Types of pathogens

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Types of pathogens
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Pathogens

• May be cellular or non-cellular.
• Cellular pathogens
• Also known as pathogenic organisms
• Include bacteria, protozoa, oomycetes, fungi,
  worms and arthropods.
• Non-cellular pathogens
• Also known as pathogenic agents
• Include viruses, viroids and prions.

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Bacteria

• Prokaryotes no membrane bound nucleus or
  organelles.
• Bacteria have a cell wall and a single major
  chromosome a circular thread of DNA double
  helix.
• Replicate by binary fission (20min).
• Some bacterial diseases that affect people are
  diphtheria, food poisoning, wound infections,
  tetanus, pneumonia, tuberculosis, meningitis, gas
  gangrene, typhoid fever, gonorrhoea and syphilis.

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Classification of bacteria

• Bacteria can be classified into different groups.
  
• They are classified on the basis of a number of
  physical and metabolic characteristics.

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Physical characteristics of bacteria

• Shape
• Bacteria can have three basic shapes round, rod
  and spiral shapes.
• A round-shaped bacterium is called a coccus.
• A rod-shaped bacterium is called a bacillus.
• A spiral-shaped bacterium is called a spirochaete.

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Physical characteristics of bacteria

• Organisation
• Although bacteria are single-celled organisms,
  they often cluster together in special ways that
  are used as a basis for classification.
• Single, in pairs (diplo), in chains (strepto),
  clustered (like grapes staphylo)

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Physical characteristics of bacteria

• Structures
• Some bacteria have flagella thin appendages
  that originate just below the bacterial wall and
  are visible with a light microscope when special
  stains are used. Flagella allow a bacterium to
  move. Bacteria without flagella cannot move
  they are said to be non-motile.
• Many bacteria have a layer called a capsule
  outside the cell wall. A capsule is made of slimy
  gelatinous material and is important in
  determining the virulence of the bacterium. The
  virulence of a bacterium is the degree to which
  it can cause disease.
• Some bacteria, particularly members of the genera
  Bacillus and Clostridium, form spores. A spore is
  a special reproductive structure formed within a
  bacterial cell. Spores are particularly resistant
  to heat and drying out.
• The cell wall of a bacterial cell is a firm,
  flexible layer that maintains the shape of the
  cell and protects the underlying protoplasm.
  Using the Gram stain, bacteria can be divided
  into two distinct groups on the basis of a
  fundamental difference in the chemistry and
  structure of their cell walls.

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Gram stains

• Developed in 1884, by the Danish bacteriologist,
  Joachim Gram (18531938).
• Gram-positive bacteria stain purple and their
  cell wall is a relatively thick layer of
  peptidoglycans, a macromolecule found only in
  bacteria.
• They are generally more susceptible to penicillin
  and sulfonamide drugs.
• Gram-negative bacteria stain pink and have a much
  more complex, multilayered cell wall, including a
  layer of peptidoglycans and an additional outer
  membrane layer of lipids.
• The outer layer of lipid compounds enables these
  bacteria to resist penicillin and other drugs. It
  also makes phagocytosis of the bacteria very
  difficult. Drugs such as streptomycin,
  chloramphenicol and tetracycline are active
  against Gram-negative bacteria.
• The Gram stain is a particularly important stain
  used to identify the bacteria that are causing an
  infection because it gives an indication of what
  drugs will be more effective in the treatment of
  a patient.

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Metabolic characteristics of bacteria

• Gaseous requirements
• Most bacteria are aerobic they grow and
  reproduce in the presence of oxygen.
• Some bacteria are anaerobic and can live in the
  absence of oxygen.
• Facultative anaerobes can survive whether or not
  oxygen is present.
• Obligate anaerobes grow and reproduce only in the
  absence of oxygen.
• Nutritional patterns
• Some bacteria are photosynthetic they use light
  as their energy source. Only some of these
  bacteria are able to use carbon dioxide as their
  carbon source.
• Chemosynthetic organisms obtain their energy from
  oxidation reactions.
• Some chemosynthetic bacteria can oxidise only
  organic compounds for their carbon source. Others
  can oxidise inorganic substances such as ammonia,
  sulfides and iron compounds and use carbon
  dioxide as a source of carbon.
• Almost all pathogenic bacteria use organic
  compounds as their source of energy and matter.

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Examples of Bacterial Disease

• Cocci
• Staphylococcus aureus causes skin and wound
  infections
• Streptococcus causes sore throat
• Bacilli
• Diphtheria throat infection caused by
  Corynebacterium diphtheriae
• Tuberculosis lung infection caused by
  Mycobacterium tuberculosis
• Leprosy skin infection caused by Mycobacterium
  leprae
• Spirochetes
• Syphilis and Lyme disease are both caused by
  spirochetes

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Protozoa

• Unicellular eukaryotic organisms.
• Can reproduce sexually and asexually.
• Three classes of protozoans have members that are
  pathogenic to animals
• Flagellates
• Sporozoans
• Sarcodinians (amoebas)

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ProtozoaN Diseases

• Flagellates
• Trypanosoma causes sleeping sickness
• Giardia causes diarrhoea
• Sporozoans
• Plasmodium causes malaria
• Sarcodinians, (amoebas)
• Entamoeba histolytica causes amoebic dysentry

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Malaria caused by Plasmodium

• The adult stage occurs in humans (the primary
  host).
• Plasmodium larvae migrate to liver and multiply
  asexually for two weeks. They leave the liver
  and pass into the bloodstream an infect red blood
  cells.
• Larvae leave the liver and infect red blood
  cells. There they grow and split into many tiny
  larvae (merozoites).
• After a time, male and female gametes are also
  formed in the red blood cells and are released
  when red blood cells burst and can be taken up
  with blood by the intermediate host, the
  Anopheles mosquito, is feeding.
• Gametes fertilise in the stomach of the female
  mosquito and develop into larvae which migrate
  through the stomach wall and into the salivary
  gland where they undergo asexual reproduction.

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Life cycleof plasmodium
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Oomycetes

• The oomycetes were originally classified as fungi
  but are now considered part of the kingdom
  Protista.
• They have motile cells (flagella), walls of
  cellulose and many cellular processes not found
  in fungi.
• Cause diseases such as blight and down mildew on
  plants.
• About 35 species which infect crops including
  potato, tomato, apple, tobacco and citrus fruits.

• Phytophthora cinnammi has destroyed much of the
  eucalypt timberland in Australia. Its spores can
  survive for years in moist soil, and are
  attracted to the roots of the plants they infect
  by a chemical released from the roots.

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Oomycetes - Phytophthora

• When Phytophthora spores land on leaf they may
  be carried by water droplets to other leaves,
  swim to a germination site, or germinate
  directly, sending out hyphae that branch out and
  invade plant tissue
• Branching hyphae (haustoria) penetrate living
  cells and absorb nutrients or release enzymes
  that digest cytoplasm into molecules that can be
  absorbed.

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Fungi

• Fungi can be unicellular or multicellular. They
  consist of eukaryotic cells with cell walls
  composed of chitin.
• Fungi are important pathogens of plants, causing
  diseases such as rusts, smuts, ergot and Dutch
  elm disease.
• Fungi that are pathogenic to humans fall into
  three main groups moulds, which are
  filamentous true yeasts, which are unicellular
  and fungi-like yeasts, which are like yeasts but
  may form long non-branching filaments.
• Moulds are multicellular fungi which invade
  tissue using hyphae while yeasts are unicelluar
  and reproduce by budding.

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Fungal Diseases

• Fungal diseases include
• Ringworm (mould)
• Atheletes foot (mould)
• Thrush (fungi-like yeast Candida albicans)
• Aspergillus infection (life-threatening for
  immunocompromised patients).
• Some fungi produce toxins that are poisonous to
  humans, for example Aspergillus species produce
  toxins (aflatoxins) that are carcinogenic (cause
  cancer). The fungus grows on peanuts and many
  grain foods.
• Other fungal products such as cyclosporine and
  penicillin have become important tools in
  medicine.

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Worms (helminthes)

• Multicellular, eukaryotic, specialized for the
  parasitic way of life.
• Mouthparts are often modified to form hooks,
  digestive systems are simple, numerous offspring
  are produced.
• Worms can be divided into two groups
  Platyhelminths and Nematodes.
• Both animals and plants can be infected with
  helminthes.

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Platyhelminths

• Are parasitic flat worms, and include tapeworms,
  hookworms and blood flukes.
• The blood fluke Schistosoma and the hydatid
  tapeworm are both examples of disease-causing
  platyhelminths that utilise an intermediate host.
• An intermediate host is the host in which larval
  or juvenile forms of a parasite exist. The
  primary host is the organisms in which a parasite
  lives its adult phase.
• Tapeworms are the most highly specialized
  parasitic flatworms.
• They have a head that attaches to the wall of the
  gut.
• A neck region, which is the region of growth,
• And a chain of segments (protglottids) which each
  contain male and female reproductive organs.
• After fertilization each segment matures into a
  bag of eggs that breaks off and passes out with
  the faeces.

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Tapeworm life cycle
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Nematodes

• Include roundworms, hookworms, and threadworms or
  pinworms.
• Nematodes are the most numerous multicellular
  animals on earth. There are nearly 20,000
  described species classified in the phylum.
• Nematodes have been characterized as a tube
  within a tube. The outside tube is the body wall
  which consists of muscle layers that are used as
  a protective covering. The inside tube is the
  digestive system.
• Nematodes range in size from 0.3mm to over 8
  metres.
• Diseases of humans caused by nematodes are
• Trichinosis often fatal disease in which worms
  invade muscle tissue. Caused by eating uncooked,
  infected pork.
• Elephantiasis swelling of tissue caused by
  blockage of lymph nodes by adult worms of the
  Wuchereria bancrofit species.
• Nematodes are also important pathogens of plants.
  They mainly attack roots.

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Arthropods

• Arthropods (insects) have been associated with
  many serious diseases.
• In most cases they act vectors of disease in
  plants and animals (they carry pathogens to the
  host), e.g.
• fleas carry bacteria Yersinia pestis (cause of
  bubonic plague)
• Specific mosquitoes carry Plasmodium (cause of
  malaria)
• A few species of insects are parasitic on mammals
  and actually cause disease or at least discomfort
  e.g. head lice, body lice, crab louse, fleas and
  ticks.
• Parasitic insects such as psyllids induce the
  formation of galls (swollen areas) on plant
  leaves.
• Tick fever (caused by cattle ticks Boophilus
  microplus) is an example of arthropods causing
  disease in animals.

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Arthropodscattle Ticks and tick fever
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Viruses

• Non-cellular agents that infect all types of
  organisms.
• Consist of either DNA or RNA surrounded by a
  protein coat and perhaps a modified membrane
  envelope.
• Obligate intracellular parasite cannot
  replicate outside of cells.
• Interaction between a virus and host cell is
  specific.
• Instructions carried by viruses direct the
  production of viral proteins and nucleic acid to
  be assembled into new virus particles.
• In most DNA and RNA viruses this process is
  similar to normal protein production, however the
  group of viruses known as retroviruses first
  produce DNA from viral RNA. HIV is an example of
  a retrovirus.
• Some viruses exit the cell by lysis of the cell.
• Enveloped virus particles are released slowly by
  budding from the cell membrane.

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Reproduction of bacteriophages
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Reproduction of a human virus
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HIV

1. Attachment virus binds to surface molecule
   (CD4) of T cell or macrophage
2. Fusion viral envelope fuses with cell membrane
   releasing contents into cell
3. Reverse transcription viral RNA is converted
   into DNA
4. Integration viral DNA is inserted into host
   chromosome (integrated DNA known as provirus and
   may stay latent for years)
5. Replication viral DNA is transcribed and RNA is
   translated to make viral proteins. Viral genome
   is replicated
6. Assembly new viruses are made
7. Release new viruses bud through cell membrane

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Viruses

• Animal Viruses
• Associated with a wide range of diseases
• DNA viruses (may be double or single stranded)
• Smallpox, cowpox, herpes, warts, common cause of
  sore throats
• RNA viruses (usually single stranded)
• Polio, hepatitis, influenza, AIDS, Ebola,
  measles, mumps
• Some viruses appear to be able to cause normal
  cells to become cancerous e.g. hepatitis B virus
  (liver cancer), Epstein-Barr virus (Burkitts
  lymphoma and nasopharyngeal carcinoma).
• Plant Viruses
• Divided into three main types each of which take
  their name from the symptoms they produce
• Yellow viruses
• Mosaic viruses
• Necrotic viruses

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Viroids

• Tiny circular single-stranded RNA molecules.
• About 1/10 the size of smallest virus.
• Have no protein coat or membrane envelope.
• Infect susceptible cells and replicate
  themselves.
• Only been associated with plant diseases in crops
  such as potatoes, citrus and coconut.

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Prions

• A group of abnormal infectious proteins that
  cause degenerative neurological diseases.
• Prions are pathogenic variants of proteins that
  are naturally produced in nerve cells and certain
  other cells. The normal "healthy" prions are
  referred to as PrPc (Prion Protein cellular).
• When a defective prion comes in contact with a
  PrPc (healthy prion) it converts the normal
  protein into a prion protein. This is the
  equivalent of the prion replicating itself.
• Prions eventually cause a cell to burst and are
  free to infect other cells. The bursting of
  nerve cells results in the holes seen in infected
  brains.

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Replication of prions
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Prion infections

• Humans might be infected by prions in 2 ways
• Acquired infection (diet and following medical
  procedures such as surgery, growth hormone
  injections, corneal transplants) i.e. infectious
  agent implicated.
• Apparent hereditary mendelian transmission where
  it is an autosomal and dominant trait. This is
  not consistent with an infectious agent.
• No treatment is available for individuals
  infected with abnormal prions.
• Prions are extremely resistant to heat and
  chemical agents.

• Animal Prion Diseases
• Bovine Spongiform Encephalopathy (BSE)
• Chronic Wasting Disease (CWD)
• Scrapie
• Transmissible mink encephalopathy
• Feline sponigform encephalopathy
• Ungulate spongiform encephalopathy

• Human Prion Diseases
• Creutzfeldt-Jakob Disease (CJD)
• Variant Creutzfeldt-Jakob Disease (vCJD)
• Gerstmann-Straussler-Scheinker Syndrome
• Fatal Familial Insomina
• Kuru