MICR 2123 Lecture Notes - Lecture 18: Macrophage, Helicobacter Pylori, Gas Gangrene

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12 Apr 2017
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Human Microbiota and Innate Immunity
Introduction
There exist a lot of microorganisms in and on our human body
oMost are harmless if they stay where they belong
oSome are even beneficial
The human body is also under constant attack from microbial invaders
oWe have barriers
Nonspecific defenses (skin, stomach acid)
For those that are successful in invading, we have adaptive and nonadaptive
immune defenses
Human microbiota
Our bodies carry 10x as many microbial cells as human cells
oBacteria are normally found at various nonsterile body sites called commensal orgnaisms
oThe consortium of colonizing microbes has been dibbed the human microbiota or
microbiome
oMicrobe populations can cause disease if they reach abnormal location
Skin
oMostly gram positive
Skin is typically dry, also epidermal secretions are salty
Gram + bacteria are more resistant to salt and dryness
oStaphylococcus epidermis
oPropionibacterium – acne
Nose and mouth
oColonization starts within hours of birth, mostly with harmless bacteria
oAll colonizers need to be able to adhere to surfaces to avoid flushing to acidic stomach
oBoth gram – and + are present. Mostly aerobes but anaerobes also appear in gingival
crevices or tonsillar pits
oEx
Streptococcus  tooth decay
Syaphylococcus auerus
S. epidermis
Respiratory tract
oMicroorganisms are constantly inhaled into the lungs
oThe ciliated mucosal epithelium of the trachea, bronchi, and bronchioles makes up the
mucociliary elevator
Sweeps foreign particles up and out of the lung
If it fails due to being overwhelmed with the number of microbial particles, infection
occurs (common cold, pneumonia)
Stomach
oVery high acidity (pH ~2, within lining pH ~5-6)
oFew microbes survive
Helicobacter pylori
Survives at pH 1
Burrows into protective mucus, may cause gastric ulcers
oEnteric pathogens must survive passage through the stomach
E. coli and Shigella are acid tolerant and have very low infectious doses
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Vibrio cholerae is more acid sensitive and has a high infectious dose. Only some
bacteria survive passage to establish intestinal infection
oDecreased stomach acidity  hypochlorhydria
Caused by malnourishment
More susceptible to enteric infections
Intestine
oThe intestine contains 1011 bacteria/gram of feces
Ratio of 1000:1 anaerobes to facultative anaerobes
oOrganisms that inhabit the intestine
Bacteroides thetaiotaomicron
Anaerobe
Breaks down complex carbohydrates in food and mucus into products that
can be absorbed by the body
Escherichia coli
Facultative anaerobe
Catabolizes gluconate, a component of mucosal polysaccharides
oComplex but balanced microbial population
Antibiotic therapy disrupts this balance and may lead to poor digestion or disease
Pseudomembranous colitis is caused by clostridium difficile
Probiotics are living microbes that are ingested to restore the natural microbial
balance
The most commonly used genera are Bifidobacterium and Lactobacillus
Urogenital tract
oKidneys are normally stable
oBladder appears to be colonized with low numbers of bacteria
oDistal urethra contains Staphylococcus epidermis and some enterobacteriacea
UPEC may cause urinary tract infections if they ascend to the bladder
oComposition of vaginal microbiota changes with menstrual cycle
Acidic secretions (pH 4.5) favor the acid resistant Lactobacillus acidophilus
Antibiotic therapy can allow Candida albicans to proliferate causing a yeast infection
Risks and benefits of microbiota
Commensal microbes benefit the human host
oMake vitamins and digest food (b. thetaiotamicron)
oProvide nutrition to colonic epithelial cells
oPrevent colonization by pathogens (compete with them for food and produce antibacterial
agents that kill them)
Opportunistic pathogens cause disease when immune system barriers are breached
oHost is said to be immunocompromised
oBacteroides fragilis (normal intestine resident)  anaerobic gas gangrene if gets into
surrounding tissues
oNeisseria meningitidis is common inhabitant of nasopharynx. Causes infectious meningitis
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