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An Overview of Parasite Diversity
Published in Eric S. Loker, Bruce V. Hofkin, Parasitology, 2015
Eric S. Loker, Bruce V. Hofkin
Members of this prominent group of unicellular eukaryotes possess a mitochondrion, but often in a highly modified form. Most have flagella. Many prominent parasites are excavates: Trypanosoma, Leishmania, Naegleria, Giardia, Histomonas, Trichomonas, and Dientamoeba. Many members of the Excavata are included in the Rogues’ Gallery and are discussed elsewhere in this book. Consider Giardia lamblia, the causative agent of giardiasis, first seen by van Leeuwenhoek using his microscope to examine his own stools in 1681. We have since learned that Giardia does not possess typical mitochondria, and it was thought that this organism might have diverged from eukaryotic stock before the ancestral eukaryote had acquired the mitochondrion by primary endosymbiosis. Primary endosymbiosis in this context refers to the acquisition of a bacterium (probably an alphaproteobacterium) by an ancestral protoeukaryote, with the metabolically versatile bacterium thereafter serving as the mitochondrion. Although Giardia is still considered to be an early diverging eukaryote, we know today that it possesses a reduced version of the mitochondrion called a mitosome. Mitosomes are double-membrane structures like mitochondria and are almost certainly derived from them, but they lack mitochondrial DNA. They are incapable of aerobic respiration, in keeping with the limited oxygen environment in which Giardia lives, but they can still produce ATP. A related group of excavates, including the genus Trichomonas, also has modified mitochondria that in this case are considered to be true hydrogenosomes (they produce H 2 as a by-product), as described above.
An Overview of Parasite Diversity
Published in Eric S. Loker, Bruce V. Hofkin, Parasitology, 2023
Eric S. Loker, Bruce V. Hofkin
Members of this group of unicellular eukaryotes often live in environments where oxygen is limited and have mitochondria that are much reduced or instead have a hydrogenosome. Flagella are prominent. Several are noteworthy parasites. Consider Giardia lamblia, also referred to as G. intestinalis or G. duodenalis (Figure 2.9A), the causative agent of giardiasis, first seen by van Leeuwenhoek using his microscope to examine his stools in 1681. We have since learned that Giardia does not possess typical mitochondria, and it was originally thought that this organism might have diverged from eukaryotic stock before the ancestral eukaryote had acquired the mitochondrion by primary endosymbiosis. Primary endosymbiosis in this context refers to the acquisition of a bacterium (probably an alpha-proteobacterium) by an ancestral proto-eukaryote, with the metabolically versatile bacterium thereafter serving as the mitochondrion. Although Giardia is still considered to be an early diverging eukaryote, we know today that it possesses a reduced version of the mitochondrion called a mitosome. Mitosomes are double-membrane structures like mitochondria and are almost certainly derived from them, but they lack mitochondrial DNA. They are incapable of aerobic respiration, in keeping with the limited oxygen environment of the small and large intestine in which Giardia lives, but they can still produce ATP. A related group of metamonads, members of the genus Trichomonas, also have modified mitochondria that in their case are considered to be true hydrogenosomes (they do not require oxygen but can still produce ATP, with H2 as a by-product), as described above. Trichomonas tenax inhabiting the mouth is associated with and can worsen periodontal disease. Trichomonas vaginalis is responsible for the common, sexually transmitted infection known as trichomoniasis or “trich,” and is the protist most commonly associated with pathogenicity in industrialized countries. A related species, Pentatrichomonas hominis lives in the large intestine and caecum and is occasionally implicated in causing diarrhea.
Therapeutic targets for the treatment of microsporidiosis in humans
Published in Expert Opinion on Therapeutic Targets, 2018
The genome of most microsporidia is highly reduced and their genome size is reported to be among the smallest of eukaryotic organisms [104]. In addition, microsporidia do not have typical mitochondria, but possess mitosomes, a mitochondrial remnant which cannot perform oxidative phosphorylation and microsporidia lack the tricarboxylic acid cycle [105–107]. This requires microsporidia to rely on other metabolic pathways such as the glycolytic pathway, pentose phosphate pathway, and trehalose metabolism for energy [108]. Thus, the enzymes involved in these pathways, which are critical for the microsporidia, are potential therapeutic drug targets.