Pathogens - bacteria

Pathogens are microorganisms that cause infectious disease. Bacteria and viruses are the main pathogens.

Bacteria

Structure of a salmonella bacterium cell

Bacteria are microscopic organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - 10 millionths of a metre.

Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill. Diseases caused by bacteria include:

• food poisoning
• cholera
• typhoid
• whooping cough
• gonorrhoea - a sexually transmitted disease

Pathogens - viruses

A hepatitis C virus showing DNA enclosed in a protein coat.

Viruses are many times smaller than bacteriabacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium.. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.

Viruses can only reproduce inside host cells, and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes.

Diseases caused by viruses include:

• influenza - flu
• colds
• measles
• mumps
• rubella
• chicken pox
• AIDS

Functions of the white blood cells

A white blood cell ingesting disease-causing bacteria.

White blood cells can:

• ingest pathogens and destroy them
• produce antibodies to destroy pathogens
• produce antitoxins that neutralise the toxins released by pathogens

In a written examination, it is easy to get carried away and waffle on about things such as invaders and battles, but stick to the point. Note that:

• the pathogens are not the disease - they cause the disease
• white blood cells do not eat the pathogens - they ingest them
• antibodies and antitoxins are not living things - they are specialised proteins

More about white blood cells

There are several different types of white blood cells, each with different functions, but they can be put into two main groups:

• phagocytes or macrophages
• lymphocytes

Phagocytes

Phagocytes can easily pass through blood vessel walls into the surrounding tissue and move towards pathogenspathogens: microorganisms that cause disease or toxinstoxins: a type of natural poison produced by an organism, often as a form of protection. They then either:

• ingest and absorb the pathogens or toxins
• release an enzyme to destroy them

Having absorbed a pathogen, the phagocytes may also send out chemical messages that help nearby lymphocytes to identify the type of antibody needed to neutralise them.

Lymphocytes

Pathogens contain certain chemicals that are foreign to the body and are called antigens. Each lymphocyte carries a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly, and makes many copies of the antibody that neutralises the pathogen.

Antibodies neutralise pathogens in a number of ways:

• they bind to pathogens and damage or destroy them
• they coat pathogens, clumping them together so that they are easily ingested by phagocytes
• they bind to the pathogens and release chemical signals to attract more phagocytes

Lymphocytes may also release antitoxins that stick to the appropriate toxin and stop it damaging the body.

Vaccination

People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogenspathogens: microorganisms that cause disease.

Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body. Vaccines can contain:

• live pathogens treated to make them harmless
• harmless fragments of the pathogen
• toxinstoxins: a type of natural poison produced by an organism, often as a form of protection produced by pathogens
• dead pathogens

These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen.

Because the vaccine contains only a weakened or harmless version of a pathogen, the vaccinated person is not in danger of developing disease - although some people may suffer a mild reaction. If the person does get infected by the pathogen later, the required lymphocytes are able to reproduce rapidly and destroy it.

Vaccines and boosters

Vaccines in early childhood can give protection against many serious diseases. Sometimes more than one vaccine is given at a time, like the MMR triple vaccine against mumps, measles and rubella.

Sometimes vaccine boosters are needed, because the immune response 'memory' weakens over time. Anti-tetanus injections may need to be repeated every ten years.

Antibiotics

Antibiotics are substances that kill bacteriabacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. or stop their growth. They do not work against virusesviruses: ultramicroscopic non-cellular organisms that replicate themselves inside the cells of living hosts: it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

How some common antibiotics work

Penicillin

A bacterium damaged and distorted by penicillin

The first antibiotic - penicillin - was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould.

Since the discovery of penicillin, many other antibiotics have been discovered or developed. Most antibiotics used in medicine have been altered chemically to make them more effective and safer for humans.

Resistance

Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteriabacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium., there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.

MRSA is methicillin-resistant staphylococcus aureus. It is very dangerous because it is resistant to most antibiotics. It is important to avoid over-use of antibiotics, so we can slow down, or stop, the development of other strains of resistant bacteria.

Cleanliness

One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in hospitals. However, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed.

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