The simplest life form known to date is the bacterium Candidatus Carsonella ruddii, which has only 182 genes. A human cell, on the other hand, has around 20,000-30,000 genes. However, this microbe cannot survive on its own and must therefore inhabit another organism, in this case a leaf flea. Through a symbiosis in mutual interest, it "saves" some genes. Some parasitic pathogens, which are then dependent on the infected organism, also do without such genes, which are actually indispensable.

 

With 1,354 genes, the free-swimming marine bacterium Pelagibacter ubique is the smallest independent, reproductive cell known to date. This currently represents a minimum number of genes for free-living microbes. However, scientists are working on an artificial microbe with a minimal genome.

Microbes generally have fewer genes than multicellular organisms. Sufficient genes must at least ensure viability and reproduction. Depending on their complexity and special characteristics, additional genes are added. For example, many microbes are characterised by their ability to adapt to different conditions, for which they need proteins and thus the genes coding for them to produce them. For example, a certain sugar can activate the genes required for its utilisation.

 

Some genes occur in several copies within a cell, which is why a large amount of genetic material does not automatically mean the presence of many different genes.

 

A special feature of microorganisms is the frequent occurrence of additional genetic information on plasmids. These are not normally required for the normal processes of the cell, but can provide advantages under certain conditions. One example of this is antibiotic resistance genes, which enable microbes with the corresponding plasmid to survive even in the presence of antibiotics.