Introduction To Microbiology – Viruses

Microbiology – Viruses


  • Viruses basically invade cells, cause colds, influenza, HIV, small pox, MMR, warts, polio and many more.
  • Some viruses are asymptomatic= don’t show symptoms
  • Viruses have particular cells they can infect, lock and key
  • Host soecificity = lock and key
  • In Latin, virus means poison.
  • Existence proven by Ivanowski (Russian Scientist) 1892 – first started with plants
  • Virus are: self-reproducing particles that enter hosts and reproduce inside cells



Is a branch of Microbiology that looks at viruses.


  • Can only be seen by electronic microscope
  • Simplest form of life
  • Neither prokaryotes or eukaryotes
  • Smaller than bacteria
  • Can’t be cultures in labs
  • Huge diversity
  • They live inside cells – feeding, reproducing and growing = they need organisms to live = cant carry out metabolic reaction alone, viruses take over ribosomes
  • Potentially infectious = dependant on Host


  • Capside = protein layer that protects RNA or DNA, composed of protein unit called capsomeres.
  • Capsides have different shapes:
    • Polyhedral
    • Rod
    • Complex

Lacks ribosomes and other replication equipment. Our bodies can recognise the antigenic properties and build antibodies.

v  Some viruses have an additional protective layer , a lipid usually (envelope) – makes it weaker to environment change, naked viruses are more stranger to pH change

v  Viral enzymes are used to infect, they are targets for antiviral therapy

v  Outside the host cell, they are unable to carry out any metabolic reaction

Classification/ Terminology

Virion: mature virus; will have nucleic acid and capsid or nucleic acid, capsid and envelope: they are capable of transmission from one host to another

Virus: intracellular infectious particle consisting of nucleic acid and protein coat (capsid)

Viroids: Infectious agent, don’t have a capsid. Only have a closed circular RNA molecule

Prion: consists only of protein : not alive, lacks genetic material. BUT it can go to a host cell and infect it.


v  Shape is used for classification

v  Genetic Material found will depend on the nature and function of the virus

v  Division based on genetic material:

  • DNA
  • RNA
  • Single or Double strand


Types of Viral Infection:

v  Acute lytic:

  • Virus infects host cell > new viruses are created > host cell dies> virus spread to neighboring cells.
  • Identifiable symptoms and signs (You know they are sick)
  • Eg common cold, mumps, flu

v  Subclinical:

  • No recognisable symptoms or signs : swollen glandsand a sligh fever
  • Can harm a fetus if mum is incontact with it during pregnancy

v  Chronic:

  • Possible no symptoms
  • Virus lives in host at a low level carrier state
  • Unlikely to be cured
  • Infectious
  • Eh HIV and HepB

v  Latent:

  • Virus lies dormant- will reactivate later
  • Common in herpes virus
  • Initial symptoms >apparent recovery> virus will still be there
  • Eg. Chickenpox, Shingles, G – fever

Infection of host:

v  Inhaled droplets : rhinovirus, influenza

v  In food or water: HepA, Norovirus

v  Direct transfer: HIV, HepB

v  Vector: Yellow Fever, West Nile Virus


v  Viruses have a host specificity, restricted range of hosts. Thank Lock and Key Fit

Adsorption = Virus attaches to host receptor

Penetration = genetic material enters host cell

Uncoating = genetic material of virus hijacks the cell, taking cell away from it’s usual job

Synthesis and Assembly =  using host cell make new virus

Release = new virus released (lysis or budding), host cell is destroyed

Infection of nearby cells


Virus- Cancer:

v  As some viruses affect dna of host cell = it most usually helps cancer,

v  Cell replicates too much and gets cancerous (after the altercation of host dna, the host cell looses control over the replication)

  • HPV and STD = cause cervical cancer
  • HepB and C = are linked to liver cancer


Once a virus has hijacked a cell and used it’s ribosomes and cellular material to reproduce, host cell will produce many viruses and infect nearby cells. Certain proteins must fit to the receptors of a host cell surface in order to infect.

The replication process is depending on Single strand or double strand.
DNA: viral DNA wants to replicate so it goes into the host nucleus = replication and production of mRNA = sits on ribsosome and produce protein. Host cell left for dead after replication.
Single Strand RNA: viral DNA is synthesised from the ssRNA using a viral enzyme called: reverse transcriptase = the new DNA is transcribed into viral mRNA protein and new viral RNA.

Some viruses sit in chromosomes and pretend to be part of us = with us in our genes forever, 5% of our genes are virus.

The more the cell replicates, the higher chance of mutation= a new virus

Important Pathogens

Rhinovirus/ Adenovirus Common cold
Arbovirus / Herpes Virus Encephalitis
EBV G-Fever
HepA-E Hepatitis
Varicella zoster Chickenpox and Shingles
Rubella German Measles, can cause arthritis later on in life, is a major problem on fetus.
Papilloma Warts

Virus – Disease

Retrovirus (RNA virus) Promotes Tumour Growth
Filovirus Bleeding and coagulation and viral hemorrhagic fever
Paramyxovirus Measles and mumps
Arenavirus Lassa fever

Hep A:

  • Caused by a virus
  • Antibiotics wont work
  • Affects liver
  • Will be in feaces a lot (checkable – any unhygienic act can be transmitted – by poo)
  • Unsanitary conditions
  • Found in water and food
  • Can be immunised against

Prevention: vaccine, past exposure, good hygiene


v  From symptoms

v  presence of other cases at the same time

v  blood test and cultures from blood samples, bodily fluid

v  PCR and ELISA = used to look at the Polmerase chain reaction = used to detect dna of other microbes.

v  Examination of blood or tissue under electronic microscope


Little way to attack viruses unless they take their own enzymes “Reversetase” then we can attack.

v  Drugs (antiviral) – but can be very toxic to human cells

v  Viruses build up a resistant to antiviral drugs very easily.

  • Inferon Drugs=Protein (synthetic)  that slows the replication process
  • Immune Globulin = antibodies produced by others
  • Vaccines = help prevent infection

Host Defence Mechanism:

v  Phagocytosis

v  Antibodies


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