Cross-resistance

thumb|321x321px Cross-resistance is when something develops resistance to several substances that have a similar mechanism of action. For example, if a certain type of bacteria develops antimicrobial resistance to one antibiotic, that bacteria will also have resistance to several other antibiotics that target the same protein or use the same route to get into the bacterium. A real example of cross-resistance occurred for nalidixic acid and ciprofloxacin, which are both quinolone antibiotics. When bacteria developed resistance to ciprofloxacin, they also developed resistance to nalidixic acid because both drugs inhibit topoisomerase, a key enzyme in DNA replication. Due to cross-resistance, antimicrobial treatments like phage therapy can quickly lose their efficacy against bacteria. This makes cross-resistance an important consideration in designing evolutionary therapies.

== Definition == Cross-resistance is the idea is that the development of resistance to one substance subsequently leads to resistance to one or more substances that can be resisted in a similar manner. It occurs when resistance is provided against multiple compounds through one single mechanism, like an efflux pump. This can keep concentrations of a toxic substance at low levels and can do so for multiple compounds. Increasing the activity of such a mechanism in response to one compound then also has a similar effect on the others. The precise definition of cross-resistance depends on the field of interest.