Although this simple approach is a convenient way to describe commonly studied species, it is far from an accurate account of the true diversity of bacterial cell shapes. Many of the best-known bacteria fall within these simple categories: Escherichia coli and Bacillus subtilis are simple rods Helicobacter pylori is a curved cell and Staphylococcus aureus is a typical coccus. We will also give a perspective on the study of cell shape going forward including experimental methods and ecological considerations required to move towards understanding not just how but why these significant transitions take place.īacterial cell shapes today and in the ancient pastīacteria are commonly classified as rods, curved cells, or spheres (cocci). The key to understanding the evolution of spherical cell shape is an understanding of the bacterial cell envelope and the bacterial cytoskeleton, particularly FtsZ, the division determinant of both spherical and rod-like cells, and MreB which is the main cytoskeletal determinant of the rod-like shape in bacteria. Here, we will summarise some recent findings on the evolution of spherical cell shape focusing on the molecular players and their roles. We have hints, based on phylogenetic and phenotypic work on extant bacteria, as to how these rod-like predecessors might have become spherical, and we can put a lower limit on the number of times that this has happened independently. If bacteria can be said to have something like a shared last common ancestor, the shape of that ancestral bacteria was most likely to have been a rod. It is remarkable then that after many hundreds of years of classifying and characterising bacteria according to shape, we have just started to approach the study of how and why cell shape evolves. Motility, DNA segregation, nutrient acquisition, waste disposal, surface attachment, size, predation, and parasitism - these are all aspects of microbial life that are significantly affected by cell shape. The shape of a bacterial cell is defined by the physical limits of the cell, but cell shape likewise imposes limits on many crucial bacterial processes. Which is a shame, because the bacteria seem to care very much.” - Young, K.D. “ To be brutally honest, few people care that bacteria have different shapes. Here we will highlight what is known of this particular transition in cell shape and how it affects fitness before giving a brief perspective on what will be required in order to progress the field of cell shape evolution from a purely mechanistic discipline to one that has the perspective to both propose and to test reasonable hypotheses regarding the ecological drivers of cell shape change. MreB is particularly relevant in the transition between rod-like and spherical cell shape as it is often (but not always) lost early in the process. The proteins that are primarily responsible for cell shape are therefore the elements of the bacterial cytoskeleton, principally FtsZ, MreB, and the penicillin-binding proteins. In order to understand its evolution, we must first understand how this trait is actively maintained through the construction and maintenance of the peptidoglycan cell wall. Rod-like cell shape appears to be original which implies that the cell wall, division, and rod-like shape came together in ancient bacteria and that the myriad of shapes observed in extant bacteria have evolved from this ancestral shape. Bacterial cell shape is a key trait governing the extracellular and intracellular factors of bacterial life.
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