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Bacteria are microorganisms of 0.3 to 2.5 µm, ubiquitous, unicellular and without nucleus, which makes them prokaryotic organisms. Their genome consists of DNA in a single chromosome and possibly extrachromosomal DNA carried by plasmids.
All bacteria form the kingdom of eubacteria, alongside the animal and plant kingdoms.
There are different elements that constitute them, found in a common way in all bacteria, or more specific to some. We can observe wall, plasma membrane, cytoplasm, capsule, flagella, spores, or pili.

Bacteria can survive independently, feed and multiply. They have a very important capacity of multiplication, and a great power of adaptation. Their mode of division is by scissiparity or by sexual reproduction (bacterial conjugation).
Bacteria are distinguished by, among other things
– the thickness of their wall (Gram + or Gram-)
– their need or not for oxygen to develop (aerobic or anaerobic)
– their shape (shell, rod, spiral,…)
– their responsibility for the development of infectious diseases or not (pathogenic or commensal germ)
– their optimal growth conditions (psychrophilic, mesophilic, thermophilic).

Not all bacteria are harmful to humans. On the contrary, some are essential to the proper functioning of living beings, in a given context. These bacteria are called commensal and are the usual hosts of a subject. On the other hand, there are bacteria known as pathogenic or opportunistic, i.e., likely to cause disease.