Multidrug resistance is the ability of disease-causing organisms to withstand a wide-variety of structurally and functionally distinct drugs or chemicals that are designed to aid in the eradication of such organisms. These organisms can be pathologic cells, including bacterial and neoplastic (tumor) cells.
Bacterial resistance to antibiotics
Bacteria have been able to adapt so that antibiotics are no longer effective. They have done this via several mechanisms.
- No longer relying on a glycoprotein cell wall.
- Enzymatic deactivation of antibiotics
- Decreased cell wall permeability to antibiotics
- Altered target sites of antibiotic
- Efflux mechanisms to remove antibiotics
- Increased mutation rate as a stress response 
Many different bacteria now exhibit multidrug resistance, including staphylococci, enterococci, gonococci, streptococci, salmonella, Mycobacterium tuberculosis and others. Additionally, some resistant bacteria are able to transfer copies of DNA that codes for a mechanism of resistance to other bacteria, thereby conferring resistance to their neighbors, who then are also able to pass on the resistant gene.
To limit the development of antibiotic resistance:
- Only use antibiotics for bacterial infections
- Identify the causative organism if possible
- Use the right antibiotic; don't rely on broad range antibiotics
- Don't stop antibiotics as soon as symptoms improve; finish the full course
- Most colds, coughs, bronchitis, sinus infections, and eye infections are viral; do not use antibiotics
To help this process governments need to legislate greater restrictions on use of antibiotics, particularly for treating animals such as battery hens.
Cancer cells also have the ability to become resistant to multiple different drugs, and share many of the same mechanisms:
- Increased efflux of drug (as by P-glycoprotein, multidrug resistance-associated protein, lung resistance related protein, and breast cancer resistance protein)
- Enzymatic deactivation (i.e. glutathione conjugation)
- Decreased permeability (drugs can't enter the cell)
- Altered binding-sites
- Alternate metabolic pathways (the cancer compensates for the effect of the drug)
Because efflux is a significant contributor for multidrug resistance in cancer cells, current research is aimed at blocking specific efflux mechanisms. Treatment of cancer is complicated by the fact that there are such a variety of different DNA mutations that cause or contribute to tumor formation as well as a myriad of mechanisms by which cells resist drugs. There are also certain notable differences between antibiotic drugs and antineoplastic (anticancer) drugs that complicate designing antineoplastic agents. Antibiotics are designed to target sites that are specific and unique to bacteria, thereby harming bacteria without harming host cells. Cancer cells, on the other hand, are altered human cells, and therefore they are much more difficult to damage without also damaging healthy cells.
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