Antimicrobial resistance occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. The term antibiotic resistance is a subset of AMR, as it applies to bacteria that become resistant to antibiotics. Resistant microbes are more difficult to treat, requiring higher doses, or alternative medications which may prove more toxic. These approaches may also be more expensive. Microbes resistant to multiple antimicrobials are called multidrug resistant (MDR).