Bacterial resistance to drugs are of two types.
A. Transferable Drug resistance
B. Mutational drug resistance.
Transferable Drug resistance occurs by means of transduction, a process where a plasmid known as resistance factor factor (rF), having genes encoding for resistance against one or multiple drugs at the same time,is transferred from one bacterium to the other and a large population of bacteria acquires this type of resistance.(Seen in case of Shigella)
Mutational drug resistance occurs in a single bacterium due to mutation in a specific enzyme ,which is the target of a drug and that leads to modification of the enzyme receptor for that drug,hence renders the drug non functional.(Seen in case of Salmonella--topoisomerase IV mutation)
Difference is that,the previous one is severe and can't be overcome using increasing dose of drug but the later is not such.
Now what happens,this 1st type of drug resistance is purely natural,occuring in between plasmid and bacteria that are contagious or not to each other.
But we human beings can lead to the 2nd type of drug resistance by unnecessary use of antibiotics,which causes frequent mutation among bacteria and renders that drug totally non functional against such bacteria. That's why it is a common sayings now a days warning people not to use abruptly the reserved antibiotics for treating critical conditions,because using those abruptly leads to mutational drug resistance.
Besides prolonged use of a drug to treat a disease ultimately leads to development of full resistance against it by the bacteria,that's why combination therapies are receiving much more importances now a days.
As for example Artemisinin and Sulphadoxine-Pyrimethamine combination which is used to treat Falciparum malaria so that individual drugs don't develop resistance to Plasmodium falciparum.