2) Koch Postulates

• Organism that derives food or other benefits from another organism without hurting or helping it (most E.coli strains)

• although beneficial bacteria inhabiting humans are usually refferd to as commensal, it is a more mutualistic relationship

• an organism in a symbiotic relationship i.e., mutual benefit

• e.g. Probiotics: Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host; e.g., E. coli Nissle 1917

• Agent causing disease or illness to its host

• e.g. organism or infectious particle capable of producing a disease in another organism (e.g. EHEC)

• infectious microorganism that is normally a commensal or does not harm its host

• but can cause disease when the host’s resistance is low

• Commensial or symbiont that is able to promote pathology only when specific genetic or environmental condiations are altered in the host (e.g. AIEC)

Robert Koch

• Koch first established by microscopy that all infected animals carried the bacterium in the blood

• Koch showed that blood from a diseased animal could transmit the disease to uninfected animals

• He used these newly infected animals to re-infect other animals and in each case observed that the progress of the disease was identical

• He developed a method to culture the bacterium from the blood of the diseased mice —> re-inoculation of the pure culture leads to disease in healthy mice

• Tuberculosis

• Cholera

1) The microorganism must (always) be found in the diseased animal, and not found in healthy animals

2) The microorganism must be extracted and isolated from the diseased animal and subsequently grown in culture

3) The microorganism must cause disease when introduced to a healthy expertimental animal

4) The microorganism must be extracted from the diseased experimental animal and demonstrated to be the same microorganism that was originally isolated from the first diseased animal

1) The microorganism must be found in the diseased animal but not in healthy animals

—> BUT lots of pathogenic organisms are found in healthy animals and humans without causing disease

2) The microorganism must be extracted and isolated from the diseased animal (and grown in culture)

—> Many pathogens do not grow in nutrient medium culture (e.g. Mollicute bacteria without cell wall)

3) The microorganism must cause disease when introduced to a healthy experimental animal

—> Some of the worst microorganisms will not produce disease in healthy animals

4) The microorganism must be extracted from the infected experimental animal and demonstrated to be the same microorganism that was originally isolated

—> Many infections, considered the underlying cause of a disease, are absent from the lesions that ultimately deploy (e.g. Streptococcus infection is the cause for rheumatic fever, however infection is long gone prior the the appearance of the lesions)