Bacterial Outer Membranes, Periplasm, and More (PDF)

2-4: Bacterial outer membranes & more Lecture Overview: • A description of structures at the cell envelope/surface other than the cytoplasmic membrane and cell wall. • Outer membranes of Gram negative bacteria (main focus), the periplasmic space, S-layers, capsules, and surface appendages. • Textboo...

2-4: Bacterial outer membranes & more Lecture Overview: • A description of structures at the cell envelope/surface other than the cytoplasmic membrane and cell wall. • Outer membranes of Gram negative bacteria (main focus), the periplasmic space, S-layers, capsules, and surface appendages. • Textbook: Chapter 2.4-2.6 The outer membrane o Gram negative bacteria have a second membrane outside the cell wall that is very different from the cytoplasmic membrane o The outer leaflet is comprised of an important molecule called LPS Textbook, Fig 2.12 The outer membrane - asymmetry o The contemporary view is that the outer leaflet of the OM is mostly (or almost exclusively) LPS, with very little phospholipid. The inner leaflet is comprised of phospholipid. It is therefore asymmetrical. Textbook image is misleading in this respect! Outer leuf let Toner ruftet Textbook Fig 2.12 From a modern research paper Modified from: May and Grabowicz, PNAS, 2018 Lipopolysaccharide (LPS) o The predominant feature of the outer membrane o Three “parts”: (1) lipid A (within membrane), (2) core polysaccharide, (3)O-specific polysaccharide (outermost component) Textbook Fig 2.13 Lipid A o Lipid A is very different from phospholipids o Hydrophobic tails anchor Lipid A in OM - base of LPS o Phosphates confer a negative charge Textbook Fig 2.13 o Lipid A molecules can vary, but the overall structure generally maintained o Important molecule sensed by the immune system. Also known as “endotoxin”. Can lead to a potent and unchecked immune response – can cause septic shock if detected in bloodstream. E. coli Lipid A (Wikipedia) O-specific polysaccharide (“O antigen”) o Polysaccharide comprised of diverse sugar subunits connected and branched in different ways o Repeating combination of sugars with variable numbers of repeats o Usually conserved within a given strain, but often highly variable from lineage to lineage (strain to strain, species to species). E.g. -- some individual species can have hundreds of different versions o Detecting O antigen using specific antibodies can be used to identify a bacterium (“serovar” O antigen. E.g. – E. coli O157) Textbook Fig 2.13 The outer membrane is anchored to the cell wall o Braun’s lipoprotein serves to connect the OM to cell wall o Small lipoprotein (protein modified to be linked to lipid). o Lipid anchors in OM, other end of protein covalently attached to peptidoglycan. Very abundant in Gram negative cells. Textbook Fig 2.12 The outer membrane: porins o The outer membrane is generally impermeable to larger molecules like proteins o But the OM is permeable to many small molecules, largely due to porins o Porins are protein channels that serve as channels for entrance/exit of small molecules o Can be specific or non-specific. Their expression often regulated (not always expressed) Textbook Fig 2.12 Functions of the outer membrane o Provides mechanical strength to cell – ionic bonds between adjacent LPS molecules via divalent metal ions o Although far more permeable than cytoplasmic membrane, the outer membrane can be an important barrier. Soaks up or blocks access to many molecules – important for antibiotic sensitivity o Protects cell wall (from lysozyme, for example) o Enables a substantial periplasmic space (see next section) The periplasm o In Gram negative bacteria, the space between inner/outer membranes. o Space mostly (entirely?) between cytoplasmic membrane and cell wall. Is there a true periplasmic space between cell wall and outer membrane? Up for debate…. * cell wall is technically i the membrene Textbook, Fig 2.12 The periplasm o Buffer between environment and cell o Contains many important proteins/functions: o Break down macromolecules for uptake as nutrients o High affinity binding protein for nutrients o Detoxify harmful compounds o Protein folding - disulfide bond formation o Smaller space between cell membrane and cell wall in Gram positive bacteria. Function/importance of G+ periplasm less clear. ↳ Next ↳ couple more all of Slides components bacteria -> have these not S-layers u o Some bacteria produce S-layers (more common in archaea, where S-layers are very common) o Rigid/permeable monolayer of protein or glycoprotein – selforganized into a repeating structure that encompasses the cell o Always the outermost layer of the cell o Can perform many of same functions as cell wall attacks - Shields o Often a protective layer (protect bacteria from bacteriophage or bacterial pathogens from host defences, for example) o Can provide a periplasmic space or keep external proteins from accessing OM or cell wall S-layers Textbook images that show examples of bacterial S-layers (Fig 2.14) Capsules and slime layers o Some bacteria produce capsules or slime layers – coats of polysaccharides around the cell surface o Capsules are organized into a matrix and attached to the cell – slime layers are loosely attached, less organized o Functions can include: adhering to surfaces, protection from host immune cells (bacterial pathogens) & protection from water loss/dehydration Textbook, Fig 2.16 Pili & fimbriae o Pili (singular = pilus) are protein filaments that extend from the surface of a cell o Most bacteria (particularly Gram -ve) produce these structures o Different pili have different functions – conjugative pili are important for conjugation (transfer of genetic material between bacterial via a pilus bridge) o Fimbria is a term used for a pilus that mediates attachment (to a surface or another cell) Electron micrographs showing a pilus involved in conjugation (above) and fimbriae (below). Textbook Figs 2.17/2.18