2 Koch & Defining Pathogens

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

An organism in a symbiotic relationship (i.e., mutual benefit, includes Probiotics: Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host; e.g., E. coli Nissle 1917)

An agent causing disease or illness to its host, such as an organism or infectious particle capable of producing a disease in another organism (e.g. EHEC)

An infectious microorganism that is normally a commensal or does not harm its host but can cause disease when the host’s resistance is low

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

Robert Koch 1845-1910

Robert Koch was the first to show, in 1876, that anthrax, a disease of sheep and other animals, including humans, was caused by a bacterium that he called Bacillus anthracis

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

• mouse used as model organism

• 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 and showed that the pure cultures could cause the disease upon re-inoculation of healthy mice

Louis Pasteur

Koch’s experiments confirmed (for the first time) the germ theory of disease proposed earlier by Louis Pasteur

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 experimental 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 anima

He also discovered, in 1882, that tuberculosis and, in 1883, that cholera are each caused by a different bacterium

lots of pathogenic organisms are found in healthy animals and humans, producing disease in only a tiny fraction of the individuals who are infected

Many pathogens do not grow in nutrient medium culture. This applies generally to common Mollicute bacteria (i.e. bacteria without a cell wall), including Mycoplasma and Ureoplasma, and also Mycobacterium leprae (causative agent of leprosy), which is yet to be cultured

some of the worst microorganisms will not produce disease in healthy animals. For certain human specific pathogenic bacteria they are no suitable animals models (e.g. Salmonella enterica serovar Typhi)

Many infections, considered the underlying cause of a disease, are absent from the lesions that ultimately develop

• For example, Group A Streptococcus infection is considered to be the underlying cause of rheumatic fever. However, infection is long gone prior to the appearance of the valvular and endocardial lesions of rheumatic fever

Stanley Falkow (1934–2018)

Premise is not in the ability to isolate a particular pathogen but rather to identify a gene that may cause the organism to be pathogenic.

• Falkow’s modifications to Koch’s original postulates explain not only infections caused by intracellular pathogens but also the existence of pathogenic strains of organisms that are usually nonpathogenic • For example, the predominant form of the bacterium Escherichia coli is a member of the normal microbiota of the human intestine and is generally considered harmless • However, there are pathogenic strains of E. coli such as enterotoxigenic E. coli (ETEC) and enterohemorrhagic E. coli (O157:H7) (EHEC)