BACTERIAL MORPHOLOGY AND GROWTH REQUIREMENTS

• Major Phenotypic Categories of Bacteria

  • Gram-negative and have a cell wall
  • Gram-positive and have a cell wall
  • Lack a cell wall (Mycoplasma spp.)
• Characteristics of bacteria used in classification and identification

  • cell morphology
  • staining reactions
  • motility
  • colony morphology
  • atmospheric requirements
  • nutritional requirements
  • biochemical and metabolic activities
  • enzymes that the organism produces
  • pathogenicity
  • genetic composition
• Basic categories of bacteria based on shape

  • Cocci (round bacteria)
  • Bacilli (rod-shaped bacteria)
  • Curved and spiral-shaped bacteria
• Bacilli


  Often referred to as rods; they may be short or long, thick or thin, and pointed or with curved or blunt ends. They may occur singly, in pairs (diplobacilli), in chains (streptobacilli), long filaments, or branched. An average sized bacillus is 1 x 3 µm. Extremely short bacilli are called coccobacilli.
• Medically important bacilli


  • Escherichia, Klebsiella, Proteus, Pseudomonas, Haemophilus
• Curved bacteria


  • Vibrio spp., Campylobacter spp., Helicobacter spp.
• Spiral-shaped bacteria


  • Treponema spp., Borrelia spp., Leptospira spp.
• Two major groups based on Gram-staining reactions

  • Gram-positive (bacteria end up being blue to purple)
  • Gram-negative (bacteria end up being pink to red)
• Gram-negative cell walls


  Thin layer of peptidoglycan, outer bilayer membrane composed of inner leaflet of phospholipids and proteins and outer leaflet of lipopolysaccharide (LPS) responsible for endotoxin activity. Integral proteins called porins form channels through both leaflets allowing glucose and other monosaccharides to move across. Periplasmic space between outer membrane and inner plasma membrane contains enzymes for breakdown of large molecules and detoxification/inactivation of antibiotics.
• Gram-positive cell wall


  Thick layer of peptidoglycan and lipoteichoic acids that anchor the wall to the cytoplasmic membrane.
• Gram staining reactions


  Gram-positive bacteria have a thick layer of peptidoglycan making it difficult to remove the crystal violet-iodine complex. Gram-negative bacteria have a thin layer of peptidoglycan making it easier to remove the crystal violet, so they are subsequently stained with safranin.
• Acid-fast bacteria


  Some Gram-positive bacteria like Mycobacterium have walls with up to 60% mycolic acid, a waxy lipid. This helps them survive desiccation and makes them difficult to stain with regular water-based dyes, requiring a special acid-fast staining procedure.

  
• Motile bacteria


  Most spiral-shaped bacteria and about 50% of bacilli are motile, while cocci are generally nonmotile. Motility is achieved through flagella, endoflagella (axial elements), and pili/fimbriae.
• Bacterial growth


  Involves an orderly increase in the number of cells, not the size of the organism. Growth is implied by the formation of colonies composed of billions of cells.

  
• Bacterial atmospheric requirements


  • Obligate aerobes
  • Microaerophilic aerobes
  • Facultative anaerobes
  • Aerotolerant anaerobes
  • Obligate anaerobes
• Capnophilic organisms


  Grow best in the presence of increased concentrations of CO2 (usually 5%-10%)
• Culturing microorganisms in thioglycolate broth (THIO)


  Sodium thioglycolate consumes oxygen, allowing growth of obligate anaerobes. Diffusion of oxygen from the top produces a range of oxygen concentrations along the depth, allowing identification of different oxygen requirements.
• Bacterial oxygen requirements in thioglycolate broth

  • Obligate aerobes gather at the top
  • Obligate anaerobes gather at the bottom
  • Facultative anaerobes gather mostly at the top
  • Microaerophiles gather in the upper part, but not the very top
  • Aerotolerant organisms are evenly spread throughout
• Bacterial nutritional requirements


  • Carbon source (autotrophs vs organotrophs)
  • Nitrogen, sulfur and phosphorus for proteins, nucleic acids, ATP
  • Inorganic ions like magnesium, potassium, calcium, iron, trace elements
• Physical requirements for bacterial growth

  Moisture and water as medium to acquire nutrients, temperature (thermophiles, mesophiles, psychrophiles), pH (alkalophiles, neutrophiles, acidophiles)
• Bacterial growth curve


  1. Lag phase (cellular activity but no growth)

  2. Exponential (log) phase (rapid cell division)

  3. Stationary phase (no overall population growth)

  4. Death or decline phase (exponential decrease in living cells)

  
• Bacterial growth phases


  1. Lag phase

  2. Exponential growth phase

  3. Stationary phase

  4. Death or decline phase

  
• Exponential growth phase


  It is in this phase that antibiotics and disinfectants are most effective targeting bacteria cell walls or the protein synthesis processes of DNA transcription and RNA translation
• Exponential growth phase duration

  About 8 hours
• Stationary phase


  The phase when bacterial cell growth reaches a plateau, where the number of dividing cells equal the number of dying cells, as the available nutrients become depleted and waste products start to accumulate

  
• Stationary phase


  This results in no overall population growth
• Stationary phase


  Phase when spore forming bacteria produce endospores and pathogenic bacteria begin to generate substances (virulence factors)

  
• Death or decline phase


  The number of living cells decreases exponentially, and population growth experiences a sharp decline
• Death or decline phase


  Due to continuous depletion of nutrients and accumulation of waste products
• Death or decline phase


  Sporulation continues during this stage - Spores are able to survive the harsh conditions of the death phase and become growing bacteria when placed in an environment that supports life
• Metric units are used to express the sizes of microbes
• Aerotolerant organisms


  do not require oxygen as they metabolize energy anaerobically. They are NOT poisoned by oxygen. They can be found evenly spread throughout the test tube.
• Autotrophs (lithotrophs)


  use inorganic compounds (e.g., carbon dioxide) as their sole carbon source.