What determines cell size? Springer. Link. Kevin D Young.
The most obvious characteristic of bacteria is that they are small. Really small. As in requiring microscopes of high magnifying and resolving power to see them. So it surprises people to learn that the volume of these normally tiny cells can differ by as much as 1. Pelagibacter SAR1.
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Thiomargarita and Epulopiscium in which some species measure over 6. Of course, large bacteria are an extreme minority, with most known bacteria falling somewhere between 0.
Another conceit is that bacteria are boring, at least in morphological terms. But this is just because most of us rarely encounter bacteria outside of what are usually brief episodes of disease, and the shapes of these common bacteria are admittedly pretty lame, being, as they are, no more than tiny cylinders. However, on a more global scale, bacterial shapes range from the plain (rods, spheres, strings) to the outlandish (branched, curved, coiled, spiraled, star- shaped), to the truly bizarre (fluted and tentacled) [5]. Given this range of possibilities, what determines the morphology of any one bacterium? The first determinant is, as always, evolutionary.
Bacteria cope with at least six fundamental selective forces that have some degree of control over the size that will best suit them to survive in particular environments. Specifically, bacteria adopt certain sizes and shapes so they can import nutrients most efficiently, meet requirements imposed by cell division, attach themselves to external surfaces, take advantage of passive dispersal mechanisms, move purposefully to pursue nutrients or avoid inhibitors, or avoid predation by other organisms [5, 6]. Fundamental to all these considerations is that bacteria must accumulate nutrients that reach them by diffusion alone [7]. A basic tenet is that for such cells to exist the ratio of their surface area to cytoplasmic volume has to be quite high. Therefore, to maximize this ratio, most bacteria produce cells in the 0. Because of this reliance on diffusion, those bacteria that reach near- millimeter size do so by employing clever morphological tricks. For example, some deploy their cytoplasm as a thin film around the outer rim of a large internal vacuole, creating a cell that looks very much like the skin of a balloon [2, 9].
Others localize tens of thousands of chromosomes around the periphery of their cytoplasm, in near contact with the cell surface, so that each genomic equivalent 'governs' a volume approximately equal to that of a more normal, smaller cell [4]. Where a particular bacterium will eventually land in this size universe depends on other selective forces, which basically revolve around a bacterium's need to put itself in position to reach any nutrients at all versus the need to defend itself against becoming a nutrient for others.
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The second determinant of bacterial morphology is mechanical, a factor that encompasses the biochemical mechanisms that do the heavy lifting of constructing cells of defined sizes and shapes. The current consensus is that morphology is determined primarily by molecular machines that synthesize the rigid cell wall. Three major types of machines are available. One, directed by the protein Fts.
Z, is responsible for nucleating the process of cell division and is shared by all bacteria, while the other, directed by the protein Mre. B and its homologues, is responsible for cell elongation in rod- shaped bacteria [1. The third, first recognized by the activity of the Cre.
S (crescentin) protein of Caulobacter crescentus, is responsible for creating the curved cells of this organism and the more regular shapes of other bacteria [1. In a series of conceptual surprises, it was realized that Fts. Z is a homologue, and perhaps progenitor, of the eukaryotic cytoskeletal protein tubulin [1. Mre. B is a homologue of actin [1. Cre. S and its relatives are homologues of intermediate filaments, a third class of eukaryotic cytoskeleton proteins [1.
Though the structural similarities are clear, these proteins have been co- opted to perform different functions in bacteria. One last curiosity deserves mention: some classic metabolic enzymes also moonlight as cytoskeletal filaments that affect bacterial shape, a discovery with potentially far- reaching implications [2. Finally, these basic tools can be modified, supplemented or differentially regulated to create morphologies from the simple to the quite complex. There is room here to give only three brief examples of how rod- shaped bacteria control their overall size by varying cell length.
The first involves Escherichia coli, a plain cylindrical rod that is normally about 1 μm in diameter and 2 μm long. In this organism, the future division site is determined by at least two mechanisms, each of which inhibits the polymerization or function of Fts. Z and thus regulates when and where cell division occurs. First, driven by the Min. D and Min. E proteins, the Min. C inhibitor oscillates back and forth between the two polar ends of the cell, taking approximately 1 to 2 minutes per cycle [2. This behavior creates a time- averaged Min.
C concentration gradient that is highest at the poles and lowest near mid- cell. As the cell elongates, the concentration near the cell's center is reduced until it becomes so low that Fts. Z can polymerize and initiate cell division.
Therefore, cell size (as measured by length) is determined by the amount of Min.C - larger amounts produce longer cells. The Sims Medieval Updates On Glen .
Conceptually, this is eerily similar to the mechanism that regulates cell length in rod shaped fission yeast, as described by Swaffer et al. Forum article (below).
Though there are biochemical differences, in this eukaryote cell length is regulated by a concentration gradient of Pom. Division is therefore inhibited until the cells become long enough so that the concentration of Pom. The second way E. Here, the Slm. A protein binds to specific DNA sequences, and the Slm. A- DNA complex prevents cell division by inhibiting Fts. Z. Interestingly, Slm. A binding sites are distributed around the chromosome except near the area where DNA replication terminates.
During chromosomal segregation the two origins are pulled to either pole, and the two termination regions remain near the cell center, where they are the last to be replicated and separated. This means that as replication ends and when segregation is almost complete there will be a dearth of Slm.
A near mid- cell, at which time Fts. Z will no longer be inhibited and can trigger division. Again, note how similar this is to the kind of mechanism that may explain how chromosomal ploidy determines cell length in yeast (see the contribution from Swafer et al. Forum article, below). Recently a third, and surprising, mechanism was discovered by which cell length is tied to the metabolic status of the cell. Bacillus subtilis, a rod shaped bacterium about 1 to 2 μm in diameter and 5 to 1. Although it sounds simple, the question of how bacteria accomplish this has persisted for decades without resolution, until quite recently.
The answer is that in a rich medium (that is, one containing glucose) B. Fts. Z (again!) and delays cell division. Thus, in a rich medium, the cells grow just a bit longer before they can initiate and complete division [2. These examples suggest that the division apparatus is a common target for controlling cell length and size in bacteria, just as it may be in eukaryotic organisms. In contrast to the regulation of length, the Mre. B- related pathways that control bacterial cell width remain highly enigmatic [1. It is not just a question of setting a specified diameter in the first place, which is a fundamental and unanswered question, but maintaining that diameter so that the resulting rod- shaped cell is smooth and uniform along its entire length.
For some years it was thought that Mre. B and its relatives polymerized to form a continuous helical filament just beneath the cytoplasmic membrane and that this cytoskeleton- like arrangement established and maintained cell diameter. However, these structures seem to have been figments generated by the low resolution of light microscopy. Instead, individual molecules (or at the most, short Mre. B oligomers) move along the inner surface of the cytoplasmic membrane, following independent, almost perfectly circular paths that are oriented perpendicular to the long axis of the cell [2. How this behavior generates a specific and constant diameter is the subject of quite a bit of debate and experimentation.