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Antibiotic resistance tests: The bacteria in the culture on the left are susceptible to the antibiotics contained in the white paper discs. The bacteria in the culture on the right are resistant to most of the antibiotics.

Antimicrobial resistance is a form of drug resistance where microorganisms such as bacteria, fungi or various protozoa are able to survive exposure to one or more antimicrobials. Antimicrobials are drugs which are used to treat bacterial as well as other microorganism infections. Thay can be divided into antibiotics which combat bacteria (and in some cases fungus and protozoa, but not viruses): antifungals; antiprotozoals such as antimalerials; andantivirals. Microorganisms that are resistant to multiple antimicrobial drugs they would normally be affected by are considered multidrug resistant (MDR) or sometimes superbugs.

Diagnosis

Resistance may occur following genetic mutation, or through various methods of horizontal gene transfer (HGT) employed various microorganisms (HGT does not occur in protozoa). Genes for resistance to antibiotics, like the antibiotics themselves, are ancient. However, the increasing rates of antibiotic-resistant bacterial infections seen in clinical practice stems from antibiotic use both within human medicine and veterinary medicine. Any use of antibiotics can increase selective pressure in a population of bacteria to allow the resistant bacteria to thrive and the susceptible bacteria to die off. As resistance towards antibiotics becomes more common, a greater need for alternative treatments arises. However, despite a push for new antibiotic therapies, there has been a continued decline in the number of newly approved drugs. Exposure to an antibiotic increases survivability of microorganisms with some restitance and selects for the survival of organisms with genes helpful against the antibiotic, promoting resistance. In this way, a gene for antibiotic resistance may readily spread through an ecosystem of bacteria, even in one individual. Once a bacterium becomes resistant to an antibiotic, it is unable to return to its previous state of vulnerability. Resistance will be passed on to all of the daughter cells of the resistant microbe. Antibiotic-resistance plasmids frequently contain genes conferring resistance to several different antibiotics.

Antibacterial resistance is considered a major health concern by the World Health Organization which states, "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."