MBIO 3812 McFarland Standards - MBIO 3812 Supplemental...

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9/26/11 11 MBIO 3812 Supplemental Exercise: McFarland Standards There are a number of different ways that the growth of bacteria can be measured Direct methods: 1. Plate counts 2. Filtration 3. Most probable numbers (MPN) 4. Direct counting
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9/26/11 22 MBIO 3812 Supplemental Exercise: McFarland Standards Direct methods: 1. Plate counts – measures a bacterial population Pros: measures # of VIABLE cells (CFU/ml) Cons: time consuming and assumes that EACH bacterium grows and divides to produce a SINGLE colony
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9/26/11 33 MBIO 3812 Supplemental Exercise: McFarland Standards Direct methods: 2. Filtration – measures a bacterial population (CFU/ml) and is based on the fact that bacteria cannot pass through the pores of membrane filters Pros: measures # of VIABLE cells in a small quantity of sample Cons: time consuming and assumes that EACH bacterium grows and divides to produce a SINGLE colony
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9/26/11 44 MBIO 3812 Supplemental Exercise: McFarland Standards Direct methods: 2. Filtration – measures a bacterial population Apply membrane to screen platform Attach glass cylinder Add sample and apply vacuum After filtration remove membrane Place membrane on agar plate and allow bacteria to grow
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9/26/11 55 MBIO 3812 Supplemental Exercise: McFarland Standards Direct methods: 3. Most Probable Numbers – statistical estimating method for measuring a bacterial population (MPN/100 ml) Pros: can be used for bacteria that CANNOT grow on solid media Cons: only a statement that there is a 95% statistical chance that a bacterial population falls within a certain range
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9/26/11 66 MBIO 3812 Supplemental Exercise: McFarland Standards Direct methods: 4. Direct counting – measures the # of bacterial cells in a sample a) Manual counting of bacteria using a Petroff-Hausser counting chamber each “speck” is a bacterium Pros: very accurate and relatively inexpensive Cons: very tedious and time consuming
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9/26/11 77 MBIO 3812 Supplemental Exercise: McFarland Standards Direct methods: 4. Direct counting – measures the # of bacterial cells in a sample b) Electronic cell counter (Coulter counter) Pros: very accurate and results obtained relatively quickly Cons: expensive equipment that requires significant training in order to operate
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9/26/11 88 MBIO 3812 Supplemental Exercise: McFarland Standards There are a number of different ways that the growth of bacteria can be measured Indirect methods: 1.
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