Microbial world

• Living cells can be classified into prokaryotes and eukaryotes
• Prokaryotes


  Organisms that do not possess a true nucleus and membrane-bound organelles (e.g. bacteria)
• Eukaryotes


  Organisms that possess a true nucleus and membrane-bound organelles (e.g. plants, animals, fungi, parasites, algae)
• It is important to know the difference between eukaryotic and prokaryotic cells
• Medically important microorganisms


  • Viruses
  • Bacteria
  • Fungi
  • Algae
  • Parasites (Protozoa & Helminthes)
• Viruses


  • Lack necessary cellular parts to replicate independently
  • Lack genes and enzymes for energy production
  • Obligate intracellular parasites
• Viruses


  • Acellular organisms
  • Outer surface is called capsid (composed of capsomeres)
  • Possess only a single nucleic acid, either DNA or RNA, but never both
• Virus classification based on presence or absence of envelope

  • Naked Viruses - whose capsids are not covered by an envelope
  • Enveloped Viruses - covered with an envelope
• Bacteriophages are special type of viruses that primarily infect bacteria
• Bacteriophages


  • They are obligate intracellular parasites
  • They are similarly shaped like other viruses
  • They play a role in the acquisition of virulence factors of certain bacteria and in the transfer of genetic material from one bacterium to another (transduction)
• Bacteria


  • Prokaryotic cells with majority having an outer covering called the cell wall (composed mainly of peptidoglycan)
  • Possess both DNA and RNA
  • Possess a nucleoid instead of a true nucleus, smaller ribosomes, and lack mitochondria
• Hans Christian Gram discovered Gram Stain in 1884
• Bacteria categorization based on Gram Stain

  • Gram-negative bacteria with cell wall (e.g E.coli)
  • Gram-positive bacteria with cell wall (e.g S. aureus)
  • Acid-fast bacteria with lipid-rich cell wall (e.g. Mycobacterium tuberculosis)
  • Bacteria without cell wall (e.g. Mycoplasma)
• Fungi


  • Eukaryotic cells with an outer surface composed mainly of chitin
  • Cell membrane is made up mostly of ergosterol
  • Possess both DNA and RNA
  • Possess a true nucleus that is enclosed by a nuclear membrane and mitochondria
• Differences between Bacteria and Fungi

  • Cell type: Prokaryotic vs Eukaryotic
  • Role in ecosystem: Producers/decomposers vs Decomposers
  • Optimal pH: Neutral vs Slightly acidic
  • Cell structures: No true nucleus/organelles vs True nucleus/organelles
  • Main component of cell wall: Peptidoglycan vs Chitin
  • Sterols in cell membrane: Absent vs Present
  • Mode of nutrition: Heterotrophic, chemoautotrophic, photoautotrophic vs Heterotrophic, aerobic, facultative anaerobic
  • Reproduction: Binary fission vs Sexual and asexual spores
• Algae


  • Eukaryotic organisms whose outer surface consists primarily of cellulose
  • Plant-like organisms capable of photosynthesis but do not possess true roots, stems and leaves
  • Do not produce significant diseases in humans
• Types of algae


  • Diatoms - unicellular algae inhabiting fresh and saltwater
  • Dinoflagellates - unicellular algae that are important members of the phytoplankton group and can cause "red tide"
• Bacterial morphology


  Classified into three basic shapes: Coccus, Bacillus, Spiral-shaped or curved
• Bacterial shapes


  • Coccus (spherical or round-shaped)
  • Bacillus (rod-shaped)
  • Spiral-shaped or curved
• Bacterial envelope structures


  • Glycocalyx - outermost covering that may be in the form of capsule or slime layer
  • Cell wall - provides rigid support and gives shape, composed mainly of peptidoglycan
  • Teichoic acids and polysaccharides in gram-positive cell walls
  • Outer membrane, lipoprotein, and periplasmic space in gram-negative cell walls
  • Lipid-rich cell wall in acid-fast bacteria
• Gram staining differentiates bacteria based on their cell wall composition
• Flagella are thread-like structures made up of the protein flagellin that project from the bacterial cell
• Acid-fast cell wall


  Unlike gram-positive and gram-negative bacteria, acid-fast organisms such as Mycobacterium tuberculosis possess an outer layer that is lipid-rich
• Cell wall of acid-fast organisms

  Composed of large amounts of waxes that are known as mycolic acids
• Inner layer of the cell wall

  Made up of peptidoglycan but because the outermost layers is lipid rich, cell walls of acid-fast organisms are hydrophobic
• This is the reason why they cannot be stained using the reagents used in gram staining
• Hydrophobic nature of their cell wall


  Protects them from harsh chemicals such as strong acids and detergents
• Schematic representation comparing gram-positive, gram-negative and acid-fast cell wall
• Top 5 students additional 2 points
• Flagella


  Thread-like structures made up entirely of molecules of the protein sub-unit flagellin, project from the capsule and are organs for motility
• Types of flagella


  • Monotrichous (single polar flagellum)
  • Lophotrichous (a tuft of flagella at one end of the bacterium)
  • Amphitrichous (flagella at both ends of the bacterium)
  • Peritrichous(flagella all around the bacterium)
• Bacteria without flagella are called atrichous
• Pili or Fimbriae


  Rigid surface appendages found on many gram-negative bacteria, fine and short in comparison with flagella, structural protein subunits are called pilins, may function for motility or adherence to cell surface (common pili) or attachment to another bacterium during conjugation (sex pili)
• Axial Filaments


  Also called endoflagella, found in spirochetes, composed of bundles of fibrils, the structures of which are similar to flagella, the filaments rotate producing movement of the outer sheath of the spirochetes propelling them forward
• Cytoplasmic Membrane


  Also called cell membrane or plasma membrane, located beneath the cell wall, encloses the cytoplasm of the cell, selectively permeable membrane that allows for transport of selected solutes, contains enzymes needed for the biosynthesis of DNA, cell wall components, and membrane lipids
• Nucleoid


  Bacteria have no true nucleus that is surrounded by a nuclear membrane, their genetic material is packaged in a structure called the nucleoid, bacteria possess a single, circular, double-stranded DNA
• Mesosomes


  Function for cell division, involved in the secretion of substances produced by bacteria
• Ribosomes


  Function for protein synthesis, bacterial ribosomes are smaller than eukaryotic ribosomes
• Granules or Inclusion Bodies


  Found in certain bacteria, serve for storage of food and energy (e.g., metachromatic granules of Corynebacterium diphtheriae or Much granules of Mycobacterium tuberculosis)
• Endospores


  Structures produced by many bacteria when they are placed in a hostile environment, composed of dipicolinic acid which confers resistance to heat, drying, chemical agents, and radiations, the process of spore production is called sporulation, when environmental conditions become favorable, the endospores revert to their vegetative state through a process called germination, some gram-positive, but never gram-negative, bacteria form spores