A 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.