Bacterial Cell Structures and Treatments 1

Bacterial Cell Structures and Treatments 1

• The cell envelope usually consists of two components:
  • an outer cell wall or peptidoglycan (gram-positive and gram-negative)
  • a cytoplasmic or plasma membrane (gram-positive and gram-negative)
  • an outer membrane (gram-negative only)
• The cell wall (peptidoglycan) is chemically inert (unreactive). It is composed of sub-units found nowhere else in nature.
• The cell envelope of gram-negative bacteria can produce symptoms of disease (endotoxins).
• The site of action of some of the most effective chemotherapeutic antibiotics is the cell wall peptidoglycan.

• Distinguishing between the different cell envelope
• The gram-stain played a major role in classifying bacteria
• Two major groups
  • gram-positive
  • gram-negative
• Gram refers to staining procedures (Christian Gram)
  • based on the cell wall structure of bacteria
  • important for susceptibility to osmotic pressure, antibiotics, etc.

• Involves three different chemical reagents
  • the primary stain (crystal violet)
  • Gram's iodine (mordant) forms an insoluble complex by binding to the primary stain
    • The decolourising agent (ethanol 95%) which acts as a lipid solvent as well as protein-dehydrating agent
  • The counter stain (safranin)
• In gram-positive, the low lipid concentration is important for the retention of the complex iodine-crystal violet: cell remains blue
• In gram-negative, the high lipid concentration found in the outer layers of the cell wall is dissolved which facilitates the release of the iodine-crystal violet complex leaving the cell colourless

• The cell wall peptidoglycan is a rigid structure which gives shape to the bacterium
• The wall also prevents osmotic rupture of the protoplast in dilute solutions
• When no cell wall remains the whole structure is called a protoplast - protoplasts always assume a spherical form even when derived from rod-shaped organisms
• The protoplast accounts for 60-80% of the dry weight of the whole cell
• When part of the cell wall remains attached to the protoplast the whole structure is called a spheroplast

• The peptidoglycan, which is the backbone of the cell wall, is a macromolecule consisting of two major sub-units (amino sugars) and amino acids
  • N-acetyl-muramic acid and N-acetyl-glucosamine that alternate to form a high molecular weight polymer
  • a chain of several amino acids is attached to each of the N-acetyl-muramic acid molecules
• The bacterial cell wall is a three-dimensional rigid, multilayered network around the organism
• Within this open meshwork are found a number of matrix materials, chiefly polysaccharides

• They include the immunologically active substances which determine the antigenic specificity of certain organisms (e.g. the C polysaccharide and M protein of species of Streptococcus)
• One group of these matrix is the techoic acids: highly acidic substances present only in the cell walls of gram-positive bacteria
• The cell wall peptidoglycan is the target of the B-lactams, glycopeptides, bacitracin, D-cycloserine and fosfomycin
  • They all inhibit cell wall peptidoglycan synthesis
  • The different B-lactams target different enzymes (penicillin-binding protein)
  • B-lactams and glycopeptides interfere with the late stage of cell maturation (cross-linking)
  • Bacitracin, D-cycloserine and fosfomycin interfere with early stages of cell synthesis

• The cytoplasm is bounded by a plasma membrane which is distinct from the enveloping cell wall
• The chemical composition of the plasma membrane is similar to that of the mammalian cell membranes and includes 40% lipid (much in the form of phospholipids), 60% protein and small amounts of carbohydrates. Sterols are absent

• The plasma membrane is the effective permeability barrier of the cell regulating the inflow and outflow of metabolites to and from the protoplast
• The membrane is semi-permeable. Generally, only low moleculae weight materials can penetrate to the inside of the cell
• The membrane contains many proteins/enzymes essential to the bacterium survival; e.g the proton motive force that generates adenosine triphosphate (ATP; energy)
• The plasma membrane is the target of very few antibiotics and is also the target of antimicrobial peptides
  • Polymyxins (polymyxin B), colistin and daptomycin. Polymyxin B and colistin affects membrane permeability. Daptomycin affects membrane function

• Periplasmic Space
  • The periplasmic space of gram-nagative bacteria contains exoenzymes called periplasmic enzymes, e.g. penicillinase hydrolyses and thereby inactivates penicillin - antibiotics
• Outer membrane
  • The outer membrane is composed of a layer of lipopolysaccharides (LPS) and one layer of phospholipids
    • LPS component (lipid A) is an endotoxin (septic shock syndrome)
    • LPS component (carbohydrate) important for identification/immunology

• Complicated three-dimensional network of fine fibrils stretching from the plasma membrane to the nuclear mass
• It lacks of elaborate endoplasmic reticulum characteristic of the eukaryotic cell
• In the bacterial cell the structures devoted to protein synthesis are the ribosomes 
  • Protein constitutes more than 50% of the dry weight of a typical bacterial cell wall
  • Almost 90% of all energy the cell utilises in synthesising cell components is used for the manufacture of proteins

• Ribosomes are characterised by their sedimentation properties when centrifuged at very high speeds in an ultracentrifuge
• Bacterial ribosomes are termed 70s ribosomes, which distinguish them from the 80s ribosomes of eukaryotic cells
• Each ribosome is made of 2 parts, and each part in turn is made of two different kinds of macromolecules
  • ribosomal protein
  • ribosomes RNA (rRNA)

• Many antibiotics inhibit protein synthesis by interacting with the bacterial ribosome:
  • Tetracylines
  • Macrolides
  • Lincosamides
  • Streptogramins
  • Aminoglycosides/aminocyclitols
  • Chloramphenicol
  • Fusidic acid
  • Mupirocin
  • Linezolid
• These target sites may be close together leading the problems such as bacterial cross-resistance

• Many bacteria form and store granules in their cytoplasm in the form of high molecular weight polymers
  • glycogen: a storage form of both carbon and energy
  • polymer of B-hydroxybutyric acid (storage form of both carbon and energy)
• Phosphate can be stored as granular inclusions of polymeric phosphate volutin
• Protein crystals are sometimes found. Bacillus thuringiensis crystals are used as insecticide 

• Bacteria lack an organised nucleus but they possess DNA
• The chromosome exists as a closed circle in all bacteria and is not surrounded by a nuclear membrane
• In addition some bacterial cells possess a short length of extrachromosomal DNA in the form of a plasmid
• Plasmids range in size from approximately 0.1 to 10% of the chromosome
• Plasmids often play an important role in the transfer of genetic material between bacteria

• Many antibiotics inhibit the synthesis of nucleic acid, often indirectly
  • Quinolones (DNA gyrase - DNA replication and transcription inhibition)
  • Rifampicin (RNA polymerase - transcription)
  • Sulphonamides and trimethoprim (folic acid synthesis)
  • Nitrofurantoin, mitrodinazole (DNA interactions)