In a microorganism like a virus or bacterium, multidrug resistance is an acquired resistance to more than two medications. Multidrug-resistant organisms present a significant public health threat. Patients infected with these organisms are harder to treat and are at increased risk of developing life threatening complications, especially if those patients have compromised immune systems that make it difficult for them to fight off infections on their own.
Microorganisms are designed to be highly adaptable so they can take advantage of changes in their environment and pass those changes onto other organisms to promote the survival of the species as a whole. This trait makes treatment of infectious organisms extremely challenging, because they can evolve faster than pharmaceutical companies can develop new treatments. A broad spectrum of antiviral, antibiotic, and antifungal drugs have been developed to tackle infections, with the idea that if one class of drugs fails, another may be effective.
Resistance to at least one class of drugs is not uncommon. With bacteria, for example, antibiotics target specific metabolic pathways in the organism and not all bacteria are identical. Thus, medications that are highly effective for some will have no effect on others. In multidrug resistance, however, organisms evolve tolerance to drugs that they would normally be susceptible to and this tolerance is passed on. If these organisms spread, they can exchange genetic material with organisms that have evolved resistance to other antibiotics, potentially resulting in an organism that is resistant to most classes of antibiotics. Multidrug resistance will allow an organism to survive in conditions that are normally hostile.
Drug resistance evolves as a result of exposure to pharmaceuticals. Limiting exposure to medications to times when it is absolutely needed reduces the ability to develop resistance, and patients must also complete courses of medication so that they do not inadvertently create drug resistance organisms. If a course of drugs is left unfinished, all of the organisms that were susceptible will die, while the organisms with some resistance will be left behind, and they will multiply and spread, potentially causing a recurrence of the infection in addition to being passed to other people.
One problem with multidrug resistance is that a patient may need to be treated with several drugs before an effective medication can be found. This takes time and patients can grow sicker while doctors search for an effective treatment. With infections that are commonly associated with drug resistance, such as tuberculosis, a doctor may culture a sample to find a medication to use, rather than trying several medications in the same patient. Some drug treatment regimens address the multidrug resistance issue by treating patients with several drugs at once.