An Introduction to Pathogenesis Flashcards
Do all pathogens possess each type of virulence factor?
Bacteria rely on specialized virulence factors to cause disease.
There are many different bacterial virulence factors.
Not all pathogens possess each type of virulence factor.
a) Some bacteria possess few virulence factors e.g., Vibrio cholerae.
b) Some bacteria possess many virulence factors e.g., Salmonella species.
c) Pathogenesis is usually dependent on the possession of multiple virulence factors, i.e., it is multifactorial.
What steps does a bacterium have to proceed through in order to infect the human body?
- The bacterium has to come in contact with the host’s body.
- The bacterium has to adhere to (and transiently colonize) or invade the host.
- The bacterium has to multiply.
- The bacterium has to evade the host’s innate immune defence system.
What are features of a bacterium that is able to cause disease?
It must possess chemical/molecular mechanisms whereby it damages the host e.g., toxins or host damaging enzymes, or it must trigger a chronic inflammatory response.
The possession of virulence factors is what allows the pathogenic bacteria to infect normal people and cause disease.
What is the abilit of an infecting organism to spread within the host body?
The ability of an infecting organism to spread within the host body, from the site of infection, is a measure of the invasiveness of the organism.
Non-invasive organisms cause localized infections.
Invasive organisms may cause systemic infections, i.e., the infection can be disseminated throughout the body, often via the blood.
What is the presence of bacteria in the blood called?
What is it called if bacteria are multiplying in the blood?
Bacteria are not normally found in the blood, i.e., the blood is a sterile site.
The presence of bacteria in the blood is termed bacteremia.
Evidence that the bacteria are multiplying in the blood is termed septicaemia.
Give a summary of virulence factors important in Salmonella pathogenesis.
There are several factors that are important in the virulence and development of pathogenesis.
Pathogenic bacteria may have a few or many virulence factors.
Genes that encode virulence factors or the enzymes that produce them (e.g., an enterotoxin) may be encoded on a plasmid or in the bacterium’s chromosome (e.g., LPS, flagella).
Robert Koch developed these postulates to demonstrate a causal relationship between a specific microorganism and a specific disease.
These postulates are still used today as a guideline, even though there are potential problems with each one of the postulates.
What are they?
- The suspected pathogenic organism should be present in all cases of the disease and absent from healthy animals.
- The pathogenic organism should be isolated from the infected animal(s) and cultivated in pure culture.
- Such a culture, when inoculated into susceptible animals, should initiate the characteristic disease symptoms.
- The pathogen should be re-isolated from the experimentally infected animals and shown to be the same as the original pathogen isolated in step 2.
What are two major impediments of Koch’s postulates?
First, certain bacterial pathogens cannot be cultivated in vitro, and sometimes there are no animal models for a specific disease.
What is the molecular version of Koch’s postulates?
- The gene(s)/factor should be present in pathogenic strains of the organism and absent from nonpathogenic strains.
- The molecular disruption, i.e., deletion of the gene(s), should reduce the virulence of the bacterial strain. Introducing the cloned gene(s) into an avirulent strain should render that strain virulent.
- The gene(s) must be expressed at some point during the infectious process in experimentally infected animals.
- Antibody raised against the virulence factor should offer some protection against infection, in an experimentally infected animal.
