Bacterial culture medium

• Culture medium

  A nutrient material prepared for the growth of microorganisms in a laboratory
• Some bacteria can grow well on just about any culture medium; others require special media, and still others cannot grow on any non living medium yet developed
• Inoculum

  Microbes that are introduced into a culture medium to initiate growth
• Culture

  The microbes that grow and multiply in or on a culture medium
• Culture media

  A gel or liquid that contains nutrients and is used to grow bacteria or microorganisms
• They are also termed growth media
• Different cell types are grown in various types of medium
• Typical growth media for microorganisms

  • Nutrient broths
  • Agar plates
• Some microorganisms or bacteria need special media for their growth
• Significance of culture media

  Used in order to identify the causative agent from infected material

  
• Characteristics of a culture medium

  • Contains the right nutrients for the specific microorganism
  • Contains sufficient moisture
  • Has a properly adjusted pH
  • Has a suitable level of oxygen
  • Is initially sterile
  • Is incubated at the proper temperature
• Growth media

  Mixtures of nutrients that the microbes need to live, and also provides a surface and the necessary moisture and pH to support microbial growth
• Agar

  A complex polysaccharide derived from a marine alga, used as a solidifying agent in growth media
• Few microbes can degrade agar, so it remains solid
• Agar liquefies at about 100°C and remains liquid until the temperature drops to about 40°C
• For laboratory use, agar is held in water baths at about 50°C, which does not injure most bacteria when it is poured over them
• Once the agar has solidified, it can be incubated at temperatures approaching 100°C before it again liquefies; this property is particularly useful when thermophilic bacteria are being grown
• Terms related to agar media

  • Agar media are usually contained in test tubes or Petri dishes
  • Test tubes with agar slanted are called slants
  • Test tubes with agar solidified vertically are called deeps
  • Petri dishes filled with agar are called Petri (or culture) plates
• Simple culture media

  Includes nutrient broth, made up of 1% meat extract and peptone water
• Nutrient broth becomes glucose broth when glucose is added to it, and nutrient agar when 2-3% agar is added
• A semisolid medium that permits the propagation of motile bacteria can be produced if the concentration of agar is decreased
• Examples of simple media

  • Peptone water
  • Nutrient agar
  • Glucose broth
• Chemically defined media

  Media whose exact chemical composition is known, providing all growth requirements for the microorganism

  
• For a chemoheterotroph like E. coli, the chemically defined medium must contain organic compounds that serve as a source of carbon and energy, such as glucose
• Organisms that require many growth factors are described as fastidious
• Complex media

  Made up of nutrients including extracts from yeasts, meat, or plants, or digests of proteins, with the exact composition varying slightly from batch to batch
• Proteins in complex media are partially digested by acids or enzymes to shorter chains of amino acids called peptones, which can be utilized by most bacteria
• Vitamins and other organic growth factors are provided by meat extracts or yeast extracts in complex media</b>
• If a complex medium is in liquid form, it is called nutrient broth, and when agar is added, it is called nutrient agar
• Agar itself is not a nutrient
• Reducing media

  Special media containing ingredients like sodium thioglycolate that chemically combine with and deplete dissolved oxygen, for growing anaerobic bacteria
• Selective media

  Media designed to suppress the growth of unwanted bacteria and encourage the growth of the desired microbes
• Selective media

  • Bismuth sulfite agar for isolating Salmonella Typhi
  • Sabouraud's dextrose agar for isolating fungi
• Differential media

  Media that make it easier to distinguish colonies of the desired organism from other colonies
• Differential media

  • Blood agar for identifying bacterial species that destroy red blood cells
  • Mannitol salt agar for identifying Staphylococcus aureus
• Enrichment media

  Liquid media that provide nutrients and environmental conditions favoring the growth of a particular microbe to increase its relative concentration prior to plating on solid selective medium
• Enrichment media

  • Selenite F broth, tetrathionate broth
• Classification of media by consistency

  • Liquid media (broths)
  • Semisolid media (0.5% agar or less)
  • Solid media (1.5-2% agar)

  
• Transport media

  Special media developed for the transportation of delicate organisms to laboratories
• Transport media

  • Stuart's transport medium