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Germ Theory of Disease

Koch’s Postulates

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Germ Theory of Disease Overview
o Some diseases are caused by microbes
 Infectious vs noninfectious
o Corollary = if you can control exposure…you can prevent the disease (yay!)

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Purpose of Koch’s Postulates
o Specific infectious diseases are caused by a specific microbe
o Help determine etiology of disease
 Can help with prevention and treatment!

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19th century Healthcare
o Lack of proper medical education
o Primitive anesthesia
o No PPE, handwashing, sanitation
o No separation between clinic, labs, morgue, operating rooms -> spread of disease





Semmelweis’s Work
o Ignatz Semmelweis
o Labeled as the savior of mothers
 Made the connection between maternal deaths from septicemia to lack of
proper handwashing
 Told hospital staff to wash their hands (but they didn’t listen…too
scandalous of an idea apparently)
o Ignored due to lack of tact; too early for his time (before Pasteur)

Origins
o Robert Koch’s experiment
 Isolated microbes from a diseased animal
 Grows microbes in pure culture
 Identifies microbes
 Microbes are inoculated into a healthy animal in the lab
 The disease is reproduced in the healthy animal
 Microbes are taken from the lab animal and grown in pure culture
 Identifies microbes to see if they match the microbes from the
diseased animal

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Koch’s Postulates
o 1) The same pathogen must be present in every case of the disease
o 2) Pathogen must be isolated and grown in a pure culture
o 3) Pathogen from pure culture must cause disease in the healthy animal
o 4) The pathogen must be isolated from inoculated animal and must match the
original pathogen
o What are some problems with Koch’s postulates?
 Many infections have the same symptoms
 Secondary infections
 Some organisms don’t get sick from certain microbes
 Clean petri plate problem
 Some microbes can’t grow in the lab

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Microbiota
o Symbiotic relationship with the host (us!)
o Microbiota protect us
 Microbial antagonism = competition with other microbes; occupy niches
 Produce aids and bacteriocins
 Probiotics = live microbes applied to or ingested into the body
 Produces benefits



Viral Cultures
o 1) Mince tissue for culturing
o 2) Disaggregation by enzymes
o 3) Inoculate cells in a fresh culture medium



Occurrence of Infectious Disease
o Incidence = portion of the population that contracts a disease during a specific
time
 NEW cases
o Prevalence = portion of the population that has a disease during a specific time
 TOTAL cases
o Sporadic diseases = disease that occurs occasionally in the population
o Endemic diseases = disease constantly presents in population
o Epidemic diseases = disease acquired by many hosts in a given area in a short
time
o Pandemic diseases = worldwide epidemic
 However, does not indicate the severity of the disease, just the spread
 Example: 1918 Influenza Pandemic

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Reservoirs of Infection
o Humans
o Animals
o Environmental
 Water and soil



Transmissions
o Direct contact
o Droplet transmission
o Indirect transmission
 Fomites
 Example: doorknob



Vehicle Transmission
o Transmission by inanimate reservoir
o Aerosols
o Food
 Fecal-oral transmission



Vector-borne Diseases
o Arthropods
 Fleas, ticks, mosquitos
o Mechanical Transmission
o Biological Transmission



Nosocomial Infections
o Hospital acquired infections (HAI)



Joseph Lister
o English surgeon and aristocrat
o 1865; made the connection between Pasteur and Semmelweis’s works
o Discovered that carbolic acid can disinfect surgical tools and decrease infections

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Other Contributors to Germ Theory
o John Snow
 1854; mapped out cholera epidemic to find the hot spots to detect the
source
o Florence Nightingale
 Crimean War 1854
 Introduce sanitation to hospitals in the war
 Clean sheets, laundry, ventilation
o Clara Barton
 Nurse during the American Civil War
 Founder of the American Red Cross

o 4) Confluent culture; cells are separated by enzymes
o 5) cryopreservation OR subculturing/passaging
o Forms viral plagues
 Turns purple host cells - white
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River’s Modified Postulates
o 1) Isolate virus from the diseased host
o 2) Cultivate virus in host cell
o 3) Proof of Filterability
 Bacteria = 1-5 micrometers while Viruses = 50-100 nanometers
 Filter pore separates the two to determine the type of infection
o 4) Production of the comparable disease
o 5) Reisolation of same virus from inoculated animals
o 6) Specific immune responses to virus
 Nucleic Acid Based Detection
 PCR, quantitative PCR, reverse transcriptase PCR
 Antibody Based Detection
 Proteins made by B lymphocytes
 ELISA
 Western Blots

Infectious Disease

o Multiple Factors
 Microbes in the hospital
 Compromised host
 Chain of transmission
 Usually the health care worker
o Examples
 UTIs, surgical site, lower respiratory, GI, bloodstream
 Breaches body’s natural barriers


Emerging Infectious Diseases
o Genetic recombination
o Evolution of new stains
o Misuse of Antibiotics
o Climate change
o Globalization
o War or Natural Disasters
o Animal Control
o Public Health Disaster

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Outbreak Information
o Etiology
o Reservoir
o Mode of transmission
o Occurrence



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