Spread and Replication Flashcards
What is a surface infection?
The microorganisms multiply at the site of entry and don’t penetrate deeper tissues. The microorganism can spread to other areas of the body via fluid flow (coughing, sneezing, flow of mucous over the GI and respiratory tract) but as soon as they penetrate into deeper tissues, it’s automatically systemic
Sometimes large areas of the body can be involved in a surface infection
Incubation time is usually less than a week
The innate immune system is usually involved with surface infections (short incubation time)
What is a systemic infection?
An infection that has spread through the blood stream or lymph, which can spread to other parts of the body
Incubation time is usually more than a week
The adaptive immune system is involved
Immune status is a factor in surface or systemic infections
What factors affect the location of infection?
Temperature
The site of budding
Systemic Spread
How does temperature affect the location of infection?
Example: Rhinoviruses can only cause upper respiratory infections (33ºC), not lower respiratory infections (37ºC)
Example: Mycobacterium leprae can only infect nasal mucosa, skin and superficial nerves
How does the site of budding affect the location of infection?
Example: influenza and parainfluenza viruses invade surface epithelial cells from the lung, but are liberated by budding from the free (external) surface of the epithelial cell, no from the basal layer where they could spread to deeper tissues
How can systemic spread affect the location of infection?
Many microorganisms fail to spread and multiply at the primary site of infection
Example: measles and typhoid do not multiply replicate at the sit of initial respiratory or intestinal infection. They spread through he body and replicate
Example: Hep A travels from the alimentary canal to the liver
Example: Mumps travels from the respiratory tract to the salivary glands
Describe the mechanism of systemic infection (stepwise invasion of different tissues)
Pathogen infects epithelial cells where it may or may not replicate. Then spreads to the blood stream and/or lymph, where it may or may not replicate. It may or may not shed from the bloodstream to the exterior. If it does not shed, then it spreads to different organs of the body where it may or may not replicate. It travels back to the bloodstream and it may or may not shed to the exterior. If it does not shed, it will travel to another organ of the body where it may or may not replicate, and then shed to the exterior
After traversing the epithelium, invading microbes face what from our immune system?
Tissue fluids containing antimicrobial substances
Local macrophages
Physical barriers of local tissue structure (more difficult for bacteria than viruses)
The lymphatic system (phagocytic and immunologic defences, as well as macrophages)
By the time symptoms have appeared, all these systems have failed
Are microbes in the blood very serious?
Viruses and small numbers of bacteria can gain easy access to the blood without causing much of an alarm. Example: transient bacteraemia are fairly common (defecating, brushing teeth, etc.) and are filtered out and destroyed by macrophages of the liver and spleen. However localization in less well-defended areas can lead to infection (viridians streptococci in endocarditis caused by congenital abnormal heart valves)
What happens if there are microorganisms free in the blood?
They are exposed to the body’s defences such as antibodies and phagocytes. However some microorganism can associate with circulating cells (e.g., EBV, rubella and intracellular bacteria such as listeria are present in lymphocytes and monocytes), which will protect and transport these microbes
Why each circulating microorganism invades characteristic target organs and tissues is not completely understood, but it may be due to what?
Specific receptors for the microorganism
Only some organs being suitable for colonization and replication
Accumulation of circulating microbes in sites where there is local inflammation, because of the slower flow and sticky endothelium in inflamed vessels
Describe spread through the nerves
Some viruses are able to travel through the axons of the peripheral nerves to the CNS (e.g., rabies, HSV, varicella-zoster virus). They travel back to the peripheral nerves during recurrent outbreaks
Host defences are unable to control viral spread through the nerves
What happens if pathogens enter the cerebral spinal fluid
Once pathogens have crossed the blood-brain barrier, they spread rapidly
Describe the spread via pleural and peritoneal cavities
Microbes can spread from one visceral organ to another through the peritoneal or pleural cavity
Injury and/or disease in the abdominal organ may lead to infections (e.g., peritonitis)
What are genetic determinants of spread and replication?
Genetic determinants of the host
Genetic determinants of the pathogen
