (6th ed.) London: Butterworth.
- Brooks, G.F., Butel, J.S., Morse, S.A. et al. (2007). Medical Microbiology (24th ed.). New York: McGraw- Hill.
- Cook, G. (2000). Manson’s Tropical Diseases (22nd ed.). London: WB Saunders Company Ltd.
- Greenwood, D., Richard, C.B.S & John, F.P. (1992). Medical Microbiology (4th ed.). Hong Kong: ELBS with Churchill Livingstone, Medical Division of Longman Group, Ltd.
- Harwood, R.F., James, M.T. (1979). Entomology in Human and Animal Health. (7th ed.) Washington State University Pulman
Cont.
Jawetz, Melnick, & Adelberg. (2007). Medical Microbiology (4th ed.). United States of America: The McGraw-Hill Companies, Inc.
Levinson, W. (2004). Medical Micribiology and Immunology. Examination & Board
Review (8th ed.). New York: International Edition Lange Medical Books /McGraw Hill Medical Publishing & Davson.
Monica, C. (1987). Medical Laboratory Manual for Tropical Countries. (2nd ed.). Volume 1. Oxford: ELBS Butterworth, Heinemann Ltd.
Monica, C. (1998). District Laboratory Practice in Tropical Countries. Part 1. Tropical Health Technology. Noida, India: Gapson Papers Ltd.
Monica, C. (2000). District Laboratory Practice in Tropical Countries. Part 2. Tropical Satish G. (1982). The short Handbook of Medical Microbiology. New Delhi. India:
Jaypee Brothers Medical Publishers PVT Ltd.
NMCP (2007). Training Course on Laboratory Diagnosis of Malaria. Malaria Control Series 17. Ministry of Health and Social Welfare.
Additional Study Notes
This section is separated from the source slides to make the lesson easier to revise.
Supplementary learning: For every microorganism or parasite, revise classification, morphology or structure, reservoir or habitat, transmission, life cycle or replication where applicable, diseases caused, appropriate specimens, laboratory diagnosis, prevention and general treatment principles taught in the course.
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