Transcript Host-Microbe Relationships and Disease

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Host-Microbe Relationships and Disease
• Classical ecological definitions
– Mutualism: organisms live together, both benefit
• E. coli in GI tract; we get Vitamin K, protection from
pathogens, it gets warm wet place to live, lots to eat.
– Commensalism: one organism benefits, the other is not
particularly benefited or harmed; to eat at the same table
• Most GI tract microbes; we provide a warm wet place
to live with food, we don’t get all that much in return.
– Parasitism: one organism benefits at the other’s expense
• Disease-causing bacteria; to them, we’re dinner.
• Classically, a “parasite” lives in or on host.
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Terms, terms, and more terms
• Contamination: presence of microbes (where they don’t
belong).
• Infection: multiplication of parasitic organisms in/on host.
– Infestation: used to describe larger organisms, e.g. lice.
• Disease: malfunction in or damage to the host.
– Many kinds of “disease”; here we discuss “infectious disease”.
– Disease is a condition of the host, not an infectious microbe.
• Pathogen: a parasite capable of causing disease
– Not all pathogens are equal as we will see..
• Pathogenicity: ability of pathogen to cause disease
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Terms, terms, and more terms-2
• Virulence: relative ability to cause disease.
– Especially variations in pathogenicity within a specific group
– Virulence can be weakened (attenuation) or increased (animal
passage); Growing pathogens on agar attenuates them.
• Normal microbiota: the microbes normally found on the
body. Since people are not “normally” sick, pathogens are
not normally consider “normal microbiota”.
– The term “flora” is to be avoided as microbes are NOT plants!
• Resident microbiota: always found on human tissues.
• Transient microbiota: come and go, can include pathogens.
Where do they live?
• Microbes live where it is “topologically outside”
– We are a tube within a tube. We have sacs
open to the outside.
• Respiratory tract:
– nasal passages, sinuses, trachea, lungs. Lungs
well protected, other areas more populated.
• GI tract: Crowded!
– Mouth is full, fewer in esophagus and stomach; toward
end of small intestine, numbers increase greatly.
– Feces consist largely of bacteria.
http://www.radiation-scott.org/deposition/respfig2.gif
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Where do they live?-2
• Skin: largest organ in the body.
– Colonized. Various factors keep the numbers down.
• Genito-urinary tract:
– Female reproductive tract colonized, especially with
Lactobacillus which is helpful and with yeast which is
occasionally not helpful.
– Lower portion of urethra contains some bacteria, but
bladder, ureters, and kidneys normally sterile.
• All areas of the body have mechanisms for keeping
us from being lunch; to be discussed soon.
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What determines whether we get sick?
• Inf Dis: the likelihood of
contracting an infectious
disease.
• N: the numbers of infecting
organisms.
•V: the virulence of the organism.
•HF: host factors, including overall health, nutritional
status, genetic background, age, immune status.
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How dangerous?
• Pathogen: causes disease.
• Opportunistic pathogen: can cause disease under the
right circumstances
– Dose in high numbers
– Host is in a weakened state, e.g. HIV infection.
– Organism gets where it doesn’t belong
• E. coli and urinary tract infections.
– Lack of microbial antagonism, e.g. superinfection
• competition for space, nutrients; bacteriocins.
• Saprobe/saprophyte: decompose dead stuff.
Whether an organism will cause disease
is not always a clear cut thing
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• Not everything in biology can be neatly classified.
There is a gradation from pathogen to opportunist to
non-infectious, and what happens depends on the
balance of these 3 factors.
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Types of disease
• Inherited diseases: caused by a faulty gene
– but susceptibility to microbes can be genetic too.
• Congenital: due to damage during development.
– but microbes can be involved in this damage.
• Degenerative diseases, due to age or lifestyle
– but microbes can be involved in some as well.
• Nutritional, endocrine, mental, immunological,
neoplastic (cancer), idiopathic; same caveat.
• Iatrogenic: caused by doctor.
– Nosocomial infections: occur in hospital.
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Types of infectious diseases
• Communicable: can be spread from one person to
another.
– Example: tuberculosis, HIV
• Contagious: highly communicable, can easily be
spread from one person to another.
– Genital herpes, measles.
• Non-communicable: are not spread from one host to
another.
– Examples: your infected appendix bursts
– You get tetanus from “rusty nail”