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Dana Philpott

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LECTURE 02: Innate barriers to infection
Epithelial cells first to encounter microorganism cells that line mucosal services
Commensal flora newsworthy
5 different levels of protection beginning with innate barriers these services provide
protective substances that protect from microbial infection
If bypassed, cellular and humoral defences are first line of defense
If the innate immune system fails rarely happens adaptive immunity system reacts
including the antibodies, cytokines, etc.
7 services where microbes try to grain a foothold to infect
Skin, internal organs are lined with epithelial cells that are in contact with external
environment (food, air, etc.) urogenital tract v. expose to environment
8 in tears, enzymes called lysosomes can break holes in bacteria clear epithelial cells
that line eyes tears contain substances that fight microbial infection
Stomach very acidic few microorganisms can withstand the acidity
Normal flora competes with different pathogens
Commensal flora that live with humans defend against pathogens
Anatomical barriers mechanical, chemical, biological
Mechanical barriers prevent infection
Squamous epithelial desquamates losing skin cells bacteria attached to skin cells will
get sloughed off along with the skin cells
Mucous membranes GI tract parastaliss squeezing push out bacteria with feces
Respiratory epithelium ciliated cells cilia push bacteria particles back up into nose and
Epithelium flushes services away from bacterial infections
Chemical factors
Sweat has antimicrobial factors that can fight off infections
Mucuous membranes low pH HCl produeced by parietal cells few can survive
Lysozyme and phospholipase A poke holes in bacteria
Surfactants in long opsonisation coat bacteria and make it easier to recognize by
macrositic (?) cells, so easier to detect
Biological factors normal flora antimicrobial substances competes for nutrients, so
pathogens cannot gain a foothold on sufaces to infect

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Recognizable microbial community in all these membranes gastrorespiratory all have
microbial community residing on these surfaces
Humans are more bacteria than human cells
Communities present in two different ppl v. different probiotics carried transiently in gut
Before two years of age, changes in microbial community by 2 years, constant presence of
Core gut microbiome
Family members eating same food, same environment
Fraternal and identical twins not genetic makeup that dictates the presence of
microbial communities but the environmental exposure that determines makeup
of microbial community
Output of these communities metabolic possibilities
oAll the microbes function to give same output despite proportions are
Relative numbers of microbial loads large intestine many reside, performing
useful functions
Composition of flora vary between certain sites
Gastrointestinal flora
Provide protective function to GI tract - bacteria take up space in tract, so
pathogens cannot have space or nutrients to gain foothold and grow on tract
Produce antimicrobial factors kill different pathogens
Structural function barrier fortification epithelial cells flora strengthens
barriers beweens epithelial cells
oMice grown in germ-free environment, weak barrier
Stimulates immune system in gut
oKeep mice in buble environments, where flora devoid, v. primitaive immune
system develop in gut need flora to develop strong immune system in gut
Bacteria can digest what we cannot
Provide nutrient (protein fatty acids) for epithelial cells metabolic gas that fuels
epithelial surface
Produces vitamins
Harmful effects of GI flora
Urinary tract infections strain of E. coli typically found in intestinal tract
oSomehow it got into urogenital tract to cause urinary infections
Perforated appendix massive infection within body cavity sepsis v. serious
when flora escape elsewhere
Altering microbial flora
Anitibiotics can perturb proportions of bacteria living in gut
oConstant use of antibiotics overgrowth et c.
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