Susceptibility and sensitivity would seem to mean the same thing. However, susceptibility usually refers to the presence of targets of antimicrobial activity within a genre or species of bacteria. “E. coli (referring to all of them) are susceptible to gentamicin.”Sensitivity is measure of the concentration of an-antimicrobial necessary to demonstrate activity against a particular isolate. “This E. coli isolate (a particular clinical case) is sensitive to (a particular concentration of) gentamicin.” Resistance takes on a variety of meanings depending on the context: