Microorganisms are everywhere
A microorganism (also spelled as microrganism) or microbe is an organism that is microscopic (too small to be seen by the human eye).
Microorganisms are manifold. Microorganisms include bacteria, fungi, archaea or protists, but not viruses and prions, which are generally classified as non-living.
Microorganisms live almost everywhere on Earth where there is liquid water, including hot springs, on the ocean floor, and deep inside rocks within Earth's crust.
Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. However, pathogenic microbes can invade other organisms and cause diseases that kill millions of people every year.
Bacteria - definition
Singular bacterium: Microscopic single-cell (unicellular) life form that exists practically everywhere in the Earth's environment, and is simpler than the cells of animals, fungi, and plants. Of about three million species of bacteria believed to exist, only about 4, 000 are known.
Bacteria - aerobic - anaerobic
Oligate aerobic bacteria, are those bacterial strains that require oxygen for their survival, growth and reproduction. In short, they need oxygen for cellular respiration (e.g. Bacillus, Pseudomonas aeruginosa, Mycobacterium tuberculosis).
Contrary to this, there are anaerobic bacteria, which live in a non-oxygenated environment throughout their life, e.g. Sulfate-reducing bacteria (SRB).
Intermediary to these two groups are facultative bacteria (e.g. E. coli, Staphylococcus)