PathogenA pathogen is defined as any organism capable of invading the body and causing disease.
Such an organism is said to be pathogenic.
Koch isolated and identified organisms such as Mycobacterium tuberculosis and Bacillus anthracis, where the isolation of the organism only occurred in the presence of disease.
It is to define these bacteria as pathogens.
They also fulfill Koch’s further definition of a pathogen that introduction of a pure culture of the organism into the healthy host can cause the disease.
This definition works well for many bacteria, but does nor fully describes the complex interactions between microbes and humans which more recent understanding has revealed.
For instance, Escherichia coli is found in huge numbers in the healthy human bowel, and could therefore be defined as non-pathogenic.
E. coli is also important cause of diarrhoeal disease and potent enterotoxins and other pathogenic determinants have been described in some strains.
E. coli can therefore behave as a pathogen or as a colonizer depending on various circumstances.
A broader definition of a ‘biological agent’ used in European Union legislation is: ‘any microorganism, cell culture or toxin capable of entering the human body and causing harm’.
Changes in medical practice mean that increasing numbers of patients are immunocompromised as the result of either disease or treatments.
In such patients organisms which are usually non-pathogenic, such as saprophytic fungi, may act as pathogens.
Intensive therapy medicine, with insertion of intravascular cannulae, allows Staphylococcus epidermitis, a normal part of the skin flora, to enter the cannula and cause blood-borne infection: behaving as a pathogen.
Definition of pathogen
