An antibiotic sensitivity (or susceptibility) test is done to help choose the antibiotic that will be most effective against the specific types of bacteria or fungus infecting an individual person.
Some types of bacteria or fungus are resistant to certain antibiotics because of differences in their genetic material (genes). Infections caused by resistant bacteria or fungi are not cured by treatment with those antibiotics.
Drug-resistant bacteria or fungi usually develop because the entire course of antibiotic treatment was not completed. Stopping drug treatment early kills only the bacteria or fungi that are sensitive to the drugs, allowing the resistant bacteria or fungi to multiply and cause an infection.