Bacterial Cell Structure and Function PDF

Bacterial Cell Structure and Function PHAR1802 Dr Laura J. Smith Image: NIAID Image: CDC/James Archer  Learning Outcomes  Describe the cellular components of bacterial cells relating structure to function  Explain...

Bacterial Cell Structure and Function PHAR1802 Dr Laura J. Smith Image: NIAID Image: CDC/James Archer  Learning Outcomes  Describe the cellular components of bacterial cells relating structure to function  Explain the importance of these features to pharmacy and the infection process  Give examples of antibiotics acting on these various components  Briefly discuss fungal and viral structure  Origins of Microbes  Analysis of ribosomal RNA shows that prokaryotic and eukaryotic cells have evolved from a common ancestral form (LUCA) along different pathways  There are two distinct groups of prokaryotic cell: › Archaeobacteria › Eubacteria  The group of bacteria most important to pharmacy is the Eubacteria Relative Size of Microbes  Elements of Viral Structure  Not considered cells  No metabolic abilities of their own  Rely completely on biosynthetic machinery of infected cell  Infect all types of cells  Smallest virus is 10 nm in diameter Images: CDC Prokaryotic and Eukaryotic Differences  Prokaryotic and Eukaryotic Differences Prokaryote Eukaryote Cell Size Typically 1-2 µM Typically 10-200 µM Nucleus No nucleus with DNA located in True nucleus containing multiple one circular bacterial chromosome linear chromosomes Flagella Simple consisting of one protein Complex structure flagellin Cell wall Chemically complex containing Simple when present often peptidoglycan containing chitin or cellulose Cytoplasmic membrane Contains hopanoids Contains carbohydrates and sterols Energy metabolism Associated with cytoplasmic Mitochondria present membrane Ribosomes 70S 80S Cytoplasm No cytoskeleton Cytoskeleton Glycocalyx Capsule/slime layer Sometimes present if no cell wall Membrane encoded organelles Absent Many examples present including golgi apparatus and lysosomes  Fungal Cell Structure  True nucleus  Cell membrane – contains ergosterol (excellent anti-fungal target)  Cell wall – contains chitin. Prokaryotic Cell Morphology Coccus (cocci) Rod (bacillus) From NIAID Image from CDC Staphylococcus aureus Escherichia coli Streptococcus pneumoniae Mycobacterium tuberculosis Spirochete Spirilla Image from CDC Image from CDC Image from NIAID Treponema pallidum Borelia burgdorferi Campylobacter jejuni Helicobacter pylori Cell Structure  The Bacterial Cell Wall  All bacteria, except for the Mycoplasmas have a cell wall/envelope  Chemically unlike structures found in other cells  Bacterial species can be divided into two groups depending on the structure of their cell wall  Gram-positive bacteria have a distinct cell wall (20-80 nm thick)  Gram-negative bacteria have a multi-layered cell envelope  Cell Wall – The Gram Stain  The differences in the cell wall Gram +ve Gram -ve structure can be observed using the Gram stain Fixation  This gives us two major classifications: Flood with crystal › Gram positives e.g. S. aureus violet › Gram negatives e.g. E.coli Fix the stain with iodine  Gram positives retain the purple colour due to their thick cell wall Decolourise with alcohol  Gram negatives lose this colour due to their thin layer of Counter- peptidoglycan and can be counter- stain with safranin stained  Cell Wall – The Gram Stain Image From CDC Image From CDC Gram negative Gram positive  Not all bacteria can be identified by the Gram stain  Mycobacterium species have a wax-like and virtually impermeable cell wall  Require a special staining technique called Ziehl-Neelson  Cell Wall - Common Features  The cell wall is necessary for all bacterial cells to cope with the high osmotic pressure in their cytoplasm  It prevents bursting (cell lysis) and helps provide rigidity and structure  Its main component is peptidoglycan, which provides this rigidity: › Composed of two sugars: N-acetylglucosamine and N-acetylmuramic acid and some amino acids › Form long chains and crosslink to produce a strong structure Cell Wall - Different Features Figure 3.15: Brock (13th Ed)  The Gram +ve Cell Wall Fig 3.18: Brock Biology of Microorganisms (13th Ed)  Thick peptidoglycan wall (90%) many sheets of peptidoglycan stacked on top of each other  Contains surface antigens including teichoic acids and lipoteichoic acids  The Gram -ve Cell Wall (Cell Envelope) Fig 3.20: Brock Biology of Microorganisms (13th Ed)  Contain a thin peptidoglycan layer (10%)  Most of the cell wall is composed of the outer membrane and often referred to as the lipopolysaccharide layer (LPS)  Contain a periplasmic space rich in proteins including enzymes to break down host cell components  Agents acting on the Cell Wall  Enzymes: › Lysozyme (attacks peptidoglycan)  Antibiotics: › Penicillin › Cephalosporins › Bacitracin › Vancomycin › Cycloserine › Carbapenems Image: CDC/Lindsley  The Cytoplasmic Membrane  Thin barrier separating the inside of the cell from the outside environment  Destruction of the cytoplasmic membrane disrupts the integrity of the cell and leads to leakage of the cytoplasmic contents and ultimately cell death  The cytoplasmic membrane consists of: › Phospholipid Bilayer › Membrane Proteins Fig 3.05: Brock Biology of Microorganisms (13th Ed)  Functions of the Cytoplasmic Membrane  Permeability barrier › Consists of an aqueous solution of salts, sugars, amino acids, nucleotides, vitamins and co-enzymes › Prevents passive leakage of solutes into or out of the cell › Functions as gateway for Fig 3.08a: Brock Biology of Microorganisms (13th Ed) transport of nutrients into, and wastes out of the cell  Functions of the Cytoplasmic Membrane  Protein anchor › Site of proteins involved in transport of substances across the membrane › Others are enzymes, catalysing bioenergetic reactions  Energy conservation › Energetically charged - site of generation and use of proton motive force Fig 3.08b and c: Brock Biology of Microorganisms (13th Ed)  Agents acting on the Cytoplasmic Membrane  Antibiotics: › Polymixin › Gramicidin › Daptomycin  Disinfectants: Image: CDC/Mills › Phenol, dettol, TCP, cetrimide, chlorhexidine  Antifungal agents: › Amphotericin B › Nystatin  The Cytoplasm  Liquid-filled space enclosed by the cytoplasmic membrane  Aqueous mix of: › Macromolecules (e.g. proteins, lipids and polysaccharides) › Small organic molecules › Various inorganic ions  Nucleoid (Bacterial chromosome - circular DNA)  Plasmids (independent DNA)  Ribosomes  Agents acting on DNA  Antibiotics: › Rifampicin  Inhibits RNA-synthesis by inhibiting RNA-polymerase › Nalidixic acid & Ciprofloxacin  Inhibit DNA gyrase thereby inhibiting cell division › Trimethoprim & Sulphonamides  Interferes with the folic acid synthesis pathway thus depriving cell of DNA precursors › Actinomycin D  Inhibits transcription by binding to DNA at the transcription initiation complex  Ribosomes  The most numerous intracellular structure  The cell’s protein-synthesising structures  Interact with cytoplasmic proteins and messenger and transfer RNA in key process of translation (protein synthesis) 30s Small subunit  70S (made up of two 70s subunits (30S and Working 50s 50S) ribosome Large subunit  Agents acting on Ribosomes and Protein Synthesis  Antibiotics: › Tetracyclines  Prevent tRNA binding to mRNA › Aminoglycosides  Distort the 30S subunit › Chloramphenicol  Prevents peptide bond formation between amino acids › Erythromycin  Interferes with the transfer of amino acids from tRNA to growing peptide chain.  Flagella  Bacteria can move by swimming or gliding motility.  Swimming is facilitated by structures called flagella.  Long thin appendages (15-20 nm) that are free at one end and attached to the cell at the other  Different arrangement of flagella, which can be used in classification. Polar Lophotrichous Amphitrichous Peritrichous  Fimbriae and Pili  Fimbriae and pili are filamentous protein structures that extend from the surface of the cell and have many functions.  The role of fimbriae: › Enable cells to stick to surfaces (including animal tissues in the case of pathogens) › Form pellicles or biofilms on surfaces › Virulence factor (e.g. Salmonella sp., N. gonorrhoeae and B. pertussis)  The role of pili: › Longer and less abundant than fimbriae › Receptors for viruses › Facilitating genetic exchange (conjugation) › Adhesion of pathogens to specific host tissues and invasion (e.g. Neisseria sp. And Streptococcus sp.) › Involved in twitching motility  Capsules and Slime Layers  Many prokaryotes secrete slimy/sticky materials (polysaccharides) on their cell surface.  Can be thick or thin and rigid or flexible. › Capsules:  Tightly formed matrix and excludes small particles  Adhere firmly to the cell wall › Slime layers:  More easily deformed layer and does not exclude small particles  Loosely attached and can be lost from the cell surface  Functions: › Attachment to solid surfaces (role in pathogenesis) › Development of biofilms › Confers resistance to phagocytosis (e.g. Enterococcus sp.) › Resistance to drying-out  https://www.youtube.com/watch?v=4hexn-DtSt4 - swimming https://www.youtube.com/watch?v=iHg15E414lk - gliding https://www.youtube.com/watch?v=NLZ3nDQM98c - twitching https://www.youtube.com/watch?v=ODYu--TNPDE Further Reading Brock Biology of Microorganisms 13th Edition Chapter 3: Cell Structure and Function in Bacteria and Archaea (Sections 3.1 – 3.7; 3.9; 3.13)