Entodiniomorphid Ciliates: A Neglected Component of the Intestinal Microbiota of Great African Apes

Entodiniomorphic ciliates (EC) form an important part of the microbial communities in the vertebrate digestive system. The best known EC live in the foregut of cattle (rumen ciliates), where they contribute to polysaccharide processing and methane production. EC of other herbivorous hosts inhabit the large intestine and are much less studied. This is also true for the EC of African great apes, our closest relatives. Although 16 species of EC have been described from the colon of African great apes, we know very little about most of them. In humans, EC are not present, but thereare theories that our ancestors had them, but with the decline in fiber intake an the invention of fire (for cooking food), EC disappeared from human intestines. Gorillas and chimpanzees in human care are fed a diet rich in starch and poor in fiber, which probably results in only one species of EC, Troglodytella abrassarti, persisting. The loss of these symbiotic ciliates, which can lead to an imbalance of the gut ecosystem, is thought to be a possible reason for the occurrence of pathologies similar to so-called civilisation diseases in great apes in human care, such as cardiometabolic diseases, which are a common cause of death. EC are anaerobic, and thanks to research on the rumen ciliates of the cattle, we know that the mitochondria of some of them have been converted into hydrogenosomes whose metabolism produces hydrogen. This hydrogen is subsequently metabolized by their prokaryotic symbionts to methane. Another important product of EN metabolism is butyrate, which is absorbed as a nutrient by vertebrate hosts. However, we still know almost nothing about the energy metabolism of EC from great apes and their syntrophic symbioses with prokaryotes, which greatly limits our understanding of the importance of EN to hosts.