<i>Tritagonist</i> 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.