In order to survive, strains of bacteria have developed resistance against 1 or more antibiotics. rendering the antibiotics against these strains of bacteria.
Through spontaneous mutation, plasmids and Transposons, bacteria are able to build resistance to antibiotics and ‘transfer’ that resistance to bacteria.
Bacteria mutate. They produce genes that enable them to resist the antibiotics they have been exposed to.
Plasmids are mini-chromosomes that contain information about the environment. Plasmids can be transferred between various bacterial cells and even from 1 species of bacteria to another, rendering the other species of bacteria immune to the antibiotics too.
Transposons are smaller than plasmids and jump from plasmids to chromosome and vice versa very easily. They spread the resistance to the antibiotic even more quickly.
In this bleak situation, there is still some hope. These resistant genes are a burden to the bacteria and will be lost if that antibiotic is not used at all for a sufficient period of time. That would make the anti biotic useful again.
Thus, antibiotics should only be used when absolutely necessary.