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Outer membrane lipoproteins: late to the party, but the center of attention | Journal of Bacteriology
The division of the bacterial world into “Gram-negative” or “Gram-positive” classes—reflecting their staining propensity in the classical Gram stain technique—had long been used to stereotype bacteria into one of two cell envelope architectures. Gram-negative bacteria were distinguished by a thin peptidoglycan cell wall and the presence of a second, outer membrane (OM) that is rich with the glycolipid lipopolysaccharide (LPS) (1). Gram-positive bacteria, on the other hand, lacked an OM and produced a thick cell wall (1). This simple staining-based distinction remains useful in many contexts, but the Gram status of a few key species has tinted perceptions of entire phyla as consisting of either OM-producing diderm Gram-negatives or monoderm Gram-positives. In fact, phyla that are widely considered Gram-positive contain species that produce a diderm architecture, complete with an OM and even LPS (2–5). For example, OM producing bacteria are present among the Bacillota (nee Firmicutes) phylum that is widely considered “Gram-positive” (2–5). Recent re-rooting of bacterial phylogeny showed that an early branching led to the Terrabacteria and Gracilicutes taxa and within these are species generally considered “Gram-positive” and “Gram-negative,” respectively (6). However, diderm cell envelope structures are commonly found among disparate Terrabacteria (2–5). This fact now supports a model where the last common bacterial ancestor was very likely a diderm cell (3, 4, 6). Today’s diderms have simply retained and adapted this ancestral diderm cell envelope architecture (2, 5). Meanwhile, Terrabacterial monoderms apparently arose through any of several independent instances of relinquishing the OM (2, 3, 5). This insight into the origins of the OM offers the chance to re-appraise how we understand OM biogenesis pathways in diverse contemporary bacterial species.
