How has tuberculois/staph infection become resistant to antibiotics?

In terms of natural selection, the ones that are not killed by the antibiotic get to survive and reproduce.

There are many antibiotics being used today: from giving them to animals to produce more to giving them to people who do not really need them. Also there are the ones that do not finish their prescriptions so some germs remain that were not killed, and when prescriptions are dumped in the toilet, the go to the sewage treatment plant and have an effect on the bacteria there which are supposed to be decomposing the fecal matter there. If the animal was treated with antibiotic, you get some in the meat or milk and bacteria become used to it.

In E. coli, it is thought that there is some conjugation between them and some salmonella, passing on some resistance