Tritagonist as a new term for uncharacterised microorganisms in environmental systems

‘Commensal’ originates from the Latin word ‘commensalis’ (‘eating at the same table’) and was introduced by Pierre–Joseph van Beneden in 1876 . In ecology, the term defines a relationship between two organisms, where one, the commensal, benefits without affecting the other. Thus, commensalism defines a one-way interaction. Originally, the term was used in a host-centric view for interactions where a ‘small’ species (for example, a microorganism) is provided with a habitat (for example, skin) or waste products (for example, methanol released by plants during their cell-wall metabolism ( Fall and Benson, 1996 )) as food by another, usually much larger host (for example, a mammal or a plant). However, the term commensal has now started to rather imply an unknown, benign function than a truly proven commensal, mutualistic, or antagonistic lifestyle ( Casadevall and Pirofski, 2014 ). In particular when considering interactions among microorganisms, it seems unlikely that a true commensal microorganism exists. It may thus be prudent to assume a function for any kind of microorganism and to reserve commensal for organisms that have been shown not to influence their interaction partner. To emphasise such a microbe-centric view and to imply functionality, we suggest the term tritagonist .