Conceptual Foundations of Cell Mortality.

The new era of microbial cell death stems from a flood of new information emanating from the mechanistic and evolutionary life sciences, philosophy, and even sociology. In the shifting landscape, longstanding cell death terminologies and concepts have rightfully been questioned. There is currently very little consensus on how these concepts should be defined. One result of this is that similar findings often prompt different explanations because of the diversity of meanings associated with the terms. In this chapter, we review terms and concepts in microbial cell death that are key to understanding cell mortality. We discuss concepts like cell death, mortality, and the distinction between endogenous and exogenous death. We examine the contentious problem of defining programmed cell death (PCD) and argue that an evolutionary concept of PCD is foundational and applies to all cells across the tree of life, including microbial taxa. Alternative conceptions that define PCD in mechanistic, developmental, and ecological terms are useful tools for dissecting the molecular mechanisms, environmental triggers, and functions of PCD, but they do not define what PCD fundamentally is. Finally, we emphasize the importance of being clear on such concepts in order to achieve an overarching cell mortality framework.