Introduction
Two groups of flagellates that live on human mucosa without entering the blood: the trichomonads (order Trichomonadida), above all Trichomonas vaginalis, the most common non-viral sexually transmitted infection, and Giardia lamblia (order Diplomonadida), a common cause of diarrhoea and malabsorption. Both are anaerobic and lack typical mitochondria, and both respond to metronidazole. The key difference: Trichomonas has no cyst; Giardia spreads by cysts.
Genus Trichomonas
Systematic position
- Phylum Sarcomastigophora → Subphylum Mastigophora → Class Zoomastigophorea → Order Trichomonadida → Family Trichomonadidae. In modern taxonomy: Parabasalia (Metamonada).
Common features
- Pear-shaped trophozoites with 3–5 anterior flagella and one recurrent flagellum that runs back along the body, forming the outer edge of an undulating membrane, supported at its base by a rod called the costa.
- An axostyle: a stiff rod of microtubules running through the middle of the cell and sticking out at the back like a tail, which may help the cell attach.
- One nucleus at the front; a parabasal body (Golgi).
- Instead of mitochondria they have hydrogenosomes, which make ATP without oxygen and release hydrogen. These organelles activate metronidazole, which is why the drug works.
- No cysts: only the trophozoite exists. It dies quickly outside the body, so it must pass directly from person to person (sexual contact, saliva, faecal–oral).
- Reproduce by longitudinal binary fission; feed by phagocytosis of bacteria and cells and by absorption.