Bacterial Structure - Foundations of Pharmacology PHRM2005 PDF

Foundations of Pharmacology PHRM2005 Dr. Ricky R Lareu Bacterial Structure 2 Key Concepts Prokaryotic (bacterial) sub-cellular structure – distinctive from eukaryotic cell structure Sub-cellular structures provide phenotypic properties important in disease and treatment The Gram stain: Gram-positive...

Foundations of Pharmacology PHRM2005 Dr. Ricky R Lareu Bacterial Structure 2 Key Concepts Prokaryotic (bacterial) sub-cellular structure – distinctive from eukaryotic cell structure Sub-cellular structures provide phenotypic properties important in disease and treatment The Gram stain: Gram-positive and Gram-negative cell walls Cell shape and growth arrangement Learning Outcomes Be able to describe the various sub-cellular components-features of prokaryotes (bacteria), with their respective functions: ▪ Components of the cytoplasm, membrane, cell wall and the extracellular compartment Describe the common and distinctive structural components between Gram-positive and Gram-negative cell walls, and their significant phenotypic properties Be able to classify bacteria based on cell shape and growth arrangement using correct terminology 3 Lecture Information Reading and reference material: For Bacteria: Mims’ Medical Microbiology 5th Ed., Chapter 2, pages 7-9 Burton’s Microbiology for the Health Sciences 10th Ed., Chapter 3, pages 30-35 4 Cellular Organisms and Acellular Particles Cellular microorganisms: Prokaryotes Archaea & Bacteria (eubacteria) Eukaryotes Eukaryota Acellular infectious particles: Viruses So small can only be seen with an electron microscope. Prions Infectious proteins Image: Oregon State University 5 Importance of Bacterial Structure Knowledge of the molecular composition and structures which make up bacteria enable us to understand how they cause disease and form the basis for antimicrobial therapies ▪ Specific structures which enhance disease-causing ability – Virulence factors ▪ Many bacteria are capable of movement ▪ Bacterial growth/multiplication requires synthesis ▪ Bacteria have mechanisms for coping with the environment – genetically controlled This lecture is a little more detailed than the textbook. ▪ Most microbiology textbooks will have these topics e.g. Burton’s Microbiology for the Health Sciences 10th Ed., Chapter 3, pages 30-35 6 General Structure Of Bacteria Cytoplasmic components Cytoplasm Nucleoid Ribosomes Cell envelope Cytoplasmic membrane Cell wall Glycocalyx (Capsule) Appendages Flagella Pili Fig. 2.1 Mim’s MM Image courtesy of MolecularExpressions 7 Prokaryotic vs Eukaryotic Cell Features Revision Definition of prokaryote: do not possess a nucleus nor membranebound organelles Don’t think that bacteria are simple! Unlike multicellular organisms, bacteria have to do everything to survive and replicate in one cell. 8 Prokaryotic Cytoplasm Cytoplasm Gel-like matrix of H2O (solvent), proteins (e.g. enzymes), vitamins, ions, nucleic acids etc. Growth, metabolism and replication occur Nucleoid ▪ Location of the genome (i.e. host DNA) ▪ a single, circular chromosome Cytoplasmic inclusions/Granules ▪ Structure for storing nutrients e.g. glycogen 9 Prokaryotic Ribosomes Machinery for translation of proteins Structure ▪ Ribosomal RNA and protein superstructures ▪ Subunit structure (30S + 50S = 70S) Location – cytoplasm or attached to the cell membrane N gonorrhoeae University of Texas Different from eukaryotes therefore good antimicrobial drug target e.g. aminoglycosides ▪ Prokaryotic is 70S; eukaryotic is 80S Very abundant ▪ ~90% of RNA in cell; 30-40% of dry weight ‘S’ is a relative ultracentrifugation value 10 Cytoplasmic/Plasma Membrane Revision Composed of phospholipids (30-40%) and proteins (60-70%), no sterols e.g. cholesterol Highly conserved in all living cells – prokaryotes and eukaryotes Separates cell content from the environment: major barrier – selectively permeable Proteins embedded in lipid bilayer ▪ Asymmetric – different structures/proteins on either side ▪ Enables both passive and active transport, secretion and energy generation Pass through lipid bilayer: lipid-soluble molecules, some water, gases (O2, CO2) Cannot pass through without transport/channels: watersoluble molecules (e.g. glucose, amino acids), ions (e.g. Ca2+, Na2+) 11 Prokaryotic Cell Wall Essential for most bacteria ▪ Determines the bacterial shape ▪ Allows survival in many different environments: protection from osmotic pressure, chemicals, acid/alkali Structure ▪ Complex semi-elastic lattice structure - porous ▪ Both rigid and elastic components Composition – unique to bacteria species ▪ main structural compound is peptidoglycan The cell walls protect bacteria from exploding (membrane rupture) in hypotonic solutions (e.g. pond water, drinking water) Bacteria can be killed by hyperosmotic shock i.e. desiccation e.g. very salty or very sweat environments i.e. those used for preserving foods: salted meat, jam, honey Fig. 3-6. Burton’s Microbiology for the health sciences Ed. 9 12 Cell Wall – Peptidoglycan Supportive 3D scaffold-like structure Large polymer of interlocking chains of alternating monomers Backbone – the “glycan”: amino sugars, derived from glucose ▪ Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) monomers Glycan polymers are connected by peptide bridges Charged amino acids – hydrophilic Sensitive to lysozyme, ampicillin and its derivatives Cleavage site of lysozyme β-lactam antibiotics (e.g. penicillin G) inhibit peptide cross-linking Fig. 3-8. Burton’s Microbiology for the health sciences Ed. 9 13 Bacterial Cell Wall Structures - Comparison Two major classes of bacteria based on cell wall structure Gram stain – Developed by Hans Christian Gram (1884) Fig. 3-9. Burton’s Microbiology for the health sciences Ed. 9 14 Gram-positive Cell Wall Structure Thick layer – 20-80 nm Hydrophilic – helps resist bile acids in gut Teichoic Acids Polysaccharides - polymers of glycerol or ribitol Gram positives only Attached covalently to NAM and add rigidity to wall Lipoteichoic acids Anchor cell wall to membrane Surface Protein Teichoic Acid Peptidoglycan Lipoteichoic Acid Plasma membrane Cytoplasm Surface Proteins Covalently attached to cell wall Important in bacterial disease 15 Gram-negative Cell Wall Structure Outer membrane Additional permeability barrier – protection Both hydrophilic and -phobic components Proteins act as pores to control movement across this barrier (porins) Peptidoglycan layer Thin Lipoproteins attach the outer membrane to the peptidoglycan Lipoprotein Porin Lipopolysaccharide Mg2+ + + + + Outer membrane Peptidoglycan Periplasmic space Plasma membrane Cytoplasm Drugs like penicillins and cephalosporins which interfere with peptidoglycan synthesis cannot easily get through the outer membrane 16 Gram–positive Cell Wall Lipopolysaccharide Lipopolysaccharide (LPS) is in outer layer of membrane Lipid portion sits in outer membrane with sugar chain (polysaccharide) outside of cell Significant properties Endotoxin Lipid A Antigenic – carbohydrate chain ‘O’ antigens Recognised by immune system and bacterial viruses (bacteriophage) 17 Bacterial Cell Wall Summary Essential for viability, survival and shape Composed of unique components not found elsewhere in nature Important site for action by antibiotics Exopolysaccharides e.g. teichoic acids, LPS - Provides ligands for adherence, receptor sites for drugs and bacteriophage, pathogenesis ▪ Causes symptoms of disease in animals ▪ Large diversity and characteristic to species and strains ▪ Provides immunological distinction and variation among bacterial strains 18 Glycocalyx Excreted material forming an extra layer outside the cell wall produced by some bacteria Help bacteria to adhere to surfaces and evade immune University of Wisconsin system → virulence factor Composition is often characteristic of a particular species or strain - used as a vaccine target Capsule Secreted polysaccharides firmly attached to the cell wall Some capsules protect bacteria from immune cells Slime Layer Polysaccharides or sometimes glycoproteins which are loosely associated with the cell wall Protects against desiccation and important in biofilms formation 19 Bacterial Biofilms A biofilm is a bacterial community living within an organic polymer matrix (glycocalyx/slime layer) adhering to a surface Creates a beneficial and protective environ. Inherently resistant to host defences since it blocks access by immune cells and antibodies Also acts as a filter to reduce diffusion of antibiotics and antimicrobial agents Example Strept. mutans is a Gram-positive bacteria that produce a slime layer that helps it to adhere to tooth enamel ▪ Eventually this builds up and other bacteria become trapped → build-up of dental plaque and eventually caries Image from Cell Chemical Biology, v19: 1503-1513 20 Bacteria Surface Appendages Flagella - Organ of motility - rotates like a propeller Made up of thousands of protein subunits known as flagellin Flagella arrangement is a species characteristic, present on: ▪ almost all spirilla, ½ all rod-shaped bacteria, few cocci Flagella are a virulence factor e.g. E. coli and Proteus spp cause frequent UTI – flagella propel bacteria up the urethra Immunity to some pathogens is through Abs. against flagella Pili or Fimbriae – fine hairlike projections, made from pilin subunits Hollow, shorter and thinner than flagella Bacteria use to attach to host surfaces, needed for infection and colonization, initiate formation of a biofilm → virulence factor ▪ enterotoxogenic E.coli ▪ Resistance to phagocytosis – S. pyogenes M protein Additionl information – for interest, not examinable http://microbeonline.com/bacterial-flagella-structure-importance-and-examples-of-flagellated-bacteria/ Microbeonline.com Images: University of Wisconsin 21 Bacteria Cell Shapes and Arrangements Most bacteria are classified according to shape ▪ coccus (plural = cocci) = spherical ▪ bacillus (plural is bacilli) = rod shaped Single Palisade Streptobacillus (chain) Spiral shaped - spirillum - spirochaetes Other types of shapes ▪ ▪ ▪ ▪ ▪ Coccobacilli – stretched out cocci Filamentous – bacilli in long threads Vibrios – short, slightly curved rods Fusiform – bacteria with pointy ends Single Diplococcus (pair) Tetrad (group of 4) Staphylococcus (cluster) Streptococcus (chain) Image copyright © motifolio.com Image copyright © motifolio.com 22 Summary Although bacteria (prokaryotes) share many subcellular features and functions with eukaryotic cells, there is an absence of organelles The cytoplasm contains the nucleoid (genome) which is a single, circular structure without a nuclear membrane The cytoplasmic membrane protects cellular biochemistry (intracellular) from the surrounding environment and is selectively permeable. The cell wall protects against mechanical damage and osmotic shock and defines characteristic cell shape (e.g. coccus, bacillus, spiral) Gram-positive and Gram-negative cell walls have distinctive features → certain phenotypic properties important in disease and treatment Extracellular structures like capsule and slime layer, flagella and fimbriae give bacteria survival advantages → greater virulence 23