bacteria

• Bacteria are single-celled microorganisms with prokaryotic cells, which are single cells that do not have organelles or a true nucleus and are less complex than eukaryotic cells
• Bacteria


  • Come in a wide variety of shapes and sizes, called morphology
  • The most common shapes are rod-like, called bacillus (plural, bacilli) form, or spherical, called coccus (plural, cocci) form
  • The rod forms vary considerably from very short rods that almost look like cocci, to very long filaments thousands of microns in length
  • Bacteria also form spiral and corkscrew, commas, oval (coccoid), and elaborately branched structures
  • The cocci often take on multi-cell forms; as two cocci joined together (diplococci), as chains of cocci (streptococci), or as tetrads (four cells in a cube)
• General cellular organization of bacterial cell


  • Appendages: Flagella, Pili, Fimbriae
  • Surface layers: External - S layer, Glycocalyx, Capsule, Slime layer
  • Cell envelope: Cell wall, Cell membrane
  • Internal: Cytoplasm, Ribosomes, Inclusions, Microcompartments, Nucleoid/chromosome, Cytoskeleton, Endospore, Plasmid, Intracellular membranes
• Flagella


  • The long helical filament is connected to the basal structure via a short, highly curved segment called the hook
  • Both the hook and the helical filament are self-assembling macromolecular structures composed of the hook protein (FlgE) and flagellin (FliC), respectively
  • Each filament may comprise as many as 30,000 flagellin subunits and can grow up to 75 μm
  • The hook is a helical assembly of 130 copies of FlgE subunits with a well regulated length of 55 nm ± 6 nm, capable of forming polymorphic supercoil structures
• Flagellar hook


  • Acts as a molecular universal joint that can transmit the torque produced by the basal body, a rotary motor, to the flagellar filament
• Basal body


  • The only structure of the flagellum that is present within the cell membrane
  • Connected to the hook of the flagellum which then connects to the long filament
  • A rod-shaped structure with a system of rings of microtubules
  • The rods present in the basal body act as a reversible motor that propels the filament in a different orientation for specific functions
  • Essential for the transfer of flagellar proteins from the cytoplasm to the hook and filament part of the flagellum during assembly
• Types of flagella


  • Monotrichous - single flagellum
  • Lophotrichous - multiple flagella at the same point in the cell
  • Amphitrichous - flagella present throughout the body of the cell, all directed in different ways
  • Peritrichous - presence of either a single flagellum or multiple flagella at either polar ends of the cell
• How bacteria move


  1. Rotation of the flagella enables bacteria to be motile

  2. Chemotaxis - movement of bacteria in response to chemical signals, consisting of a series of tumbles and runs toward or away from source of stimuli
• Fimbriae


  • Shorter than flagella and straighter, smaller, hairlike appendages, only on some gram-negative bacteria
  • Function: adhere, not involved in motility, one of the invasive mechanisms on bacteria, some pathogens cause diseases due to this (antigenic characteristic), prevent phagocytosis
  • Attached to the host cell
• Pili


  • Necessary for bacterial conjugation resulting in the transfer of DNA from one cell to another
  • Have been implicated in the ability of bacteria to recognize specific receptor sites on the host cell membrane
  • Two bacteria can reproduce by transferring genetic material through the pili
• Glycocalyx


  • An additional layer outside of the cell wall, can come in the form of a slime layer (loosely associated glycoproteins) or a capsule (thick, rigid, and smooth)
  • Guards against desiccation, protects the cell from phagocytosis, can be used for attachment
• The slime layer of Gram-positive Streptococcus mutans allows it to accumulate on tooth enamel, one of the causes of cavities
• Other bacteria in the mouth become trapped in the slime and form a biofilm, eventually leading to a buildup of plaque