Most of the antibiotic resistance genes in bacteria (recipient) are acquired from the other bacteria (donor) carrying those genes by direct gene transfer through cell contact. This process is called as conjugation. Sometimes bacteria get infected with viral particles which could also pass on the gene from one bacterium (donor) to another (recipient). This process is called as transduction. Environmental factors also play a role by mutating DNA during the process of replication. Nowadays, antibiotics are over-use or misuse for humans, animals and plants. So in order to survive in such environment bacteria are developing resistant to antibiotic to protect themselves.
