Question: When was micro-organisms discovered?
About the article "Microscope", section "Rise of modern light microscopy".

Candidate passages:
Pathogenic micro-organisms include bacteria, fungi, and viruses.
They kill or debilitate their host and are relatively host-specific.
Various microbial insect diseases occur naturally, but may also be used as biological pesticides.
When naturally occurring, these outbreaks are density-dependent in that they generally only occur as insect populations become denser. 
Discoveries and inventions.
Rock medicine.
Hamaker believed remineralizing the world’s soil with rock dust, a quarrying by-product, could revitalise barren soil and reverse climate change.
Rock dust nourished soil micro-organisms whose protoplasm is the basis of all living things.
When mixed with compost, the dust created rich, deep soils which could produce high growth vegetation free from pests and predators, at an accelerated rate.
The idea was later confirmed by agricultural scientists such as Arden Andersen, who showed how high sugar and mineral levels in soil gave immunity to soil bacteria, stopping insect and fungal attacks.
For Hamaker and Andersen, minerals were the primal food for micro-organisms which provided life and health for the soil. 
Small aquatic insects, micro crustaceans and a variety of planktonic organisms. 
As an ecologist and zoologist he was interested in unusual and small habitats, which were only poorly studied or completely unknown concerning their biodiversity.
In such micro-habitats, he often discovered nematodes and mites as well as some other organisms.
He supervised students, who studied these organisms in detail.
That way, his students became specialists for designated animal groups.
They formed the so called "Stammer-School".
Besides free living and non-parasitic organisms, Stammer also made research in the field of parasitology.
The evolutionary biologist Günther Osche started his career with a nematode-research (genus Rhabditis) having Stammer as his scientific supervisor .
Stammer was honored with several species, which were named after him, for example: Nitocrella stammeri Chappuis, 1938; Zausodes stammeri Jakobi, 1954; Lohmanella stammeri Viets, 1939 or Histiostoma stammeri Scheucher, 1957. 
The aim of the current regulations is to keep the number of micro-organisms low enough so that the probability of contamination of Mars (and other targets) is acceptable.
It is not an objective to make the probability of contamination zero.
The aim is to keep the probability of contamination of 1 chance in 10,000 of contamination per mission flown.
This figure is obtained typically by multiplying together the number of micro-organisms on the spacecraft, the probability of growth on the target body, and a series of bioload reduction factors.
In detail the method used is the Coleman-Sagan Equation.formula_1.where   formula_2 = the number of micro-organisms on the spacecraft initially  formula_3 = Reduction due to conditions on spacecraft before and after launch  formula_4 = Probability that micro-organisms on the spacecraft reach the surface of the planet  formula_5 = Probability that spacecraft will hit the planet - this is 1 for a lander  formula_6 = Probability of micro-organism to be released in the environment when on the ground, usually set to 1 for crashlanding.
formula_7 = Probability of growth.
For targets with liquid water this is set to 1 for sake of the calculation.
Then the requirement is formula_8The formula_9 is a number chosen by Sagan et al., somewhat arbitrarily.
Sagan and Coleman assumed that about 60 missions to the Mars surface would occur before the exobiology of Mars is thoroughly understood, 54 of those successful, and 30 flybys or orbiters, and the number was chosen to endure a probability to keep the planet free from contamination of at least 99.9% over the duration of the exploration period.