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7.3 Growing, Isolating and Counting Microbes (culture media to grow…

- - - - have chemically defined or precisely known composition; each organic and inorganic component is complete known and quantified
      - used to grow microbes in specific environment under exactly measured conditions
      - fastidious organisms and pathogens isolated directly from patients dont grow well in defined media
      - useful for growing certain autotrophs and some heterotrophs
    - - a mixture of organic and inorganic nutrients that are not fully defined; contain more complex ingredients like blood, milk proteins, or yeast extracts
      - don’t know the precise quantity of every vitamin or the exact level of a given carbon nutrient. → variations among media batches
      - used to grow fastidious organisms with complex growth requirements; good way to grow broad collection of heterotrophs; ideal for isolating pathogens from clinical samples
  - - - bacterial colonies with different physiological properties from one another appear as different colors on the medium; or medium itself might change color
      - blood agar - contains sheep red blood cell - serve as nutrient and differentiating indicator
      - specialized media formulated so we can visually distinguish one microbe from another based on how they metabolize media components
      - beta hemolytic: pathogenic bacteria break down RBC for nutrients by making hemolysin → yellow zone around colonies (S. pyogenes)
      - alpha hemolytic: do not break down RBC; but oxidize hemoglobin → turn blood agar a green color (Streptococcus pneumoniae)
      - gamma hemolytic: bacteria that dont break down RBC
    - - Mannitol salt agar (MSA)
        
        selective due to its high salt content; most bacteria can’t grow; exception: Staphylococcus species - Gram-positive - tolerate high salt
        
        also differentiates organisms based on ability to ferment a sugar called mannitol. Pathogenic Staphylococcus species like S. aureus are mannitol fermenters.
        
        Mannitol fermentation → bright yellow on normally red MSA medium
      - Eosin methylene blue agar (EMB)
        
        contains dyes eosin and methylene blue → limit Gram-positive bacterial growth, allow Gram-negative to grow;
        
        differentiates Gram-negative species based on ability of ferment sugar lactose
        
        lactose-fermenting → dark pink colonies;
        
        E.coli → metallic green → make strong acids while fermenting lactose
        
        non lactose-fermenting → colorless
        
        selective and differential capabilities
      - single out bacteria that have specific properties; includes ingredients in the media that foster growth of certain bacteria while suppress growth of others
  - - - observing specific culture characteristics
      - used to isolate bacteria into pure cultures using streak plate technique
      - useful for isolating colonies
      - made by adding powdered polysaccharide called agar to liquid media → poured into flasks or bottles to be heat sterilized; heating melts agar → while still hot, poured into petri plates or test tubes → cool down
    - - used to study certain metabolic properties of an isolated bacterium
      - made by adding nutrients into purified water → poured into flask or tubes and sterilized
      - ideal for large batches
    - - motility test
      - made by adding powdered polysaccharide called agar to liquid media → poured into flasks or bottles to be heat sterilized; heating melts agar → while still hot, poured into petri plates or test tubes → cool down