CAS BI 114 Lecture Notes - Lecture 6: Candida Albicans, Exponential Growth, Genitourinary System

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Principles of Infection
Disease State v. Non-Disease State
Disease - any type of deviation from health
Path- (prefix) - disease
Pathogen - a disease causing agent
Not all diseases are infectious (ex: cancer, diabetes, arthritis)
Epidemic - occurs when the rates of disease rise above what is expected
ex: obesity epidemic v. Zika virus epidemic
Humans as Hosts
There are many non-pathogenic microbes living in/on us at all times
We get an infection when
1. Microbes cross to a new, previously un-colonized/non-competitive area and throws off the
healthy balance (ex: UTI)
They do not have to compete with other bacteria for nutrients, leading them to reproduce
exponentially
2. Microbes acquire new disease-causing traits (ex: E. coli food poisoning)
3. Microbes grow in higher numbers than usual (ex: yeast infections)
4. New microbes are acquired (ex: Zika virus)
5. Defenses are down (ex: shingles)
Can be through stress, immunosuppressive drugs or agents
Humans as Hosts
Stages: Contact, Colonization, Infection, Disease, Spread
Who gets sick, when and why?
Everyone is exposed to microbes at all times
But factors can vary
1. The pathogen varies by virulence
Virulence - the relative severity of disease by a pathogen (ex: influenza is more virulent than the
common cold)
Virulence is determined by a pathogen’s ability to: establish itself in the host or cause damage
2. How much of the pathogen did you get exposed to? (infectious dose)
3. Did you get exposed at the right place? (ex: a respiratory pathogen will have no effect on skin-to-
skin contact because it needs to be coughed or inserted into the body, which is highly unlikely)
4. Do you have a genetic predisposition to the microbe?
5. Do you have previous exposure to the microbe or one that is similar?
6. How healthy are you? (ex: nutritionally deficient, lack of sleep, etc.)
Pathogens and Virulence
We can get sick from all sorts of microbes
True Pathogens - capable of causing disease in even the healthiest of people (ex: flue, rabies, HIV)
Opportunistic Pathogens - only cause disease when the host’s defenses are compromised (ex:
candida albicans (yeast), several pneumonia-associated infections)
Step 1: Getting in and Getting Established
Getting in - Transmissibility
How able is a pathogen to cross from one host to another?
If a pathogen can survive outside the body, endure dry air/sunlight, it can get to another host easily
Transmissibility is directly tied to the portal of entry
Getting in - Portals of Entry
Skin, respiratory system, urogenital system, GI tract, blood
Pathogens that dwell in the GI tract must travel by the oral-fecal route
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Transmissibility will increase if these pathogens can tolerate watery environments (ex: drinking
water supplies)
Pathogens that dwell in the blood must travel through a vector or through the exchange of blood
(ex: donation, needles)
Vectors are usually blood-sucking insects or anthropods (ex: ticks, mosquitos, bed bugs
Pathogens that dwell in the respiratory system are most transmissible if they can travel through the
air
Airborne droplets are the best method (ex: coughs, sneezes)
Pathogens that dwell in the reproductive system are the least hardy of pathogens
Must have a warm, moist environment and direct transfer between one host to another
Often a slow and infrequent transmission
Inoculating Dose - how many/much microbes does it take to initiate infection?
Varies from pathogen to pathogen
ex: tuberculosis requires only 10 cells while cholera requires 1 billion
Step 2: Hunkering Down
Pathogens need to be able to stick onto the surface of host cells
Sometimes it needs to be able to invade host cells or tissues
Being inside the host can be a tumultuous place (blood flows quickly, digestive tract flows quickly,
secretions, etc.)
A firm grip is important!
Methods to grasp on: fimbriae, suface proteins, adhesive slimes, capsules, suckers, hooks, barbes
Step 3: Survival
Host cells (immune cells) will try to defend itself by eating/disabling pathogens
The more mechanisms a pathogen could have for
Avoiding detection
Avoiding engulfment (phagocytosis) by immune cells
Surviving on the inside of an immune cell (after engulfment)
Pathogens with these survival strategies will exist in the host for a longer time, increasing the
likelihood of being passed on
Step 4: Causing Disease
Some pathogens cause more damage than others
Symptoms of infection are often the result of the immune system working
Symptoms - sneezing, sweating, coughing, diarrhea, chills, vomiting
Pathogens can cause damage in two ways:
Directly - secreting damaging chemicals
Indirectly - inducing the host’s immune defenses (particularly if the immune response is excessive
or inappropriate)
Harmful chemicals:
Enzymes are secreted from bacteria to aid in infection (ex: mucinase - digests mucus allowing
bacteria to get to cells, bacterial kinases - dissolves blood clots so that bacteria can get further into a
damaged tissue)
Toxins are specific chemical products of microbes that are poisonous to other organisms
Endotoxins are components of the bacterial cell wall (LPS) that triggers inflammation and fever
Exotoxin refers to any type of toxin released by a pathogen
Toxins are named for the specific location of action
Neurotoxin - nervous system
Enterotoxin - GI tract
Hemotoxin - blood
Nephrotoxin - kidneys
Exotoxins - specific secreted protein binds to specific tissue target
Endotoxins - Outer membrane component causes fever, malaise, aches and shock
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