Why do drug-resistant strains with the same genotype have different drug-resistant phenotypes?

That is related to how many drug-resistant plasmids are replicated in bacteria, the strength of bacteria and the expression of drug-resistant plasmids. The higher the copy number of drug-resistant plasmid, the greater the expression and the stronger the drug resistance. In addition, culture conditions may also affect their expression, because inappropriate conditions are equivalent to adding a selection pressure to bacteria. At this time, the expressed bacteria will not only produce drug resistance, but also produce resistance to adverse environment.