Microbiology

• Microbial growth


  Increase in the number of cells
• Microbial replication

  1. Mother cell elongates

  2. Duplicates genetic material

  3. Septum forms

  4. Separates into 2 daughter cells
• Microbial populations

  • Increase exponentially
  • Graphed on a logarithmic scale
• Mean generation time

  Time taken for one cell to divide
• Bacteria

  • Can be cocci or rod-shaped
  • Cocci can be in chains or clusters
  • Plane of division determines arrangement
• Size

  • Smaller cells have higher surface to volume ratio and more efficient nutrient uptake
  • Leads to more rapid growth rate
• Physical factors influencing microbial growth

  • Gaseous atmosphere
  • Temperature
  • pH
  • Osmotic pressure
• Chemical factors influencing microbial growth

  • Water
  • Energy and electron source
  • Carbon
  • Macronutrients/micronutrients
• Aerobes

  • Require molecular oxygen for aerobic respiration
  • Use oxygen as terminal electron acceptor
• Anaerobes

  • Prefer absence of oxygen for anaerobic respiration or fermentation
  • Use exogenous terminal electron acceptor
  • No electron transport chain, use substrate-level phosphorylation
• Oxygen requirements of microbes

  • Obligate aerobes
  • Facultative aerobes
  • Aerotolerant anaerobes
  • Strict anaerobes
  • Microaerophiles
  • Capnophiles
• Oxygen toxicity

  • Converted to superoxide free radicals which can damage cells
  • Aerobes convert superoxide to hydrogen peroxide which is then broken down
• Temperature ranges

  • Minimum temperature where growth slows/stops (8 degrees C)
  • Optimum temperature for efficient growth (37 degrees C)
  • Maximum temperature where growth stops and bacteria die (65 degrees C)
  • Bacteria may go dormant or die at minimum temperature
• Temperature classifications

  • Psychrophiles
  • Psychrotolerants
  • Mesophiles
  • Thermophiles
  • Hyperthermophiles
• Optimum pH

  • Bacteria prefer pH 6-9
  • Fungi prefer pH 6-9 but also tolerate more acidic environments
  • Archaea can grow in extreme pH environments
• Osmotic pressure

  • Microbes prefer isotonic solutions
  • In hypertonic environments, water flows out of cell
  • In hypotonic environments, bacteria swell but don't burst due to rigid cell wall
  • Plasmolysis kills bacteria
• Halophiles


  Prefer higher salt content environments
• Bacteria are 95% water
• Temperature affects

  Water availability, which affects osmotic pressure
• Energy sources

  • Phototrophs (derive energy from sunlight)
  • Chemotrophs (derive energy from oxidation of chemical compounds)
  • Lithotrophs (chemotrophs deriving energy from reduced inorganic substances)
  • Organotrophs (chemotrophs deriving energy from reduced organic substances)
• Carbon sources


  • Autotrophs (utilise inorganic carbon, CO2)
  • Heterotrophs (utilise organic carbon from proteins, lipids, carbohydrates)
• CHONSP make up 96% of all living organisms
• Staphylococcus


  • Facultatively anaerobic, undergoes aerobic respiration and fermentation
  • Divides to give irregular clusters of cocci
  • Found on skin and mucous membranes of warm-blooded animals
  • Halotolerant
  • Some are part of normal microbiota, some cause disease
• Complex media


  Easy to prepare, unknown exact composition, contains sources of nutrients like glucose, beef extract, yeast extract, peptones
• Defined media


  Exact composition known, contains specific amino acids, sugars, vitamins
• Transport media


  Maintains viability of organisms, does not contain carbon, nitrogen or organic growth factors to stop replication, contains buffers and salts
• Enriched media


  Contains general nutrient supplements, encouraging for most bacteria
• Enrichment media


  Specifically encouraging, contains supplements that give a competitive edge to a desired microbe
• Selective media


  Specifically inhibiting, encourages growth of some bacteria while inhibiting others
• Differential media


  Provides visual indicators to distinguish bacteria, e.g. colour change
• Blood agar


  • Both enriched and differential, contains 5% animal blood, haemolysins break down red blood cells
• E. coli


  • Gram negative rod, facultatively anaerobic, motile, commonly found in lower intestine of warm-blooded animals
  • Most are harmless, some are pathogenic
  • Mean generation time of 20 minutes
  • Not dependent on media as it can synthesise all 20 essential amino acids
  • Part of normal microbiome, aids digestion and produces Vitamin K2
• Bacterial population growth

  1. Lag phase

  2. Exponential phase

  3. Stationary phase

  4. Death phase

  5. Long-term stationary phase
• Stationary phase


  Caused by lack of nutrients and buildup of toxic materials
• Long-term stationary phase


  Extended by release of nutrients when bacteria die
• Bacterial population

  2 to the power of number of generations
• Rate of bacterial growth

  Depends on rate of nutrients
• Digestive system

  Example of a chemostat
• Primary metabolites

  Essential for growth, produced in logarithmic phase
• Secondary metabolites

  Not essential for growth, accumulate in stationary phase