Principles of Infectious Disease II Flashcards
<p>What are the most "successful" pathogens?</p>
<p>Often those pathogens that limit host damage = balanced pathogenocity</p>
Whate are “Normal Flora”?
- microbial flora that inhabit various body sites of healthy individuals
- profoundly affect metabolism and host physiology, including immune system development and homeostasis - “prime” the immune system
- help exclude dangerous microbes (this balance can be upset by antibiotic treatment or hygiene practices)
- some normal flora represent a potential reservoir for disease
Fun fact: there are more bacteria in your colon than cells in your body!
Ebola virus vs. Human Cytomegalovirus - which is a more “successful” pathogen?
Human Cytomegalovirus (HCMP, type of herpesvirus) because it infects over 90% of some populations, once you have it it stays. It does not cause much disease, but can be passed on readily.
Rapidly fatal infections tend to be self-limiting in a population (ex: Ebola virus).
Side note: HIV has a long phase with little disease but plenty of virus production. Also “successful.” Good host-pathogen balance.
<p>Steps of scientific proof to identify pathogenicity</p>
<p>1 - ASSOCIATE
2 - ISOLATE
3 - INOCULATE
4 - RE-ISOLATE: the agent must be recoverable from the susceptible host</p>
Whate are the four steps of scientific proof to identify pathogenicity (Koch’s Postulates)?
1 - ASSOCIATE: the infectious agent must be present in every disease case
2 - ISOLATE: the agent must be isolated from the disease case and grown in pure culture (when appropriate)
3 - INOCULATE: the specific disease must be reproduced from pure culture in a healthy susceptible host inoculated with the agent
4 - RE-ISOLATE: the agent must be recoverable from the susceptible host
What are the general features of infection?
Pathogens must: 1 - gain ENTRY into the host 2 - PERSIST in the host 3 - CAUSE DISEASE 4 - DISSEMINATE
- each of these steps require the pathogen to overcome the host defenses
- *collectively - factors that facilitate these steps are called VIRULENCE FACTORS
For host entry, a pathogen must…
Pathogen must colonize host through portal of entry, which are heavily defended:
skin, conjunctiva, oropharynx, URI, lower respiratory tract, stomach, intestinal tract, urinary tract
*in some cases, pathogen may enter tissues directly (ex. blood, mosquito)
Host Entry (Infection-Step 1)
ADHERENCE (of bacteria, viruses) to host cells
- bacteria adhere to host cells or extracellular matrix via adhesins
- viruses use receptors (ex: gp102 binding to CD4 receptor on CD4 T cell, or to macrophage)
<p>Once within the body, what defenses does the invader encounter?
(What are the barriers to persistence of the microbe?)</p>
<p>-macrophages
- phagocytes
- neutrophils
- complement
- type 1 interferon response (for viral infection)
- fever (harder for microbes to replicate?)</p>
Invasion (Infection-Step 2)
- pathogens can invade beyond epithelial surfaces, they can move through or between cells
- can involve initial replication in epithelial cells, followed by release of progeny into circulation to infect other cell types
- “Trojan horse” - transport by immune cells. Macrophages and dendritic cells can become infected and migrate to other sites (such as lymph nodes) to spread infection
Bacteria:
-many bacteria encode INVASINS that promote uptake and survival
-this allows bacteria to live and replicate within host cells, though bacteria are free-living
Ex: molecules that
-cause host cell endocytosis
-prevent phagosome acidification
-cause phagosome rupture for escape to cytosol
-degrade cell-to-cell junctions
*some bacteria inject factors into host cell to make it more permissive to infection
What are methods of persistence (during infection-step 3) against host defenses by pathogen within host?
- anti-phagocytic activity
- antigenic variation
- hideout
- induction or inhibition of apoptosis
What is anti-phagocytic activity?
- bacteria are readily engulfed by neutrophils and macrophages
- many pathogens resist uptake due to the production of the POLYSACCHARIDE CAPSULE
- may also inhibit killing activity of the phagocytes (Mycobacterium)
What is antigenic variation?
- some bacteria (such as N. gonorrhea) have elaborate mechanisms to alter their surface proteins to avoid antibodies
- viruses are masters of antigenic variation through simple natural selection during the course of infection
What is hideout (method of persistence)?
agent may occupy niche to avoid detection and/or elimination (ex: herpesvirus becomes latent - quiescent state with limited replication; HSV latent in neurons)
What is induction or inhibition of apoptosis (method of persistence)?
- some agents may induce host cell apoptosis to prevent killing (macrophages killed by Salmonella, for example)
- for some host cell types, apoptosis is a defensive response, and pathogens actively block it (epithelial cells and N. gonorrhea)