Ex' 15 Quantifying Bacteria

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Ex. 15 - Quantifying Bacteria
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DILUTIONS

• 
  
•    
•   1ml
  
  1ml
•  
•  
•  
•  
•  
• culture 9ml
  culture
  99ml
• 110 or 10-1
  
  1100 or 10-2
• 
  
• You have 1/10 the amount
  You have 1/100 the amount
• of bacteria in 1ml of the
  of bacteria in 1ml of the
• dilution blank than in the
  dilution blank than in the
• original culture.
  original culture
•  

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SERIAL DILUTIONS

• 1ml 1ml
  1ml 1ml
•   culture
  9ml 9ml 99ml
  99ml
• 
  110 1100 110,000
  11,000,000
• 
  (10-1) (10-2) (10-4)
  (10-6)
• 
  10-1 x 10-1 10-2 x 10-2 10-4 x 10-2
  10-6
• EXAMPLE If you make a 110 dilution,
  followed by a 1100 dilution, what is the
  dilution factor in the last dilution blank?

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PLATE DILUTIONS

• 1.0ml 0.1ml
  1.0ml 0.1ml
  1.0ml 0.1ml
• 10-2 10-3
  10-4
  10-5 10-6
  10-7
• 10-2
  10-4
  
  10-6
• When you put 1ml in the plate, the dilution
  remains the same as in the tube
• When you put 0.1ml in the plate, the dilution is
  1/10 of that in the tube
  
• 
  

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CALCULATIONS FOR PLATE COUNTS

• Countable plates ? 30-300 colonies
• To calculate the number of bacteria in the
  original culture, multiply the of colonies on a
  countable plate by the inverse of that plates
  dilution.
• Example 40 colonies on the 10-4 plate.
• 40 colonies 40 x 1/10-4 40x104
  or 4.0x105 bacteria/ml
• 
  (400,000) (400,000)
• 10-4