Finally, didermic firmicutes appear to retain ancestral systems for the biogenesis of their outer membranes. Moreover, these two groups have envelope appendages (such as flagella and pili) that resemble the envelope appendages of other diderms (in other phyla) more than they resemble those of their close monodermic relatives. Notably, and unusually, most of the genes required for the biogenesis of the outer membrane clustered in a large genomic region in both groups. to make inferences about the nature and evolution of their didermic envelopes. Endospores are shown as cells within cells.Ĭomparative analyses of the genomes of Negativicutes and Halanaerobiales also allowed Antunes et al. Ancestral sporulating diderms (similar to the Negativicutes and the Halanaerobiales) convergently gave rise to classical sporulating monoderms (e.g., Bacillus and Clostridium), which lost the capacity to form endospores in some linages (e.g., Lactobacillus). ( B) Major transitions between bacterial cell plans within the Firmicutes phylum. show that the ancestral didermic cell plan of the Firmicutes phylum has been lost at least five times. Most lineages lost their outer membranes to become monoderms (thick gray lines), but the Negativicutes and the Halanaerobiales retained the ancestral didermic cell plan (thick green lines). ( A) Didermic firmicutes have a cytoplasmic membrane (shown in blue), a peptidoglycan cell wall (gray) and an outer membrane (green), whereas monodermic firmicutes have a cytoplasmic membrane and a peptidoglycan cell wall, but no outer membrane. Antunes et al. These results suggest that the monodermic firmicutes evolved at least five times from an ancestral and more complex didermic cell plan ( Figure 1). Instead, the outer membrane of the didermic firmicutes appears to have been inherited vertically from a distant ancestor. Furthermore, they demonstrate that the biosynthetic machinery for synthesizing their LPS has not been transferred between them nor acquired from elsewhere. By analyzing the genomes of more than 200 members of the phylum, they showed that the two didermic groups – the Negativicutes and the Halanaerobiales – are not each other's closest relatives and are, instead, more closely related to one or more of the monodermic groups. focused on the Firmicutes, a phylum that contains a mixture of monoderms and diderms. Now, in eLife, Simonetta Gribaldo of the Institut Pasteur and co-workers – including Luísa Antunes and Daniel Poppleton as joint first authors – report that monodermic bacteria evolved from ancestral didermic bacteria not once but multiple times by losing the outer membrane from their cell envelopes ( Antunes et al., 2016).Īntunes et al. However, it is possible that diderms could have evolved from monoderms ( Dawes, 1981 Tocheva, 2011). It is generally thought that the monodermic cell plan evolved from the more complex didermic cell plan in a single simplification event (see, for example, Cavalier-Smith, 2006). The evolutionary relationships between monoderms and diderms have remained uncertain for many years. However, at least two phyla comprise diderms that do not have LPS. The remaining phyla contain bacteria with two cell membranes, and most of these “diderms” have large molecules called lipopolysaccharides (LPS) in their outer membranes. The tree of life contains about 30 bacterial phyla, but only three of them contain bacteria that are surrounded by a single cell membrane, which are also known as “monoderms”. These different reactions to the stain reflect fundamental differences in the cell envelopes of these bacteria: Gram-positive bacteria usually have a single cell membrane that is encased by a thick wall made of a polymer called peptidoglycan, whereas Gram-negative bacteria tend to have two membranes with a thin wall of peptidoglycan sandwiched between them. The Gram stain is a violet-colored dye that is retained by Gram-positive bacteria but not by Gram-negative bacteria. For more than a century bacteriologists have used the Gram stain reaction to classify bacteria.
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