Bacterial Cell Structures and Treatments 2

Bacterial Cell Structures and Treatments 2

Bacterial Cell Structures and Treatments 2

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Flagellum: Bacterial Motility

  • The bacterial flagellum is made up of three parts:
    • The filament is composed of several chains of protein twisted together into a helix that has a hollow core
    • The hook is attached to the near end of the filament
    • The basal body is fastened to the hook, anchoring the flagellum to the cell wall and cytoplasmic membrane
  • Average length of 15-20um
  • 98% of total weight of flagellum is a protein called flagellin
  • Rotation 200-1000 rev/sec
  • Flagella proteins constitute the H-antigens of motile bacteria
  • Arrangements of flagella on the bacterial cell help characterise the species and so are often useful in identifying an organism
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Pili (Fimbriae)

  • Hair-like appendages
    • very fine, hair-like surface filaments
    • smaller than flagella (0.5um in length)
    • consist of protein sub-units wound around one another generating a hollow core
    • number of pili per cell varies from 1 to 400
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Pili (Fimbriae)

  • Pili can be separated into a number of types based on their function
    • adherence: to each other and to foreign cells (e.g. red blood cells)
    • antigenic differences, e.g. at least 5 antigenic distinct stereotypes of piliate Salmonella spp. are known
    • gene transfer by conjugation: F or sex pili
    • attachment sites for bacteriophages, e.g. sex pili
    • chemotaxis: may keep bacteria near the surface of liquids or where oxygen is most available
    • virulence (enhances the ability of bacteria to cause disease): toxin coregulated pilus (TCP); a thin, flexible, filamentous appendage, e.g. Vibrio cholerae
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Slime Layer/Capsule

  • The capsule is a loose-fitting, gelatinous structure that surrounds some bacteria
  • In nature, capsular material often forms a network of fibrils of polysaccharide-containing material outside the cell wall: the glycocalyx
  • The bacterial capsule has many roles:
    • Virulence: prevents phagocytosis (e.g. from macrophages)
    • Protection: contain water which protects bacteria against desiccation
    • Protection: exclude bacterial viruses and most hydrophobic toxic materials such as detergents
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Bacterial Endospores

  • Endospores have a very distinctive structure that is often seen inside certain species, e.g. Bacillus spp and Clostridium spp
  • Spores have a unique structure formed within the vegetative cell
  • The process of spore formation is called sporulation
  • It is not a reproductive process
  • Spores enable the survival of the bacterium (genetic material) during harsh conditions, e.g. lack of food, water, chemical and physical stress
  • Spores are usually resistant to a variety of physical and chemical agents that kill the vegetative cell
    • highly dehydrated, no metabolic activity, constituents (e.g. small acid soluble proteins, dipicolinic acid), structure
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Bacterial Endospores

  • Spores are used as biological indicators for sterility assurance 
    • Sterility assurance: ensure a process has killed all vegetative bacteria and spores
    • Sterilisation: process that kill all microorganisms including spores

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Bacterial Endospores

  • Germination is the process by which a spore reverts into a vegetative bacterium
    • Germination occurs when environmental conditions will support bacterial growth (food, water)
    • Germination can be forced to occur using certain chemicals called germinants (e.g. D-alanine)
    • Germination is a multistage process
    • During germination, the outgrowing cell becomes susceptible to chemical and physical processes
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Intracellular Parasites: Rickettsia

  • Rickettsia are aerobic, small (0.3 x 1.2um) gram-negative bacilli
  • Rickettsia are non-motile, non-spore forming bacteria
  • Rickettsia are highly pleomorphic bacteria that can present as cocci (0.1um in diameter), rods (1-4um long) or thread-like (10um long)
  • Microbial structure
    • peptidoglycan layer
    • lipopolysaccharide
    • no flagella or attachment proteins
    • surrounded by a loosely adherent slime layer
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Intracellular Parasites: Rickettsia

  • They contain DNA, RNA, enzymes for the Kreb cycle and ribosomes for protein synthesis
  • Rickettsia utilises host cell ATP, coenzyme A and nicotinamide adenine dinucleotide. They are obligate intracellular parasites
  • They divide by binary fission (6-10 hour generation time) until destruction of the host
  • Intracellular location
    • free in the cytoplasm (Rickettsia)
    • in cytoplasmic vacuoles (Coxiella and Ehrlichia)
    • in the nucleus (Rickettsia rickettsii)
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Intracellular Parasites: Rickettsia

  • Rickettsia species are carried by many ticks, fleas and lice and cause disease in humans such as typhus, rickettsialpox, Boutonneuse fever, African tick bite fever, Rocky Mountain spotted fever, Flinders Island spotted fever and Queensland tick typhus (Australian Tick Typhus)
  • The genus Rickettsia is named after Howard Taylor Ricketts (1871-1910), who studied Rocky Mountain spotted fever in the Bitterroot Valley of Montana, and eventually died of typhus after studying that disease in Mexico City
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Intracellular Parasites: Rickettsia

  • Attach to surface of cell
  • Promote their own entry by phagocytosis
  • Rickettsia escape the phagosomes to the host cell cytoplasm
  • Most rickettsia initially proliferate at the sites of inoculation and then spread to endothelial cells lining the small blood vessels
  • Treatment: inhibited by antibiotics
    • Tetracyclines: doxycycline
  • Prevention
    • vaccination (limited - a possible change with DNA vaccines)
    • reduced exposure to tick-borne disease
    • appropriate sanitary hygiene
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Intracellular Parasites: Chlamydia

  • Chlamydia are obligate intracellular bacteria
    • they lack the metabolic pathway to produce their own high-energy phosphate compounds ('energy parasites')
  • They exist in two distinct forms:
    • the small (300-400nm) extracellular infectious elementary body
    • the larger (800-1000nm) intracellular non-infectious reticulate body
  • Microbial structure
    • internal and external membrane similar to gram-negative bacteria
    • no peptidoglycan later
    • lipopolysaccharide
    • no flagella and non-piliated
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Intracellular Parasites: Chlamydia

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Intracellular Parasites: Chlamydia

  • Elementary bodies are metabolically inactive and represent the extracellulae C.trachomatis growth form. Elementary bodies are highly infectious 
  • Once ingested into a phagosome, fusion of the phagosome with the host lysosome is prevented
  • The elementary body reorganises within the phagosome into a metabolically active (not infectious) reticulate body
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Intracellular Parasites: Chlamydia

  • Treatment: inhibited by antibiotics
    • Tetracyclines: doxycycline 100mg twice daily for 7 days
    • Macrolides: azithromycin 1g: single dose
  • Prevention
    • safe sex
    • not sharing sex toys
    • appropriate sanitary hygiene
  • Contact tracing
    • treatment of sexual partners
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Mycobacteria

  • Cell wall  - highly hydrophobic
    • Basic layer: peptidoglycan linked to an arabinogalactan wall (copolymer of arabinose and galactose) esterified to mycolic acid structure
  • Mycolic acid
    • High molecular weight (60-90 carbons) 3-hydroxy fatty acid
    • Many types identified in mycobacteria
  • Other lipids
    • 25% of the dry weight is free lipids located outside the outer layers
    • Lipids include waxes, species-specific mycosides (complex glycolipids and peptidoglycolipids), lipopolysaccharides and cord factor (6,6"-dimycolate of trehalose)
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Mycobacteria

  • Cord factor is associated with the parallel assignment of rows of bacilli (cord formation), a characteristic of virulent strains of M. Tuberculosis
  • People most at risk of developing TB disease
    • close contacts of an infectious case
    • those who have lived in places where TB is still common
    • those whose immune system is weakened by HIV or other medical conditions
    • people who experience chronic poor health through lifestyle factors such as homelessness, alcoholism and drug abuse
    • young children and very elderly people are more susceptible
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Mycobacteria

TB control in England and Wales

  • Prompt and effective treatment of identified cases
  • Examination of contacts of cases
  • Screening for those at high risk of tuberculosis
  • A national BCG (Bacillus Calmette-Guerin) vaccination programme
  • A course of anti-TB drugs lasts for at least 6 months because the medicine is most effective against bacillia that are 'awake' and growing
  • Anti-TB drugs are always prescribed in combination to reduce the risk of the TB bacilli becoming resistant to one or more of them
  • It is vital that the medication is taken as prescribed. Taking anti-TB medication in the wrong dose, intermittently or for too short a time can result in the development of the drug resistance
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Mycobacteria

TB control in England Wales

  • First line drugs: isoniazid, rifampicin, pyrazinamide, ethambutol
  • Newly diagnosed
    • initial phase: isoniazid + rifampicin + pyrazinamide + ethambutol (2 months)
    • continuation phase: isoniazid + rifampicin (4 months)
  • Re-treatment
    • isoniazid + rifampicin + pyrazinamide + ethambutol and streptomycin (2 months)
    • isoniazid + rifampicin + ethambutol (5 months)
  • Treatment needs to be fully supervised (Directly Observed Therapy - DOT) in patients who cannot be relied upon to comply with the treatment regimen
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