The bacterial envelope is one of the oldest and most essential cellular components and has been traditionally divided into Gram-positive (monoderm) and Gram-negative (diderm). Recent landmark studies have challenged a major paradigm in microbiology by inferring that the last bacterial common ancestor had a diderm envelope and that the outer membrane (OM) was lost repeatedly in evolution to give rise to monoderms. Intriguingly, OM losses appear to have occurred exclusively in the Terrabacteria, one of the two major clades of bacteria. In this Review, we present current knowledge about the Terrabacteria. We describe their diversity and phylogeny and then highlight the vast phenotypic diversity of the Terrabacteria cell envelopes, which display large deviations from the textbook examples of diderms and monoderms, challenging the classical Gram-positive–Gram-negative divide. We highlight the striking differences in the systems involved in OM biogenesis in Terrabacteria with respect to the classical diderm experimental models and how they provide novel insights into the diversity and biogenesis of the bacterial cell envelope. We also discuss the potential evolutionary steps that might have led to the multiple losses of the OM and speculate on how the very first OM might have emerged before the last bacterial common ancestor.

细菌包膜是最古老和最重要的细胞成分之一，传统上分为革兰氏阳性（单皮）和革兰氏阴性（二皮）。最近的里程碑式研究挑战了微生物学的一个主要范式，它推断最后一个细菌共同祖先有一个 diderm 包膜，并且外膜 （OM） 在进化中反复丢失以产生单皮。有趣的是，OM 丢失似乎只发生在 Terrabacteria 中，Terrabacteria 是细菌的两大分支之一。在这篇综述中，我们介绍了有关 Terrabacteria 的当前知识。我们描述了它们的多样性和系统发育，然后强调了 Terrabacteria 细胞包膜的巨大表型多样性，这与教科书上 diderms 和 monoderms 的例子有很大的偏差，挑战了经典的革兰氏阳性-革兰氏阴性分歧。我们强调了 Terrabacteria 中 OM 生物发生所涉及的系统与经典 diderm 实验模型之间的显着差异，以及它们如何为细菌细胞包膜的多样性和生物发生提供新的见解。我们还讨论了可能导致 OM 多次丢失的潜在进化步骤，并推测第一个 OM 是如何在最后一个细菌共同祖先之前出现的。