by Manuel Sánchez Angulo
This articles is reprinted in translation from the exceptional blog Curiosidades de la Microbiología, courtesy of the blogmaster, Manuel Sánchez. We thank him most sincerely for giving us his permission.
If there is one thing that draws attention to the study of eukaryotes, it is the enormous diversity of their morphology, from the largest sequoias to the smallest radiolaria. Prokaryotes, on the other hand, seem rather simple and boring. If you open page 109 of the 7th edition of Murray, Rosenthal, and Phaller's "Medical Microbiology" you will find that there are only four cell types: cocci, bacilli, curved bacilli, and spirochetes. In fact, that's because bacteriology has traditionally been a medical discipline, and those are the typical morphological types of pathogenic bacteria. What also helped to firm up the idea that the bacteria are either bacilli or cocci is that the model microorganisms of molecular biology are Escherichia coli, Bacillus subtilis and Streptococcus pneumoniae. When one observes bacteria in the environment, however, the diversity of sizes and shapes found has nothing to begrudge that of other living beings.
This is precisely what is described in the article "Diversity Takes Shape: Understanding the Mechanistic and Adaptive Basis of Bacterial Morphology" by Kysela, Randich, Caccamo, and Brun. The authors not only offer a catalog of the various bacterial forms, but they also discuss their importance in evolution. Recall that in biology one of the most important paradigms is that of structure-function. Thus, the morphology of a living being has probably helped it survive in the habitat it occupies. The shape of bacteria has been shaped by natural selection, so understanding how that diversity may have come about can give us insights into how bacteria work.
Bacterial morphology was initially used as an important taxonomic criterion. Any bacteria with distinguishing characteristics, such as having a spiral form, possessing prosthecae (distinct cellular appendages), being phototrophic and helical, grouped in clusters, etc., were lumped together in higher or lower taxa. However, with the arrival of molecular phylogeny, many of those taxa had to be "restructured." In the article, the authors give the example of the Betaproteobacteria Rhodocyclus tenuis and Rubrivivax gelatinosus that previously were included within the genus Rhodospirillum in class Alphaproteobacteria. This is as if spiders had been erroneously been classified as insects.
Combining metagenomic and morphological data results in a phylogenetic tree that illustrates how the shape of bacteria in different phyla has evolved (see Fig. 1). Shapes such as helical (green) or filamentous (red) appear scattered throughout the tree. This indicates that this type of shape arose independently and repeatedly in evolution. In such a case where two different lineages come to have the same morphology, is that form generated by a convergent molecular strategy? Or is it because the species have similar lifestyles?
In other instances, a given morphology becomes grouped in a region of the tree, for example, the branches of the Actinobacteria (purple) or the appendages of the Caulobacteria (blue). Here, the morphology has been inherited from a common ancestor to all that grouping. The questions then are somewhat different: is the persistence of this form due to selective pressure? If not all members of that group are equal, how and why can morphological variations arise?
What is clear is that we don't know why some bacteria have a certain shape and not another. In general, morphological features can be attributed to adaptation to selective forces as diverse as nutrient acquisition, surface adhesion, dispersion, predator evasion, or colonization of a host. If we look at the shapes that appear in the phylogenetic tree, some are easy to associate with a certain lifestyle. For example, helical shapes are optimal for swimming in viscous liquids; large filaments usually are an adaptation to avoid being swallowed by protists; branching in an aquatic bacterium modifies its buoyancy, allowing it to control depth and thus to be placed in a nutrient-rich zone of the water column. Some shapes even serve to fulfill more than one function. Herpetosiphon is a bacterium that forms long filaments that intersect each other. This prevents it from being devoured by protozoa, but it also allows it to create a network with "holes" where hydrolytic enzyme secretions serve to lyse other bacteria, on which it feeds.
But how do bacteria acquire their shape?
The truth is that we know rather little, since what is known is based on the study of a few bacteria: the cocci Streptococcus pneumoniae or Staphylococcus aureus; the rod-shaped Escherichia coli or Bacillus subtilis, and the curved/spiral-shaped Helicobacter pylori, Caulobacter crescentus, or Borrelia burgdorferi. Not much is known about the rest of bacterial shapes. So perhaps we should rethink the question and figure out if there is a simpler one that we can answer. For example:
What aspect of morphogenesis do all bacteria have in common?
- All have peptidoglycan walls (except for mycoplasmas, thermoplasmas, and perhaps a few others) and their shape, at the molecular level, is determined by this macromolecule. So it is not surprising that the biochemical machinery responsible for the synthesis of the wall has been preserved among the various bacterial phyla. At the molecular level, this enzymatic equipment is localized in a certain "zone" of the bacterium, which is where it exerts its function. This could be called "zonal synthesis"
- The wall synthesis machinery must not only ensure that shape is maintained and the size of the bacterium increases due to elongation, it must also allow cell reproduction by septation. Changing the orientation of peptidoglycan synthesis permits transition from elongation to septum formation. But it is necessary that both processes remain coordinated in space and time to ensure the survival of the bacteria.
We could say that these are the "ground rules" that all bacteria have to follow. Perhaps each bacterial group has some specific rules that explain morphological differences. Or perhaps, one might expect that if two bacteria have a bacillary form it will be because they both follow the same set of rules. The best example is Escherichia coli (Gram negative) and Bacillus subtilis (Gram positive). Both elongate thanks to placement of their enzymatic equipment in areas that follow a spiral pattern, and thus synthesize the new peptidoglycan following this path.
The fact that two phylogenetically distinct bacteria have solved the problem in the same way points to the 'solution' having appeared long ago. But when we observe in detail what happens in other taxonomic groups (see Fig. 3), what at first sight seems to be an evolutionary conservation is actually an evolutionary convergence. Within each species we find a myriad of shapes. Observing an evolutionary convergence tells us something else: if the zonal synthesis machinery is repositioned, a great diversity of bacterial forms can be generated, all of them derived from the rod-shaped morphology. For example, if the zonal synthesis is restricted to only one of the poles, we can obtain a prostheca such as in C. crescentus. In contrast, if the zonal synthesis is distributed along the length of a filament, we get the branching observed in the Actinobacteria. The various bacterial shapes result from using the same basic rules, just differently.
Combining single-cell genomics with high-resolution microscopy, the research group of Yves Brun has been able to study the evolution of the prostheca among species of Caulobacterales. The phylogenetic tree below shows that the polar prostheca (present in C. crescentus) appeared first. In some species this prostheca has become a zone specialized for budding (Hirschia baltica), but in others this zone has been lost and the bacterium has returned to a rod shape (C. segnis). In other species, the zonal synthesis was repositioned in a subpolar region (Asticcacaulis excentricus), and from this group it was re-positioned to a bilateral position, thus increasing the number of prosthecae (A. biprosthecum). Upon closer examination of this transition, the researchers found a protein called SpmX that is responsible for coordinating the synthesis of peptidoglycan in the prosthecae. This protein has a domain that varies in each of these species, and directs the location of the zonal synthesis.
Could the great diversity of bacterial shapes be explained by zonal synthesis of peptidoglycan ? Indeed, it is quite likely that the combination of genomic methods and new microscopic imaging technologies will allow novel experimental approaches to answer this question. Such studies may tell us why bacteria look the way they do. They may even address the problem of relating bacterial shape to the forces of selection that act on it. One could certainly say that this field of research is taking shape.
Extended legend to Fig. 1. Black dots denote ancestral nodes of selected major taxa: DT Deinococcus-Thermus; Ac Actinobacteria; Cf Chloroflexi; Cn Cyanobacteria; Fi Firmicutes (inclusive of Mollicutes); Sp Spirochetes; PVC Planctomycetes, Verrucomicrobia, Chlamydiae; Cb Chlorobi; Bd Bacteroidetes; α, β, γ, δ, ε, Proteobacteria subdivisions. 1 Bifidobacterium longum. 2 Streptomyces coelicolor (mycelial [=multicellular] filament with hyphae and spores). 3 Corynebacterium diphtheriae (two cells, dumbbell and club shapes). 4 Herpetosiphon aurantiacus (filament of multiple cylindrical cells). 5 Calothrix (filament of multiple disk-shaped cells). 6 Mycoplasma genitalium. 7 Spiroplasma culicicola. 8 Lactococcus lactis (predivisional cell). 9 Borrelia burgdorferi. 10 Gimesia maris (previously Planctomyces maris, predivisional cell with proteinaceous stalk). 11 Prosthecochloris aestuarii. 12 Pelodictyon phaeoclathratiforme (filament of multiple trapezoidal cells). 13 Spirosoma linguale. 14 Muricauda ruestringensis (appendage includes nonreproductive bulb). 15 Desulfovibrio vulgaris (two cells, helical and curved shapes). 16 Helicobacter pylori. 17 Caulobacter crescentus (predivisional cell). 18 Hyphomonas neptunium (predivisional cell). 19 Rhodomicrobium vannielii (filament of multiple ovoid cells, one is predivisional). 20 Prosthecomicrobium hirschii. 21 Simonsiella muelleri (filament of multiple curved cells). 22 Nevskia ramosa (two cells with bifurcating slime stalk). 23 Beggiatoa leptomitiformis (filament of multiple, giant cylindrical cells). 24 Thiomargarita nelsonii (single giant cell). 25 Escherichia coli. 26 Mariprofundus ferrooxydans (single cell with metal-encrusted stalk). Bacterial schematics are not to scale. Species names are colored according to morphology as indicated in the key. Colored dots are appended to indicate species with multiple morphologies. Names of species depicted in schematics are emphasized in large, bold font.