Antimicrobial resistance (AMR) occurs when a microbe acquires a genetic mutation making it resistant to the effect of one or more antimicrobial agents that were once effective.[2] This genetic mutation may be acquired spontaneously or by gene transfer. Bacteria can become resistant to antibiotics, for which the term antibiotic resistance is used. Microbes that are resistant to multiple antimicrobials are termed multidrug resistant (MDR) (or, sometimes in the lay press, superbugs).[3]

The increasing rates of antibiotic resistant infections are due to antibiotic use both within human and veterinary medicine. Any use of antibiotics can increase selective pressure in a population of bacteria that promotes resistant bacteria to thrive and the susceptible bacteria to die off. As resistance to antibiotics becomes more common, a greater need for alternative treatments arises. Despite a call for new antibiotic therapies, there has been a continued decline in the number of newly approved drugs.[4] Common types of drug-resistant bacteria include MRSA (methicillin-resistant Staphylococcus aureus), VRSA (vancomycin-resistant S. aureus), ESBL (extended spectrum beta-lactamase), VRE (vancomycin-resistant Enterococcus) and MRAB (multidrug-resistant A. baumannii). Viruses, fungi and parasites can also become resistant to agents to which they were once susceptible. Resistance may take the form of a spontaneous or induced genetic mutation, or the acquisition of resistance genes from other bacterial species by horizontal gene transfer via conjugation, transduction, or transformation. Infection by resistant microbes may be community acquired" or healthcare associated.

Antibiotic resistance poses a grave and growing global problem: a World Health Organization report released April 2014 stated, "this serious threat is no longer a prediction for the future, it is happening right now in every region of the world and has the potential to affect anyone, of any age, in any country. Antibiotic resistance—when bacteria change so antibiotics no longer work in people who need them to treat infections—is now a major threat to public health."[5] Genes for resistance to antibiotics, like antibiotics themselves, are ancient.[6]:457–461