One or two membranes? Diderm Firmicutes challenge the Gram‐positive/Gram‐negative divide

How, when and why the transition between cell envelopes with one membrane (Gram‐positives or monoderms) and two (Gram‐negative or diderms) occurred in Bacteria is a key unanswered question in evolutionary biology. Different hypotheses have been put forward, suggesting that either the monoderm or the diderm phenotype is ancestral. The existence of diderm members in the classically monoderm Firmicutes challenges the Gram‐positive/Gram‐negative divide and provides a great opportunity to tackle the issue. In this review, we present current knowledge on the diversity of bacterial cell envelopes, including these atypical Firmicutes. We discuss how phylogenomic analysis supports the hypothesis that the diderm cell envelope architecture is an ancestral character in the Firmicutes, and that the monoderm phenotype in this phylum arose multiple times independently by loss of the outer membrane. Given the overwhelming distribution of diderm phenotypes with respect to monoderm ones, this scenario likely extends to the ancestor of all bacteria. Finally, we discuss the recent development of genetic tools for Veillonella parvula, a diderm Firmicute member of the human microbiome, which indicates it as an emerging new experimental model to investigate fundamental aspects of the diderm/monoderm transition. How, when and why bacteria transitioned between one membrane (monoderm) and two membranes (diderm) is an important and still open issue in evolutionary biology. We discuss here that at least in the Firmicutes, the monoderm cell envelope architecture could have arisen from multiple independent losses on an ancestral outer membrane (pink crosses), while it was retained in two lineages, the Halanaerobiales and the Negativicutes. We put forward a scenario where the loss of the outer membrane might have occurred by an accident during sporulation which destabilized its attachment and did not lead to immediate lethality if rapidly compensated by mutations increasing the cell wall thickness.