Bacteria Chapter 10: Intro to Pathogenesis Flashcards
What are the steps bacteria go through to infect human body?
- bacteria comes in contact with host’s body
- bacteria adhere to (and transiently colonize) or invade the host
- bacteria multiply
- bacteria evade host’s innate immune defence system
What does a bacterium require to cause disease?
chemical/molecular mechanisms that damages host (ie. toxins, host-damaging enzymes) or triggers chronic inflammatory response
Microorganisms’ Mechanisms of Pathogenesis
- exposure to pathogen
- adherence to skin or mucosa
- invasion through epithelium
- colonization and growth production of virulence factors
- a) toxicity: toxin effects are local or systemic
b) invasiveness: further growth at original and distant sites - tissue damage, disease
What is bacteremia?
presence of bacteria in blood
What is septicaemia?
evidence that bacteria is multiplying in blood
What are some virulence factors?
- injectosome
- endotoxin
- anti-phagocytic proteins
- O antigen
- flagellum
- H antigen
- Vi capsule antigen
- cytotoxin
- type I fimbriae
- siderophores
- enterotoxin
What does enterotoxin cause?
diarrhea
What does endotoxin in LPS layer cause?
fever
What does O antigen do?
inhibits phagocyte killing
What does H antigen do?
- adherence
- inhibits phagocyte killing
What does Vi capsule antigen do?
- inhibits host cell protein synthesis
- Ca2+ efflux from host cell
- adherence
What do Koch’s postulates tell us?
determines whether a relationship exists between a particular organism and a disease
Have Koch’s postulates been useful?
yes — have been used as a guide to determine the causes of many important diseases, which led to development of treatments for prevention and cure of many infectious diseases
What are Koch’s postulates? (4)
- suspected pathogenic organism should be present in all cases of the disease, and absent from healthy animals
- pathogenic organism should be isolated from the infected animal(s) and cultivated in pure culture
- when this culture is inoculated into susceptible (and healthy) animals, it should initiate the characteristic disease symptoms
- 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 2 major impediments that can prevent definitive proof of causation using Koch’s postulates?
- certain bacterial pathogens cannot be cultivated in vitro
- sometimes there are no animal models for a specific disease
What does the molecular version of Koch’s postulates tell us?
proves that a particular virulence factor or gene is involved in pathogenesis of an organism
What are Koch’s molecular postulates?
- gene(s)/factor should be present in pathogenic strains of the organism, and absent from non-pathogenic strains
- i) molecular disruption (ie. deletion of gene) should reduce virulence of bacterial strain
OR ii) introducing cloned gene(s) into avirulent strain should render that strain virulent
- 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
What are some molecule targets that cause disease?
- genes encoding toxins
- pili
- receptors
Why are the Molecular Version of Koch’s Postulates important?
although two examples of a bacterium may be from the same species, there may be
significant differences in their DNA that would result in one being a pathogen, and the other not being a pathogen
ie. Streptococcus pneumonia can be a pathogen or non-pathogen – pathogen has genes in its genome that code for enzymes involved in synthesis of a capsule, but non-pathogenic strains don’t
Are there any situations where Koch’s postulates may not be applicable?
1st postulate: (pathogen is found only those who are ill)
- there are people that carry pathogenic bacteria as part of their normal flora (carriers) – bacteria do not cause disease to them, but may cause disease in other people if bacteria gets transmitted to them
- bacteria of normal flora can cause disease if they get into a part of the body where they are normally absent
