The role of antibiotics in the evolution of microorganisms.

Bacteria were the first living beings to appear on our planet: the most ancient fossils available, all of them were procaryotic microorganisms, developed 4, 5 billion years ago. The paleomicrobiological studies made on that kind of fossils, which are by now several hundreds in each continent, proved bacteria to have constantly evolved and to have originated the modern Eubacteria as well as the Archebacteria and the Cianobacteria. These last appeared about 2 billion years ago and, having acquired the oxygen-type photosynthesis, have caused the formation of a large amount of organic material, afterwards used by the younger organisms, and have modified the atmosphere introducing oxygen in it and conditioning in this way the other living being's evolution. From Bacteriaceae and Cyanobacteria derive the eucaryotic microorganisms (algae, fungi, protozoa, mould) and, little by little, all the other organisms both vegetable and animal subjected to the evolutionary pressures. Nevertheless bacteria undergo more frequently than all the others the evolution law because of their short reproductive time; this is the reason why bacteria are favourite compared with the other organisms. In fact each species is subjected to a genetic mutation every 10(5)/10(6) generations: in the vegetables and animals the consequences of a genetic mutation will be evident after millennia whereas in the bacteria the mutation happens in a short time. We ourselves have witnessed the revolution which took place in the bacteria populations during the last half century when numerous antibiotic-resistant strains appeared.(ABSTRACT TRUNCATED AT 250 WORDS)