Pathogens Flashcards
Microbes functions
-Nitrogen fixing bacteria help with plant growth
-Important for decreasing atmospheric CO2
-Help break down waste
Disease causality- what makes a microbe pathogenic?
**Koch’s postulates
- Microbe must be found in all organisms suffering from the disease, but should not be found in healthy organisms
- Microbes must be isolated from a diseased organism and grown in pure culture
- Cultured microbes should cause disease when introduced into healthy organisms
4.The microbe must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original causative agent
Koch’s postulates and pathogens
According to Koch, very few organisms are pathogens.
>pathogenicity is host specific. A microbe for one species can be pathogens but to another species can be commensal
Strict pathogens
-Rabies
-Yersinia pestis (plague)
Pathogen
An organism capable of causing disease
Virulence
The relative ability of an organism to cause disease
Virulence factor
A property of an organism which allows it to establish within a host and/or cause disease
Pathogenesis
The processes and host-organism interactions which leads to disease
Host microbe interactions
**spectrum
1.Benefit (colonization)
2.Indifference (latency)
3. Damage (disease)
When do infections occur?
1.Overwhelming pathogen load
2. Compromised host defences
What does disease result from?
1.Production of toxins
2. Invasion of tissues
Fundamentals of pathogenesis
1.Associate
2. Multiply
3. Evade
4. Damage co-opt
5. Transmit
Pathogenesis associate stage
Enter body or reach site of infection (anatomical location, cell type or receptor)
Pathogenesis multiply stage AND evade stage
Acquire and utilize nutrients, multiple and reach a critical number while avoiding host defence
>some pathogens have a capsule which will help them evade complement and phagocytosis
Pathogenesis damage co-opt stage
Damage from pathogen or host response
>may include hijacking host processes
Pathogenesis transmit stage
Pathogen must get out, survive, infect and repeat
Virulence factors
- Essential virulence genes- cause damage to host
- Virulence associated genes- required for expression, secretion, or processing virulence factors
- Virulence lifestyle genes- allow organism to colonize host or reach site of infection
MSCRAMMs
-microbial surface components recognizing adhesive matrix molecules- specific proteins which bind to host ligands
>also may be anti-phagocytic by preventing opsonization
**staphs and streps
Fimbrae
-help pathogen with attachment
> specific to species (ex. F4 fimbriae in e coli- bind to intestinal epithelium in piglets up to 8wks old vs. F5 fimbriae in e coli- bind to epithelium of distal small intestine of calves during first few days old)
Flagella
-Used to reach the site of infection
-Can be used to help determine pathogen (ex. Motility Test)
Siderophores
-free iron is a limiting nutrient even in blood
-these chelators allow bacteria to capture Fe3+ in a very low concentration environment
Capsule
-polysaccharide structure
-interferes with innate immune system by preventing phagocytosis and attachment of complement
-also protects from bacteriophages
Secretion systems
-Structures which transport molecules across the cell envelope
ex. Type 3 secretion system of salmonella used to inject effector molecules into host cells (needle-like!)
Toxins
-Wide variety of actions on target receptors/cells/tissues
>includes endotoxins (Lipopolysaccharides) and exotoxins (botulinum toxin, cholera toxin)
Superantigens
Antigens which are able to non-specifically bind to and activate the T-cell receptor, resulting in a cytokine storm= massive systemic inflammation, shock, death
Enzymes
used by bacteria to break down and invade tissues AND/OR break down molecules for use in their own metabolism
Degree of pathogenicity
Non-binary
Accidental pathogen to opportunist facultative to obligate pathogen
What effects the degree of pathogenicity
-Whether the organism is pathogenic?
-Is the host susceptible to infection?
-Is the environment permissive to infection?
Damage-Response Framework
Spectrum between too little response/overwhelming infection AND too much response/damaging inflammation or allergy
>damage to host is high for either SO want something in the middle between the two
Saprophytes examples
- Rhodococcus equi
-cause pneumonia in foals, otherwise healthy 12 week olds, most likely due to decline in maternal immunity; also seen in immunocompromised people - Aspergillus
-ubiquitous in nature
-decomposing matter
-cause of pneumonia and air sacculitis in birds
-allergic airway in other horses
Opportunistic pathogens
-most clinically relevant organisms
-these pathogens gain access to normally sterile site leading to infection
Opportunistic pathogen examples
-Staphylococcus spp= mastitis, UTI, surgical site infections
- Dermatiphilus congolensis= rain scald occurring when wet mascerated skin is traumatized
-E coli (bacteremia/sepsis in most species, UTI)
Primary vs secondary infections
-important to look at why a patient has the infection. Is it secondary to another condition?? Or as a result of poor management/environment??
Epidemiological triad
- Environment
- Susceptible host
- Virulent agent
Environments role on disease
-agent and host need to be in same environment
-favourable conditions
-presence of vectors/reservoirs
-changes that facilitates spread
-change that alters host susceptibility
Hosts role in disease
-herd immunity
-individual susceptibility
-nutrition
-co-morbidities
-age
Agents role in disease
-possess needed virulence factors
-sufficient numbers
-change in virulence?
Zoonosis
An infectious disease caused by a pathogen that has jumped from a non-human animal to a human
*reverse zoonosis- from human to animal
When does Koch’s postulates fall short?
-When you are pathogen that cannot be cultured
-one pathogen does not always lead to one disease
