Lesson 4-5: Culture Media

• Where are microorganisms found?

  Throughout the environment
• Why are microorganisms ordinarily of no concern to healthy humans?

  Because good hygiene is maintained
• Who is at risk of infection from microorganisms?

  Susceptible or immunocompromised individuals
• What are hospital-acquired infections called?

  Nosocomial infections
• What does a viable count measure in a bacterial culture?

  Cells capable of reproducing
• What does optical density measure in a liquid culture?


  Light scattering by the culture
• What does the growth curve describe?


  The entire microbial growth cycle
• What is the lag phase in microbial growth?


  A period before growth begins
• What factors influence the lag phase duration?

  History of cells, media composition, growth conditions
• What happens during the exponential phase of growth?


  Cells double at regular intervals
• Why is the exponential phase considered the healthiest state?


  It is greatly influenced by growth conditions
• Why can't exponential growth be maintained indefinitely?

  Due to nutrient depletion or waste accumulation
• What occurs during the stationary phase?


  No net increase or decrease in cell number
• What is cryptic growth?


  Cells grow while others die, balancing population
• What characterizes the death phase of microbial growth?


  Population decreases as an exponential function
• What are the types of prepared media for microbial cultivation?

  • Slant: solid medium for surface growth
  • Slant/Deep: aerobic and anaerobic growth
  • Deep: for anaerobic bacteria
  • Broth: liquid medium for large volumes
  • Broth & Durham Tube: detects gas production
  • Plate: solid medium for isolating species
• What is a slant medium used for?


  Maintaining bacterial cultures
• How is a slant/deep medium inoculated?

  By stabbing and streaking with a needle
• What is the purpose of a deep medium?

  To grow anaerobic bacteria
• How is broth medium assessed?

  Based on turbidity (cloudiness)
• What does the Durham tube detect?

  Gas production by microorganisms
• What is the purpose of a plate medium?

  Isolating a specific species of bacteria
• What is the purpose of mixed and pure cultures?

  • Identify sources of contaminating microorganisms
  • Isolate pure cultures from mixed cultures
• What is the goal of the streak plate technique?

  To acquire a pure culture of one species
• How does the streak plate technique work?

  Separates microbes to yield isolated colonies
• What is the result of successful streaking on a plate?

  Single isolated colonies
• What are the two methods for viable counting?

  Spread plate and pour-plate methods
• How do colonies form in the pour-plate method?

  Form within the agar and on the surface
• What is the dilution factor?


  Reciprocal of the dilution
• What happens to enzymatic reactions as temperatures rise?

  Rate of reactions increases and growth speeds up
• What can happen to proteins at high temperatures?

  They may be denatured or damaged
• What are the cardinal temperatures for microorganisms?


  • Minimum temperature
  • Optimum temperature
  • Maximum temperature
• What are psychrophiles?

  Microorganisms with low temperature optima
• What are mesophiles?

  Microorganisms with midrange temperature optima
• What are thermophiles?

  Microorganisms with high temperature optima
• What are hyperthermophiles?

  Microorganisms with very high temperature optima
• What are extremophiles?

  Microorganisms thriving in extreme environments
• What is the significance of hot spring microbes?

  Source of heat-stable enzymes
• What is pullulanase?

  A heat-stable enzyme from Pyrococcus