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Mechanisms of Disease – Microbiology Online Lecture

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Principles of Disease


I. Pathology – study of disease

A. Etiology – cause of disease

B. Pathogenesis – how disease develops

C. Infection – invasion of the body by pathogen

D. Disease – the structural and functional changes infection causes on the body

E. Virulence – degree of pathogenicity

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F. True pathogen – causes disease in healthy person, i.e. cold virus

G. Opportunistic pathogens – usually not a problem, but can become pathogenic

1. Displaced to different area of body

a. E. coli in the large intestine is beneficial, but disease causing in the urinary tract

b. Staph on skin enters through wound

2. Disruption of normal flora

a. Candida albicans in vaginal yeast infection (antibiotics, douching)

b. Clostridium difficile (C. diff) in large intestine (antibiotics)

3. Immunocompromised

a. Immune defenses are low due to primary infection

1) Leads to Secondary Infection

a) Pneumocystis pneumonia in AIDS patient

b) Pneumonia after flu

b. Chemotherapy, mental stress, physical stress (injury, surgery), etc


II. Classifying Infectious Diseases

A. Symptoms – subjective

1. Pain, feeling tired

B. Signs – objective

1. Swelling, fever

C. Diagnosis of Disease

1. Made by evaluation of symptoms, signs and lab tests

D. Communicable Disease

1. Spread from person to person, directly or indirectly via fomite

2. Chicken pox, herpes

E. Non-communicable Disease

1. Clostridium tetani – step on rusty nail


III. Occurrence of a Disease

A. Incidence – number of people in a population that developed disease in a specified time; number of new cases


B. Prevalence – number of people in population who have the disease in a specified time; both old and new cases

1. In US in 2007, the incidence of AIDS was 56,300. The prevalence was 1,285,000.

C. Frequency of Occurrence

1. Endemic disease – always in population (common cold)

2. Epidemic disease – high incidence in given area in short amount of time (flu)

3. Pandemic disease – occurs worldwide (AIDS)


IV. Extent of Disease/Infection

A. Acute disease – develops rapidly and lasts short time (flu)

B. Chronic disease – develops slowly, lasts longer (tuberculosis, hepatitis B)


C. Local infection – invading microbe limited to small area (skin abscess)

D. Systemic infection – body wide (flu)

E. Septicemia (blood poisoning) – systemic infection; pathogens in blood


V. Stages of Disease

A. Incubation Period

1. Time between initial infection and first appearance of signs/symptoms

2. Depends on virulence of pathogen, number of pathogens, resistance of host

B. Prodromal Period

1. Early, mild symptoms (aches, malaise)

C. Period of Illness

1. Disease is most severe

2. Fever, chills, myalgia (muscle ache)

3. Either person fights off disease (immune system) or they die

D. Period of Decline

1. Symptoms subside

2. Fever decreases, person begins to feel better

E. Period of Convalescence

1. Person regains strength; return to pre-disease state


VI. Sequence of Disease

1. Source of Pathogen (Reservoir)

a. Where does it come from?

2. Transmission of Pathogen

a. How is it spread

b. Direct Contact, Indirect Contact, Vector

3. Invasion of Pathogen into host

a. How does it get into host and multiply within host

4. Development of Disease (Pathogenesis)

VII. Source of Pathogen / Reservoirs of Infection

A. Human Reservoirs

1. Spread by people with signs and symptoms

a. Cold / Flu

2. Carriers – spread by people without signs and symptoms

B. Animal Reservoirs

1. Rabies in bats

C. Non-living

1. Water and Soil

VIII. Transmission

A. Contact Transmission

1. Direct Contact – “person to person”

a. Most common

b. Cold, flu, staph, hepatitis A, STD’s, etc

2. Indirect Contact

a. Fomite – pens, cups, toys, money, eating utensils, handkerchiefs, towels

3. Droplets

a. Mucus droplets

b. Sneezing, coughing, talking

c. Flu, pneumonia

B. Vehicle Transmission

1. Transmission via air, water, food, body fluids, etc

C. Vector Transmission

1. Insect bite

2. Malaria (mosquito carries protozoan), Lyme disease (tick carries bacterium)


IX. Invasion of Pathogen into Body

A. Portals of Entry

1. Skin

2. Mucous membranes – epithelial lining

a. GI tract

b. Respiratory tract

c. Urogenital tract


B. Adherence (adhesion)

1. Mechanism of pathogen attaching to our cells

2. Adhesins (ligands) on pathogen cell surface bind specifically to receptors on our cell


a. Adhesins found on glycocalyx, pili, fimbriae, or flagella

C. Penetration

1. Pathogens can enter due to physical damage / opening in skin or mucous membrane

a. Use enzymes (hyaluronidase and collagenase) to spread through tissues


2. Pathogen can adhere and enter directly into cells

a. Pathogenic E. coli have adhesins on fimbriae to adhere to cells of small                                                 intestine, then             induce intestinal cells to take them up (endocytosis) where they

multiply inside of cells. Non-pathogenic E. coli that reside in the intestines do not

have these adhesions.

X. Damage

A. Pathogens use your tissues for nutrients and produce toxins

B. Some pathogens enter cells directly, most release toxins

C. Toxins

1. Poisonous substances produced by microbes

2. Most often primary factor of pathogenesis

3. Can produce fever, diarrhea, shock, CV issues, death

4. Exotoxins and Endotoxins

D. Exotoxins

1. Soluble proteins produced inside of Gram+ cells

2. Released by bacteria

3. Affect specific cell structures/functions; nerves, GI tract

3. Botulism and staphylococcal food poisoning

a. Due to exotoxins, not bacteria themselves

4. Naming Exotoxins

a. Neurotoxins, cardiotoxins, hepatotoxins, leukotoxins, enterotoxins

b. Diphtheria toxins, tetanus toxins

1) These toxins are inactivated and administered to us (vaccination)

a) Stimulates us to produce antitoxins without causing disease

b) Immunity is produced


G. Endotoxins

1. Lipopolysaccharides (outer cell wall) of Gram – bacteria

2. Released when Gram- bacteria die

3. Cause our macrophages to release cytokines

4. Cytokines produce general effects

a. Fever, chills, weakness, aches, shock, death

5. Septic Shock

a. Life threatening drop in blood pressure (BP) caused by Gram- bacteria

a. Phagocytes release Tumor Necrosis Factor (TNF)

b. TNF damages capillaries; lose fluid; drop in blood pressure


XI. Portal of Exit

A. Specific routes pathogens leave body to spread

1. Secretions, discharges, excretions, shed tissue

B. Examples

1. Respiratory

a. Mouth and nose

b. Coughing, sneezing, runny nose

c. Pneumonia, chickenpox, measles, mumps, flu, colds

2. GI tract

a. Feces – cholera, salmonellosis, amebic dysentery

b. Saliva – rabies, mononucleosis

3. Genitourinary tract

a. Secretion of vagina and penis – STD’s

4. Skin / Wound Infection

a. Ringworm, warts, herpes


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