12 - Communicable Diseases

Types of pathogens

• bacteria

• viruses

• fungi

• protoctista

Bacteria

• small proportion of bacteria are pathogens so cause communicable diseases

• prokaryotes

• no membrane bound nucleus/organelles

• can be classified by their basic shapes

• or by their cell walls

  • gram positive bacteria (purple-blue)

  • gram negative bacteria (red)

  • cell wall affects how it responds to antibiotics

Viruses

• smaller than bacteria

• basic structure is genetic material surrounded by protein

• invade living cells

• the genetic material of the virus takes over the biochemistry of the host cell

• reproduce rapidly

• develop adaptations to their host

• pathogenic

Protoctista

• small percentage act as pathogens

• cause communicable diseases in both animals and plants

• parasitic

• need a vector to transfer them

Fungi

• can’t photosynthesise

• digest food extracellularly before absorbing the nutrients

• saprophytes (feed on dead and decaying matter)

• when reproducing, they produce millions of tiny spores which spread across huge distances

How do pathogens damage the host tissues directly?

• viruses take over the cell metabolism

  • genetic material is inserted into the host DNA

  • virus uses the host cell to make new viruses

  • the cell bursts and the viruses spread to infect other cells

• protoctista take over cells and break them open

  • digest and use cell contents as they reproduce

• fungi digest living cells and destroy them

How do pathogens produce toxins which damage host tissues?

• bacteria produce toxins that poison or damage the host cells

  • causes disease

  • break down cell membranes, damage or inactivate enzymes or interfere with the genetic material to prevent mitosis

• fungi produce toxins that affect host cells and cause disease

Plant diseases

• bacterial - ring rot

• virus - tobacco mosaic virus

• protoctist - potato/tomato blight

• fungi - black sigatoka

Animal diseases

• bacterial - tuberculosis & bacterial meningitis

• virus - HIV & flu

• fungi - ring worm & athlete’s foot

Direct transmission of pathogens (animals)

• direct contact (contagious disease)

  • microorganisms or bacteria transmitted through kissing, contact of bodily fluids, direct skin-to-skin contact

• inoculation

  • through a break in the skin

  • from an animal bite

  • through a puncture wound / sharing needles

• ingestion

  • contaminated food or drink

  • transferring pathogens to the mouth from hands

Indirect transmission of pathogens (animals)

• fomites

  • inanimate objects such as bedding, socks or cosmetics

  • transfer pathogens (mainly fungal)

• droplet infection

  • minute droplets of saliva/mucus are expelled from the mouth and may contain pathogens that others breathe in

• vectors

  • transmits communicable pathogens from one host to another

  • often animals (mosquitoes, fleas, dogs, foxes, and bats)

  • water can also act as a vector

Direct transmission of pathogens (plants)

• direct contact of a diseased plant with a healthy plant

Indirect transmission of pathogens (plants)

• wind

• water

• animals

• humans

• soil contamination

Factors affecting transmission of communicable diseases in animals

• overcrowded living/working conditions

• poor nutrition

• compromised immune system

• poor disposal of waste

• climate change

• culture & infrastructure

• socioeconomic factors

Factors affecting transmission of communicable diseases in plants

• planting varieties that are susceptible to disease

• over-crowding

• poor mineral nutrition

• damp, warm conditions

• climate change

Natural active immunity

• immunity which results from the response of the body to the invasion of a pathogen

• production of T and B cells

Natural passive immunity

• immunity given to an infant mammal by the mother through the placenta and colostrum

Artificial passive immunity

• immunity which results from the administration of antibodies from another animal

Artificial active immunity

• immune system is stimulated to make its own antibodies

• vaccine

Vaccines

• killed / inactivated bacteria and viruses

• weakened strains

• toxin molecules

• isolated antigens from the pathogen

• genetically engineered antigens

• small amounts injected into blood

• primary immune response is triggered

• in the future, the secondary immune response will be triggered

Pharmocogenics

• interweaving knowledge of drug actions with personal genetic material

Antibiotics

• selective toxicity

• interfere with metabolism of bacteria but don’t affect human cells

• becoming less effective as bacteria are becoming resistant to more antibiotics

What are the physical plant defences against pathogens?

• callose - polysaccharide

  • synthesised and deposited between the cell walls and the cell membrane in cells next to infected cells

  • act as barriers

  • lignin is added to make it stronger

  • blocks sieve plates in the phloem

What are the chemical plant defences against pathogens?

• plant can produce chemicals that repel vectors or kill invading pathogens

  • insect repellents

  • insecticides

  • antibacterial compounds

  • antifungal compounds

  • anti-oomycetes

  • general toxins

What are the non specific animal defences to keep pathogens out?

• skin

  • prevents entry of pathogens

  • produces sebum - oily substance that inhibits growth of pathogens

• tracts are lined with mucus membranes

  • secrete mucus to trap microorganisms

    • mucus contains lysozymes (destroy bacterial and fungal cell walls)

    • contains phagocytes to remove remaining pathogens

• lysozymes in tears and urine

How does blood clotting work?

• platelets come in contact with collagen in skin or damaged blood vessel wall

• secrete thromboplastin (triggers a cascade reaction)

• secretes serotonin (causes smooth mussel in vessel walls to contract to reduce blood supply to area)

• clot dries out

• forms a scab

How does the inflammatory response work?

• mast cells are activated

• release histamines and cytokines

• histamines

  • make blood vessels dilate

  • causes localised heat and redness

  • heat prevents pathogens reproducing

  • force more blood plasma out

  • known as tissue fluid

  • causes swelling

• cytokines

  • attract white blood cells (phagocytes)

  • dispose of pathogens

What are the non specific animal defences to get rid of pathogens?

• fevers

  • increase body temperature to inhibit pathogen reproduction

  • specific immune system works faster

• phagocytosis

  • phagocytes build up at site of infection

  • pus is produced - dead neutrophiles and pathogens

What are the stages of phagocytosis?

1. pathogens produce chemicals to attract phagocytes

2. phagocytes recognise non-human proteins

3. phagocyte engulfs the pathogen and encloses it in a vacuole (phagosome)

4. phagosome combines with a lysosome - forms a phagolysosome

5. enzymes from the lysosome digest and destroy the pathogen

Macrophages and neutrophils

• neutrophils just engulf and destroy

• macrophages

  • combines antigens with glycoproteins called the major histocompatibility complex (MHC)

  • MHC complex moves the pathogen antigens to the surface membrane

  • becomes an antigen presenting cell (APC)

  • antigens stimulate other cells involved in specific immune response

What are other helpful chemicals involved in the immune response?

• cytokines

  • produced from phagocytes that have engulfed a pathogen

  • inform other phagocytes that the body is under attack

  • can increase body temperature

• opsonins

  • bind to pathogens

  • ‘tag’ them so they can be recognised

What is the structure of an antibody?

• Y shaped glycoproteins called immunoglobulins

• bind to a specific antigen

• made up of 2 types of chains - heavy chains and light chains

• held together by disulfide bridges

• antibodies bind to antigens with a ‘lock and key’ mechanism

• binding site is 110 amino acids known as variable region

• rest of the antibody is the constant region

• when antigen and antibodies bind it forms an antigen-antibody complex

• has a hinge region to provide the molecule with flexibility

How do antibodies defend the body?

1. antibody in antigen-antibody complex acts as an opsonin

2. antibody is therefore engulfed by phagocytes

3. pathogen can’t invade host cells

4. antibodies act as agglutinins - causes pathogens to clump together

What are the types of T lymphocytes?

• T helper cells

  • produce interleukins that are a type of cytokine

  • stimulate activity of B cells and other T cells

• T killer cells

  • produce perforin which kills pathogen

  • makes holes in the cell membrane

• T memory cells

  • part of immunological memory

  • will divide rapidly to form lots of clones of T killer cells if pathogen is detected again

• T regulator cells

  • suppress the immune system

  • stop the immune response once the pathogen has been eliminated

  • ensure the body does not set up an autoimmune response

What are the types of B lymphocytes?

• plasma cells

  • produce antibodies to a particular antigen

• B effector cells

  • divide to form plasma cell clones

• B memory cells

  • provide immunological memory

Where do lymphocytes mature?

• T - thymus gland

• B - bone marrow