Bacterial Cell Shape: Some Features of Ultrastructure, Evolution, and Ecology

Abstract

The bacterial cell shape and size are influenced by many selective forces. Therefore, on one hand, there is a wide morphological diversity of bacteria, and, on the other hand, a certain shape is strictly defined for one or another group of them. Why are they rods, cocci, spiral and filamentous? A number of studies have shown that the rod shape seems to be the optimal in some of the most important parameters, e.g., the efficiency of intracellular transport and active movement in aquatic environment. Most researchers are of the opinion that rods are the ancestral form of all other shapes of bacterial cells. Cocci presumably formed as a result of the loss of some cytoskeletal proteins or are the result of “reductive division.” A spiral shape can form as a result of controlled lysis of some cross-linked peptide chains between the glycan strands of the peptidoglycan, or it can be maintained simply due to the spirally oriented elements of the cytoskeleton. One possible advantage of such a shape is effective movement in viscous media, including the internal media of other living organisms. Filamentous and branching forms can probably be considered an adaptation for an increase in the nutrient-absorbing surface area of the cell, and it is also beneficial for fixing in the soil, functional differentiation, swarming, etc. The vibrio cell shape is formed by the limitation one-sided growth and, for a number of physical reasons, it appears to be the most effective form for chemotaxis. The morphological diversity of gram-positive (monoderm) and gram-negative (diderm) bacteria is markedly different. Differences in the cell-wall structure are apparently the limiting factors for some cell shapes in a certain group of bacteria: e.g., spiral in gram-positive, filamentous in gram-negative. The cell shape can change during the bacterial life cycle, especially in terms of unfavorable environmental conditions. For example, the SOS-response filamentation of cells or the coccoid formation by many gram-negative species, are functionally comparable to sporulation, typical for gram-positive ones. At the same time, the cell shape is inherited and characterizes taxa of a sufficiently high rank, which indicates the great adaptive value of this trait in evolution.

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