Decoding the mechanisms of resistance to antibiotics is essential in fighting a phenomenon, which is amplifying everyday due to the uncontrolled excessive and many times unjustified use of anti-microbial substances. At present it has become a matter of public health, together with the resistance of Mycobacterium tuberculosis to tuberculostatic or the spreading of the AIDS virus which not only affects the European countries but the entire globe. This paper presents the genic mutations taking place at the level of bacterial chromosome and inducing the resistance to antibiotics.