One could write a book on this... Genetic resistance mechanisms are, for instance mutations in genes of target sites (e.g. rpoB mutations (rifampicin), gyrase and topoisomerase genes (quinolones), alterations in ribosomal genes (aminoglycosides, tetracylines), …). Resistance may be acquired (genes coding for β-lactamase enzymes for example, or the mcr-1 gene, conferring colistin resistance) or may be a result of upregulation of genes (efflux pumps), down regulation (porins) or metabolic bypass (sulfonamides). Just to mention a very few of the mechanisms. I would suggest to go with one of the numerous review articles on that topic.

(Answer was given to a general question on resistance genes; question was modified thereafter))

There are a lot of papers/review articles out there that talk about antibiotic resistance mechanisms in different bacteria!