Micro-organisms and disease Unit 4

4.1 Micro-organisms

  • Micro-organisms are tiny living organisms, generally too small to see without a microscope.

  • They play key roles in decay and nutrient recycling.

  • They are used in food production (injera, ergoo, ayib) and alcoholic beverages (beer, wine, tej).

  • Genetic engineering is increasing their usefulness.

  • Some cause diseases in plants, animals, and humans.

    Types of Micro-organisms

  • Bacteria:

    • Single-celled organisms, much smaller than animal/plant cells.
    • Made of cytoplasm, membrane, and cell wall (not cellulose).
    • Genetic material not in a nucleus; some have flagella or slime capsules.
    • Various shapes (rod, round, comma, spiral).
    • Some cause disease, many are harmless, and some are helpful.

  • Viruses:

    • Smaller than bacteria, with regular geometric shapes.
    • Protein coat enclosing genetic material.
    • Can only reproduce by taking over living cells.
    • All naturally occurring viruses cause disease.

  • Moulds and Yeasts:

    • Fungi obtain food from dead or living organisms.
    • Serve as decomposers, breaking down material and recycling nutrients.

  • Yeasts:

    • Single-celled organisms with a nucleus, cytoplasm, membrane, and cell wall.
    • Reproduce by asexual budding.

  • Moulds:

    • Made of hyphae (thread-like structures).
    • Hyphae contain cytoplasm and many nuclei but are not individual cells.
    • Reproduce asexually via fruiting bodies containing spores.

    Germ Theory of Disease

  • The germ theory states that micro-organisms cause disease.

  • In the 17th century, Anton van Leeuwenhoek designed the first microscope.

  • Robert Koch improved microscopes in the 19th century, allowing clear viewing of bacteria.

  • Louis Pasteur showed that micro-organisms cause fermentation in beer and wine.

  • Pasteur disproved spontaneous generation, showing that growths come from existing organisms.

  • He boiled broth, sealed containers, and observed that it remained clear until exposed to air.

  • He identified micro-organisms causing diseases like anthrax, rabies, and diphtheria.

  • He weakened or killed microbes to create vaccines.

    The Immune System

  • White blood cells defend against infective micro-organisms.

  • Pathogens carry antigens (unique protein molecules) on their surfaces.

    • When pathogens enter the body, antigens trigger the immune system.
    • White blood cells (lymphocytes) produce antibodies to disable pathogens.
    • Phagocytes engulf and digest disabled pathogens.

  • After having a disease, the immune system 'remembers' the antigen and produces antibodies immediately upon re-exposure, providing immunity.

  • This is known as acquired immunity.

  • Some pathogens (e.g., HIV/AIDS) cannot be dealt with by the body.

    Control of Micro-organisms

  • Sterilization: Killing all micro-organisms to make an object safe.

    • Achieved through:
    • High temperatures/heat
    • Disinfectants
    • Antiseptics

    Using Heat

    • Boiling: Objects in boiling water (100∘C)({100}^{\circ}C). Most cells killed in 10 minutes, but some viruses/bacteria (hepatitis viruses, Clostridium) require hours.
    • Autoclaving: Uses high pressure to raise the boiling point of water to 121∘C121^{\circ}C. Cooking for 15–45 minutes sterilizes equipment.
    • Ultra High Temperature (UHT): Heating food to 135∘C135^{\circ}C to 150∘C150^{\circ}C for 2–6 seconds to kill all micro-organisms and sterilize it; results in long shelf life.
    • Pasteurization: Heating food to 71.6∘C71.6^{\circ}C for at least 15 seconds or 62.9∘C62.9^{\circ}C for 30 minutes to destroy pathogens and reduce spoilage organisms, but does not sterilize.
    • Dry Heat: Using temperatures of 171∘C171^{\circ}C for an hour, or 160∘C160^{\circ}C for two hours, in special ovens to sterilize scientific equipment; incineration also kills micro-organisms.

    Chemical Approach

  • Antiseptics and disinfectants kill micro-organisms on the skin and in the environment, reducing the spread of disease.

    • Disinfectants:

    • applied to inanimate objects to kill micro-organisms.

    • ideal disinfectant characteristics include:

      • fast-acting
      • effective against all infectious agents, without harming tissues
      • penetrates materials
      • easy to prepare
      • stable
      • not unpleasant

    • Common disinfectants: Bleach and calcium hypochlorite

    • Antiseptics:

    • chemical agents applied to living tissue to kill micro-organisms, thus used on skin

    Culturing Micro-organisms

    • Involves growing large numbers of micro-organisms to study them.
    • Requires a culture medium (carbohydrate, mineral ions, protein, vitamins).
    • Often uses an agar medium or a broth in a culture flask.
    • Warmth and oxygen are essential; bacteria can grow and divide rapidly under the right conditions.
    • Strict health and safety procedures are necessary to avoid contamination or mutation.

    Identifying Bacteria Using Simple Staining

    • Staining makes bacteria more visible (e.g., crystal violet stains all bacteria purple).
    • Gram stain: A differential stain to classify bacteria based on cell wall composition (Gram-positive vs. Gram-negative).
      Staining helps doctors decide how to treat an illness.

    Antibiotics

    • Drugs that kill bacteria without harming human cells.
    • Circulate in the blood to reach body tissues.
    • Penicillin was the first antibiotic.
    • Anti-fungal chemicals attack fungal cells.
    • Antibiotics are ineffective against viruses.

    Artificial Immunity

    • Vaccination (immunisation): Injecting dead or weakened pathogens to trigger the immune response without causing serious effects.
    • White blood cells develop antibodies, providing protection against future exposure.
    • Used against polio, tetanus, tuberculosis, measles, etc.
    • Artificial passive immunity: Injection of antibodies for immediate protection (e.g., tetanus shot).
      Mother gives certain amount of passive immunity to her unborn baby via Placenta and breastmilk especially colostrum.
    Vaccine Production

  • Live vaccines: Weakened micro-organisms stimulating the immune system without causing the disease.

  • Dead vaccines: Killed micro-organisms that are cultured and grown.

    • Using surface antigens of disease-causing micro-organisms, removing risks linked to using the actual micro-organisms themselves for safer and virtually no risk.

    Key Concepts

    • Infectious diseases are caused by micro-organisms.
    • White blood cells produce antibodies.
    • Bacteria can be cultured in the laboratory.
    • Disinfectants and antiseptics kill micro-organisms.
    • Antibiotics destroy bacteria and anti-fungal drugs destroy fungal cells.
    • Vaccination provides protection against dangerous diseases.