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