A theme of evolution is that sensors are better than armor. Mammals have very thin skin, with hair that has both sensory and protective roles. The reptilian and dinosaurian armor has been left far behind, in favor of a wide variety of short-range sensors innervating the skin, and long-range sensors, all choreographed with large brains. Among bacteria, a loosely analogous process took place in the development of Gram negative lineages.
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| Gram positive bacteria (purple) have a very thick armor of peptidoglycan, while Gram negative bacteria (lighter pink) have a second outer membrane that keeps out the Gram stand, and surrounds a much thinner peptidoglycan wall. |
The Gram stain is a complex antibacterial (and purple) molecule that binds to the outer peptidoglycan wall of bacteria. This is a meshwork which is constructed outside the plasma membrane, which is the key chemical, ionic, and electrical barrier between the cell and the outside. Like the lignin cell walls of plants, the peptidoglycan wall helps to protect the cell from physical abuse and from osmotic shock / swelling. Many antibiotics, like penicillin, impair the construction of this wall, causing the target cells to lyse. Since the construction takes place on the outside, access by antibiotics is easy, and a great deal of microbial warfare has happened at this interface.
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| Diagram of the periplasmic space. LPS is lipopolysaccharide, composing much of the outermost leaflet. IM is inner membrane, where sensors to its own stress as well as stresses on the peptidoglycan and outer membrane reside. LPP is Braun's lipoprotein. The porin is an example of a semi-selective outer membrane channel that lets in some nutrients and ions while keeping out other, larger chemicals. |
But what if some bacteria came up with an innovation to construct another membrane on the outside, encasing the peptidoglycan layer within a protective semipermiable membrane that keeps (at least some) antibiotics out? These are the Gram negative bacteria. They have an outer membrane with a specially robust outward-facing lipopolysaccharide (LPS) surface, and a very thin peptidoglycan layer, before you get to the cell's critical plasma membrane. While the Gram positive bacteria have a huge, thick armor-like peptidoglycan layer, Gram negative bacteria have a much thinner but more complicated structure, whose construction, homeostasis, and sensory capabilities are still under study. Our mitochondria are descendents of Gram negative bacteria, and also have a double-membrane, which makes the transport of the many nuclear-encoded proteins that compose them a rather involved process.
https://www.ncbi.nlm.nih.gov/books/NBK26828/
What keeps the outer membrane in place? That turns out to be the most abundant protein in bacteria like Eschericia coli, which is the prototypical Gram negative bacterium- Braun's lipoprotin. This protein serves as the strut/rivet that spans between the outer membrane and the thin peptidoglycan layer. If this protein is engineered to be longer, this tiny space widens in proportion.
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| Increasing the length of Braun's lipoprotein (lpp) widens the periplasmic space, in this case from 25 nm to 28.5 nm. In the second panel, a mutant which renders lpp incapable of attaching to the peptidoglycan layer renders the periplasmic space disorganized and the outer membrane prone to vesiculation. |
The whole space between the inner and outer membranes is called the periplasm, or periplasmic space. It has evolved not only for protection, but to host many processes best kept outside the cytoplasm, like pre-digestion of some nutrients, ionic and redox control, scavanging for iron and other key nutrients, stabilization of the flagellum, not to mention the construction and maintenance of its own components, including the outer membrane and the peptidoglycan wall.
This all requires quite a bit of sensory capacity, for instance to sense when the outer membrane needs more lipids to accommodate cell growth, or how thick to make the peptidoglycan. Very little of these sensory capacities are understood, but a recent paper discussed one sensor, RcsF, that somehow senses stress on both the outer membrane and peptidoglycan. This protein sticks a finger into the outer membrane, and either spans the entire periplasm or moves across it upon stress events. It then interacts with proteins on the inner, or plasma, membrane- IgaA and RcsC- which transmit its stress signal to a cascade of interior protein phosphorylation events that end up reducing cell movement and activating protective programs against stresses such as acid and membrane insufficiency, and increasing biofilm production and synthesis of proteoglycan and lipopolysaccharide.
The authors showed specifically that if the Braun lipoprotein was lengthened, widening the periplasm, then RcsF would only work (protecting against antibiotic treatment) if it was similarly lengthened. That indicates that RcsF spans at least between the outer membrane and the peptidoglycan layer, if not all the way to the inner membrane. The latter would be a total distance of 25 nm, which is equivalent, in a protein alpha helix, to 166 amino acids, which is longer than the entire RcsF protein. So some kind of migration or transfer through the periplasm must be taking place during the sensing event. It also suggests that one thing RcsF may be sensing specifically is the distance between the peptidoglycan and the outer membrane. Other stress sensors and mediators exist, so there remains a great deal to learn here. While Gram negative bacteria may not have a brain, they have a very smart skin that actively protects and defends them.
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