Pathogens are microorganisms - such as bacteria and viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens by producing antibodies that destroy the infectious microorganisms, and antitoxins to neutralise toxins created as a by-product.
Viruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.
spherical shaped virus showing a cross-section through the core
A hepatitis C virus showing DNA enclosed in a 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:
Pathogens contain certain chemicals that are foreign to the body. These chemicals are called antigens. Certain white blood cells, called lymphocytes, can produce specific antibodies to kill a particular pathogen.
Antibodies are proteins. They can neutralise pathogens in a number of ways. For example, they can:
bind to pathogens and damage or destroy them
coat pathogens, clumping them together so that they are easily ingested by white blood cells called phagocytes.
Each lymphocyte produces 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 to kill the pathogen.
One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweiss realised the importance of cleanliness in hospitals. Semmelweiss insisted that doctors should wash their hands before examining patients, something that was not common at the time. This policy greatly reduced the number of deaths from infectious diseases in his hospital. Unfortunately, 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.
Antibiotics are substances that kill bacteria or stop their growth. They do not work against viruses because they live and reproduce inside cells. It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.
Penicillin, the first antibiotic, 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 and developed.
Different antibiotics work in different ways. It is important that specific bacteria should be treated using specific antibiotics.
Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, 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.
The appearance of resistant strains of bacteria means that vaccinations and antibiotics may no longer work. As people are not immune to it, and there is no effective treatment, a resistant strain will spread rapidly. New antibiotics must be developed as a result.
The action of antibiotics and disinfectants can be investigated using cultures of microorganisms (populations of microorganisms that have been grown for a purpose). It is important that the cultures are uncontaminated by other microorganisms, so sterile conditions are needed:
the Petri dishes, nutrient agar jelly and other culture media must be sterilised
the inoculating loops used to transfer microorganisms must be sterilised (usually by passing the metal loop through a Bunsen burner flame)
the lid of the Petri dish is sealed with sticky tape to stop microorganisms from the air getting in and contaminating the culture.