The History of Microbiology (Key events that advanced our

understanding of microorganisms)

Microbes have been around for a long timeThought 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

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

People were aware of an invisible force that affected food and involved microbes.

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 contact2. Transmission via inanimate objects3. 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 matterproponents: putrefaction is a result of chemical eventsopponents: 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 bottlesAbbe 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 PROBLEM2. FORM A HYPOTHESIS AFTER GATHERING ALL

INFORMATION ON THE TOPIC3. DEVISE EXPERIMENTS TO TEST THE VALIDITY OF

THE HYPOTHESIS4. OBSERVE RESULTS OF THE EXPERIMENT5. INTERPRET THE DATA6. DRAW CONCLUSIONS7. 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 itshould

Pasteur’s Scientific Method

2. FORM A HYPOTHESIS AFTER GATHERING ALL INFORMATION ON THE TOPIC

Observed that structures in the air that resembled microorganisms found in putrefied materials

Passed air through cotton/ released trapped materials in liquidand observed microorganisms via microscopy

HYPOTHESIS: Microorganisms found in putrefying foodoriginate from microbes in the air—foods protected from theairbourne 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 fora 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 productsToday: 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 ReductionTime = the lengthof time a bacterialpopulation must be exposed at a giventemperature tokill 90% of the population

Caveats to using lower temperatures for the “eradication”

of microorganismsThe thermal death time depends on the size of the initial bacterialpopulation

The high sugar, protein and fat content of some products decreaseheat 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 metabolicallyinert 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 anda 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 pressurecooker,

Endospores

Accumulation of Calcium, small acid soluble spore proteins (SASPs) and Dipicolinic acid that forms a gel like structure which excludes water

Contractionof the cortex st.Vresults in a shrunken

protoplast

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.

High Temperatures (121oC) for at least 15 minutes—longer for largervolumes that take longer to reach this temperature

Endospores require approximately 5 minutes for a decimal reduction

The autoclave (cont’d)