a microscopic organism consisting of a single cell or cell cluster; 

this includes viruses, however they are not cellular 

-microscopes 

-culture-based

-molecular

the first to describe microbes 

-he described fruiting molds

-coined the term "cell"

the first to describe bacteria

-called them "wee animalcules"

Discovered... 

-living organisms discriminate between optical isomers

-alcoholic fermentation

disproved theory of spontaneous generation

-developed vaccines for anthrax, fowl cholera, rabies

-defeat of the theory of "spontaneous generation" 

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-demonstrated links between microbes and infectious diseases

-cultured anthrax(disease in cows) and tuberculosis

-known for Koch's Postulates

1. The disease-causing organism must always be present in animals

suffering from the disease but not in healthy animals.

2. The organism must be cultivated in a pure culture away from

the animal body.

3. The isolated organism must cause the disease when inoculated

into healthy susceptible animals.

4. The organism must be isolated from the newly infected animals

and cultured again in the laboratory, after which it should

be seen to be the same as the original organism.

developed the enrichment culture technique, he used this to isolate the aerobic nitrogen-fixing bacteria Azotobacter from soil

-described the first virus 

microorganisms are isolated from natural samples

using highly selective techniques of adjusting nutrient and incubation

 conditions to favor a particular metabolic group of organisms.

-demonstrated that specific bacteria are linked to specific biogeochemical transformation

(Sulfur & Nitrogen Cycle) 

-proposed chemolithotrophy

the oxidation

of inorganic compounds to yield energy.

the identification and analysis of

genetic material (DNA) in living cells