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One of the major unanswered questions in evolutionary biology is when and how the transition between diderm (two membranes) and monoderm (one membrane) cell envelopes occurred in Bacteria. The Negativicutes and the Halanaerobiales belong to the classically monoderm Firmicutes, but possess outer membranes with lipopolysaccharide (LPS-OM). Here, we show that they form two phylogenetically distinct lineages, each close to different monoderm relatives. In contrast, their core LPS biosynthesis enzymes were inherited vertically, as in the majority of bacterial phyla. Finally, annotation of key OM systems in the Halanaerobiales and the Negativicutes shows a puzzling combination of monoderm and diderm features. Together, these results support the hypothesis that the LPS-OMs of Negativicutes and Halanaerobiales are remnants of an ancient diderm cell envelope that was present in the ancestor of the Firmicutes, and that the monoderm phenotype in this phylum is a derived character that arose multiple times independently through OM loss., Competing Interests: The authors declare that no competing interests exist.

Bacteria with a single cell membrane have evolved from ancestors with two membranes on multiple occasions within the Firmicutes phylum., Competing Interests: The authors declare that no competing interests exist.

Acetohalobium arabaticum Zhilina and Zavarzin 1990 is of special interest because of its physiology and its participation in the anaerobic C(1)-trophic chain in hypersaline environments. This is the first completed genome sequence of the family Halobacteroidaceae and only the second genome sequence in the order Halanaerobiales. The 2,469,596 bp long genome with its 2,353 protein-coding and 90 RNA genes is a part of the Genomic Encyclopedia of Bacteria and Archaea project.