Bacterial resistance refers to the ability of a bacterium to develop a tolerance for a particular antibiotic. The occurrence of resistance can occur through spontaneous mutation in the DNA of the bacterium and/or the transfer of antibiotic-resistant genes. There are several mechanisms that have developed in bacteria to achieve antibiotic resistance. Currently there are three mechanisms that are understood. The bacterium develops the ability to chemically modify the antibiotic so that the antibiotic no longer recognizes the bacterium. In another mechanism, the bacterium has developed the ability to inactivate the antibiotic physically removing it from the cell. The third mechanism involves the ability of the bacterium to modify the target site on the bacterium so that the antibiotic is not recognized. It turned out that the most common mechanism is the enzymatic inactivation of the antibiotic.
Most of the resistance genes are found encoded in plasmids, which are a form of extrachromosomal DNA in bacteria. Some bacteria have the ability to transfer plasmid genes to other bacteria by conjugation, a form of sexual reproduction in bacteria.
Studies suggest that the development of resistance is inevitable when a new antibiotic is introduced. Indeed, multi-resistant strains of bacteria are widespread and, in general, there is currently no medical arsenal to combat them.
Ultimately, antibiotic resistance may be inherent in bacteria, obtained by mutation of its own DNA and/or acquired from another bacterium directly (conjugation) or by means of a bacteriophage (bacterial virus). Self-resistance can occur due to the absence of an antibiotic transport system, the absence of a target for binding the antibiotic or a hidden cell wall (which is protected by an outer membrane as a barrier to penetration, as in gram-negative bacteria).