Metronidazole is a (5-)nitroimidazole drug. This means anti-protozoal with anti-bacterial benefits. A combination drug. It is very effective against some obligate anaerobic bacteria. It has anti-inflammatory properties that reduce swelling and when swelling is caused by parasites it is often chosen as the appropriate medication but it is primarily anti parasitic. In fact metronidazole is a very rare example of a drug developed against a parasite which is now used as often as an antibacterial med.
The antibacterial activity of metronidazole was only however discovered by accident in 1962 when metronidazole cured a patient of both trichomonad vaginitis and bacterial gingivitis
It is used to treat intestinal worms and ameba in people and animals. It has been used for decades on protozoan parasites like Entamoeba histolytica. It was first used and is still used to cure dysentry in people caused by intestinal parasites. It is the medication usually used to treat amebic abscesses as well. It is used to treat Entamoeba histolytica, which is one of the most serious of all parasitic infections of humans. It is also used to treat trichomonas -another parasite that that is an STD.
Metro is trichomonacidal and amebicidal in action. This means it is readily taken up by anaerobic organisms and cells. Its selectivity of anaerobic (only) bacteria is because anaerobic bacteria have the ability to absorb it like they do nitrogen, and in doing so they reduce metronidazole to its active form intracellularly. It does not work on aerobic bacteria which utilize oxygen and reject it.The electron transport proteins necessary for the chemical reaction that mutates the DNA and therefore kills the organism are found only in anaerobic bacteria and protozoan parasites. Reduced metronidazole disrupts DNA's helical structure, thereby inhibiting bacterial nucleic acid synthesis.
Because of this distinctive method it has immuno-suppressive and anti-inflammatory properties. It is used to treat some obligate anaerobic bacteria that cause for example Crohn's disease. It is often referred to as an antibiotic but this name excludes it's original anti-protozoal properties for which it has been used widely and is still used for and it is therefore not an ideal definition.