The definition of Pathogen
A pathogen is defined as any organism capable of invading the body and causing diseases. Such an organism is said to be pathogenic.
Robert 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 easy 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 a health host can cause the disease.
This definition works well for many bacteria, but does not fully describe 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 an important cause of diarrheal disease and potent enterotoxin and other pathogenicity determinants have been described on some strains.
E. coli can therefore behave as a pathogen or as a colonizer, depending on various circumstances.
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 epidermis, a normal part of the skin flora, to enter the cannula and cause blood borne infection: behaving as a pathogen.
The definition of Pathogen
