Microbiology (Anna)

• Microbial growth


  An increase in the number of cells (not cell size)
• Binary fission

  Multiplication (reproduction) occurs as a result of growth - cell grows until it divides into 2 new cells
• Generation time (doubling time)


  The time taken for a population to double
• E. coli generation time = 20 min under optimal conditions
• Generation time range = 10 min to several days
• Binary fission

  1. DNA replication

  2. Cell elongation

  3. Completion of septum with formation of distinct walls

  4. Septum formation

  5. Cell separation
• Cell cycle


  The time it takes a newly formed cell to replicate its DNA, elongate & divide
• Cell cycle

  1. C phase - when the cell's genome undergoes replication to form 2 identical daughter DNAs

  2. D phase - when the daughters' DNA separates & moves towards opposite ends of the elongating cell, construction of the cytoplasmic membrane & cell wall at the midpoint of the cell
• For an organism with a generation time between 20 & 60 min: C phase takes approx. 40 min for the one round of DNA replication, D phase (delay) is a 20 min period between the termination of DNA replication & cell division
• A species is only maintained through continued growth of the population
• Population growth

  Estimated by the change in cell number or biomass per unit time
• Microbial growth cycle

  1. Lag phase - Cells synthesising essential constituents for growth

  2. Exponential phase (log phase) - Cell numbers double with each generation

  3. Stationary phase - Depletion of essential nutrient(s) and/or waste product accumulation, no net increase/decrease in cell number

  4. Death phase - Death rate > rate of reproduction
• Exponential growth


  Growth in a species that takes place at a constant rate per time period, described by the equation: N_t = N_0 (2^n)
• If we started with 64 cells, and had 4 generations, the number of cells would be 1024
• Logarithmic scale


  Allows us to see what is happening in the early stages of growth as clearly as in the later stages
• Single cells in genetically homogeneous microbial cultures exhibit marked phenotypic heterogeneity, adding to the fitness of the whole population
• Endospore formation


  Exponential growth ceases & vegetative cell → non-growing, resistant endospore ('sporulation')
• Endospores are produced inside ('endo') endospore-forming bacteria during sporulation (e.g. Bacillus & Clostridium), disseminate cells & give protection during poor environmental conditions
• Clumps of bacteria adhering to surface

  • Surface
  • Water currents
  • Migrating clump of bacteria
  • Biofilms
• Requirements for microbial growth

  • Temperature
  • pH
  • Osmotic effects
  • Oxygen
  • Pressure
• As temperature rises, reactions proceed more rapidly & growth becomes faster, but above certain temperature proteins denature & cell functions fall sharply to zero
• Groups of microorganisms in relation to temperature optima

  • Psychrophiles: 0°C–20°C
  • Mesophiles: 12°C–45°C
  • Thermophiles: 40°C–80°C
  • Extreme thermophiles: 65°C–113°C
• pH effects

  Largely based on changes in the nature of proteins, enzyme function & nutrient transport into the cell
• Groups of microorganisms in relation to pH

  • Acidophiles: pH 0–5
  • Neutrophiles: pH 5–8
  • Alkalophiles: pH 9–11
• Osmotic effects


  Typically, the cytoplasm has a higher solute concentration than the surrounding environment, thus the tendency is for water to move into the cell (positive water balance)
• Groups of microorganisms in relation to osmotic effects

  • Osmotolerant - grow over wide range of osmotic concentrations
  • Osmophiles - live in environments high in sugar as solute
  • Halophiles - require high salt concentration
  • Halotolerant - do not require salt but can grow in its presence
  • Nonhalophiles - not able to grow in salty environments
  • Xerophiles - able to grow in very dry environments
• Groups of microorganisms in relation to oxygen concentration

  • Obligate (strict) aerobes - require O2 for growth
  • Obligate (strict) anaerobes - inhibited or killed by O2
  • Facultative anaerobes - grow with/without O2 (better with)
  • Microaerophiles - require low levels of O2
  • Aerotolerant anaerobes - don't use O2 but can tolerate its presence
• Pressure

  • Barophiles - adapted to high pressures (extreme barophiles up to 1,000 atm)
  • Barotolerant organisms - grow at high, but not very high pressure (~400 atm optimum)
  • Barosensitive organisms - die at high pressure (majority of microbes in mammals)
• Microorganisms grow quickly under ideal conditions
• Always refer to population growth, four main phases in a typical growth curve
• Range of physical requirements for growth
• Next time: Requirements for microbial growth - Part 2
• Microbial growth


  The process by which microorganisms increase in number and size
• Requirements for microbial growth

  • Physical requirements
  • Chemical requirements (nutrients)
• Physical requirements for microbial growth

  • Temperature
  • pH
  • Osmotic effects
  • Oxygen
  • Pressure
• Chemical requirements (nutrients) for microbial growth

  • Macronutrients
  • Micronutrients (trace elements)
  • Growth factors
• Macronutrients


  Nutrients required in large amounts
• Micronutrients (trace elements)


  Nutrients required in trace amounts
• Growth factors

  Organic compounds needed in relatively small amounts by certain organisms
• Examples of growth factors

  • Vitamins
  • Amino acids
  • Purines
  • Pyrimidines