Label or Concept – What Is a Pathobiont?

An increasing number of microorganisms are classified as 'pathobionts' (i.e., organisms that can cause harm under certain circumstances) but there exist no universally used criteria for this definition. In particular, the term is often used for categorizing disease-associated taxa without proof of 'causality'. This creates confusion and distracts from explicitly searching for beneficial functions of these organisms that they may in fact have. Here, we discuss why this term in its current use, and its apparent simplicity, may obscure the complexity of microbe–host and microbe–microbe interactions that define (the status of) the gut ecosystem.