The problem of resistance was identified early in the application of antibiotics and was initially thought to be an adaptation of the bacteria to the drug with metabolic changes. Bacteria acquire resistance only after exposure to the drug, and resistance disappears when the drug is not present. Once resistant, all the bacteria in the lesion are resistant. However, later studies have found that bacterial resistance is often due to spontaneous mutations. The mutant strains themselves generally appear unrelated to the drug. However, antibiotics can affect drug resistance in two ways: first, certain antibiotics act as mutagens to promote mutation. The second is due to the coexistence of spontaneous mutant strains and sensitive strains, although the entire flora is sensitive, but when the application of antibiotics is not reasonable enough to kill these mutant bacteria, then the sensitive strains are killed, drug-resistant strains multiply to become the dominant bacteria, the entire flora will also become drug-resistant flora. The antibiotics kill the sensitive bacteria, retain the resistant bacteria, bacterial resistance is “selected” out, this is the antibiotics “selection pressure”.