Infectious Diseases - Pathogens

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  • Infectious Diseases: Pathogens
  • Bacteria
  • Fungi
  • Viruses
  • Protozoa
  • Parasites
  • How pathogens are spread
  • How pathogens cause disease
  • Growth of pathogen populations
  • Quiz

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Bacteria are possibly the most numerous types of organisms on the planet. There are about 60 million people living in the United Kingdom. Many hundreds of times more than that number of bacteria will be living in your intestines at this moment!

Bacteria are very simple single-celled organisms. They contain a small amount of loosely-coiled DNA but do not have any visible nucleus. Like plant cells, they have a cell membrane and a cell wall but their cytoplasm does not have the structures seen inside other plant or animal cells. They range in size but are usually no bigger than 10 micrometres (millionths of a metre) in length. 80 million bacteria could easily lie down on one side of a pound coin.

Most bacteria are not harmful to humans as they live on, and digest, dead plants and animals. This is vitally important as it helps to recycle nutrients that would otherwise be locked up in organisms after they have died. Humans make use of these types of bacteria in the treatment of sewage and other wastes.

For many centuries, bacteria have been used in the production of cheese and yoghurt while more recently other types of bacteria have been used in biotechnology to produce useful chemicals and medicines. Human insulin is produced by genetically engineered bacteria and the enzymes found in biological washing powders are also produced by useful bacteria

Not all bacteria are harmless. Some are pathogens and cause diseases including cholera bacteria, Salmonella and E.coli (food poisoning), dysentery bacteria, and MRSA

The structure of bacteria

Bacterial cells are smaller than plant and animal cells. There are many different types of bacteria but their cells all have some components in common:

  • Cell wall
    to maintain the shape and stop it from bursting. Not made of cellulose.
  • Bacterial (chromosomal) DNA
    controls the activities of the cell and cell division
  • Plasmid (plasmid DNA)
    small circular pieces of extra DNA controlling particular functions of the cell
  • Flagellum (plural flagella)
    for movement (not all bacteria have them)
The components of a bacterial cell

The components of a bacterial cell

Different types of bacteria

Shape Description
spherical Spherical (ball-shaped) bacteria are called cocci. They can be separate, clumped together in groups or form long chains.
rod Rod-shaped bacteria are called bacilli. Some have flagella to help them move around
spiral Spiral bacteria are called spirilla. They don't form groups.
curved rods Some bacteria look like a thin, crescent moon or a comma. They are called vibrio and should not be mixed up with protozoa that sometimes look similar but are hundreds of times bigger
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Medicine that acts against bacterial infections. Penicillin is an example of an antibiotic.
Protein that is produced by lymphocytes (white blood cells) and that attaches to a specific antigen.
Molecule on the surface of a pathogen that identifies it as a foreign invader to the immune system.
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
The use of biological organisms or enzymes to create, break down or transform a material
To cut apart, or separate, tissue especially for anatomical study.
Exponential growth
If something is growing exponentially the larger the quantity gets, the faster it grows
Micro-organism that can grow in long tubes called hyphae or as single cells. Fungi have a nucleus, cytoplasm and a cell wall.
Herd immunity
If a high percentage of a population is immune to a disease the disease cannot be passed on because it cannot find new hosts.
Infection caused by the human immunodeficiency virus (HIV). It attacks and destroys the immune system.
Hybridoma cells are formed by fusing a specific antibody-producing cell with a type of cancer cell that grows well in tissue culture
Immune system
The body's natural defence mechanism against infectious diseases.
A process which gives immune resistance to a particular disease. The human or animal is exposed to a harmless antigen in order to raise antibodies and provide an immune memory.
A type of white blood cell that make antibodies to fight off infections.
A type of white blood cell that consumes dead pathogens that have been killed by antibodies.
Organism that feeds off another living host and causes it some damage. An example of a parasite is a tapeworm that lives in the digestive system of a host organism.
A micro-organism that causes disease.
Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.
A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.
Protozoa are one-celled animals
A spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavourable conditions.
A poisonous or toxic substance - produced by pathogens.
A small amount of dead or weakened pathogen is introduced into the body. It prepares the immune system to prevent future infections with the live pathogen.
Medicine that contains a dead or weakened pathogen. It stimulates the immune system so that the vaccinated person has an immunity against that particular disease.
The smallest of living organisms. Viruses are made up of a ball of protein that contains a small amount of the virus DNA. They can only reproduce after they have infected a host cell.
Opportunistic Infections
An infection that would not normally cause disease in a healthy person but which can take hold when a person's normal immune defences are reduced.