The History of Microbiology
(Key events that advanced our
understanding of
microorganisms)
Microbes have been around for a long time
Thought to be the first living entities on
earth::photosynthetic bacteria were responsible for
generating oxygen for the support of life
Cyanobacteria—photosynthetic microorganisms were the first O2-evolving
organisms on Earth and may have been responsible for oxygenating the
atmosphere
People were aware of an invisible force that
affected food and involved microbes.
Evidence for the production of beer, wine and bread
Evidence for food preservation with salt and drying. (Ships
undertaking long passages to the Americas used these practices)
Francis Bacon (1561-1626) Had a theory that freezing should
retard the spoilage of chickens—died testing the theory
Production of beer, wine and bread @ 2400 BCE
People were aware that an invisible force caused sickness
and death
1300’s height of the first plague: implementation of
quarantenaria by the Italians
More evidence for human awareness of
disease causing microorganisms
Girolamo Fracastoro 1546 (200 years after the first plague)
Philosophical treatise “De Contagione et Contagiosis Morbis”
--some diseases are caused by the transmission of germs
3 possible modes for the transmission of contagion
1. Direct contact
2. Transmission via inanimate objects
3. Transmission from a distance
--contagion or disease processes and the putrefaction of food are
analogous.
Germs have a form::invention of the
microscope
Johannes and Zacharias Jansen (spectacle makers) –1590—two
convex lenses in sequence magnifies and image
Galileo (Italian astronomer) –1624 improved precursor to the
light microscope
Anton van Leeuwenhoek (cloth draper) –1674 designed own
microscopes, first to describe bacteria, yeasts and protozoa—
”wee animalcules”
Ernst Abbe (German physicist) --1882 developed better lenses
for microscopes, developed the oil immersion lens
Paul Ehrlich –1881 staining with methylene blue
Christian Gram –1884 differential staining with methylene
blue and safronin.
Leuwenhoek’s microscope and
drawings of some of his “wee
animalcules”
Hooke’s microscope (progenitor to the
modern day microscope) and his
observations
Hooke was a contemporary of Leuwenhoek and invented the
microscope around the same time
The controversy regarding “spontaneous
generation” in food spoilage
The belief that living forms sprang from non-living matter
proponents: putrefaction is a result of chemical events
opponents: putrefaction is caused by “germs”
Francesco Redi – 1668 Showed that flies were responsible for
maggots found on the surface of rotting meat.
John Needham –1748 organic infusions spoiled in stoppered
bottles
Abbe Lazzaro Spellanzani—1765 boiled the organic infusions,
infusions remained “barren” for a long time unless the seal was
broken
Louis Pasteur –1861—trained as a scientist and used “scientific
method” to address the issue.
Scientific Method
1. STATE THE PROBLEM
2. FORM A HYPOTHESIS AFTER GATHERING ALL
INFORMATION ON THE TOPIC
3. DEVISE EXPERIMENTS TO TEST THE VALIDITY OF
THE HYPOTHESIS
4. OBSERVE RESULTS OF THE EXPERIMENT
5. INTERPRET THE DATA
6. DRAW CONCLUSIONS
7. ALWAYS CAREFULLY DOCUMENT DESIGN AND
RESULTS OF THE EXPERIMENT!!!
Pasteur’s Scientific Method
1. STATE THE PROBLEM
A. Food allowed to stand for some time spoiled
B. Wine and beer do not always come out the way it
should
Pasteur’s Scientific Method
2. FORM A HYPOTHESIS AFTER GATHERING
ALL INFORMATION ON THE TOPIC
Observed that structures in the air that resembled micro
organisms found in putrefied materials
Passed air through cotton/ released trapped materials in liquid
and observed microorganisms via microscopy
HYPOTHESIS: Microorganisms found in putrefying food
originate from microbes in the air—foods protected from the
airbourne microbes should not putrefy.
1861—Louis
Pasteur refutes
the theory of
spontaneous generation
(good example of a
controlled experiment)
Some of
Pasteur’s “swan
necked” flasks
Pasteur’s Scientific Method
4. DRAW CONCLUSIONS
The souring of milk, beer and wine is due to microorganisms
Microorganisms in these products can be killed by heating
Some advancements from these studies and conclusions:
1867—pasteurization –heating food to 55-60oC for
a short time
Pasteurization is commonly used to
remove pathogenic organisms from
food products
Historically: Used to kill organisms causing brucellosis,
typhoid fever and tuberculosis in dairy products
Today: Used to kill Salmonella spp and Escherichia coli O157:H7
Doesn’t kill all of the organisms
Process: Passing raw milk through a heat exchanger such that
small volumes of milk are briefly heated to 71oC for 15 seconds,
the milk is rapidly cooled and maintained at the low temperature
The effect of temperature on the viability of
a typical mesophillic bacterium
Decimal Reduction
Time = the length
of time a bacterial
population must be
exposed at a given
temperature to
kill 90% of the
population
Caveats to using lower
temperatures for the “eradication”
of microorganisms
The thermal death time depends on the size of the initial bacterial
population
The high sugar, protein and fat content of some products decrease
heat penetration and increase the resistance of the organisms in
these products to heat
Endospores are moderately heat resistant—contain compounds
that are resistant to heat, have little water activity, are metabolically
inert and are surrounded by a thick keratin coat.
John Tyndall and Ferdinand Cohn—1877
Discovery of heat resistant forms of
bacteria
John Tyndall: bacteria have a heat labile or heat sensitive form and
a heat resistant form
Cohn –the heat resistant forms of bacteria form endospores
endospores become vegetative after one heating process.
Repeated heating and cooling eliminates viable organisms—
Tyndallization
Charles Chamberland—1881 designed an autoclave –large pressure
cooker,
Endospores
Contraction
of the
cortex st.V
results in a
shrunken
protoplast
Accumulation of Calcium, small acid soluble spore proteins (SASPs)
and Dipicolinic acid that forms a gel like structure which excludes water
Endospores:
Note how these structures
are resistant to heat, irradiation and chemicals
compared to vegetative cells and note how they
are low in water content and metabolically inert
*
The Autoclave—
an effective means for
killing heat resistant
bacteria and endospores
The chamber is heated under low
atmospheric pressure conditions such that
it is possible to increase the chamber
temperature to 121oC.
The autoclave (cont’d)
High Temperatures (121oC) for at least 15 minutes—longer for larger
volumes that take longer to reach this temperature
Endospores require approximately 5 minutes for a decimal reduction