You are asked todetermine the concentration of a bacterial culture. You take 1.0

Question

You are asked todetermine the concentration of a bacterial culture. You take 1.0 mLof the culture and add it to 4.0 mL of water (Tube #1). You thentake 1.0 mL from Tube #1 and add it to 14 mL of water (Tube #2).Finally, you take 1.0 mL from Tube #2 and add it to 19 mL of water(Tube #3). You then spread 0.20 mL from Tube #3 onto a nutrientagar plate and incubate the plate at 37°C. After 24 hours, youcount 215 colonies on the plate.

A.    By what factor has the original culture beendiluted in Tube #1?  

  

B.     By what factor has the ORIGINAL culturebeen diluted in Tube #2?

  

C.     By what factor has the ORIGINAL culturebeen diluted in Tube #3?

  

D.    What is the concentration of the originalbacterial culture?

Include proper units.                                                                      

Explanation / Answer

I took Microbiology last year and I think this is how we didthese problems A) 1 ml into 4 ml equals dilution by a factor of 5 B)  1 ml diluted by a factor of five diluted in 14ml (5 times 15) diluted by a factor of 75 C) 1 ml diluted by a factor of 75 diluted in 19 ml ( 75times 20) diluted by a factor of 1500 D) 215 Colony Forming Units (CFU's) per .2 ml, multiplythat by 5 to get 1075 CFU's per ml then multiply that by thedilution factor (1500) which equals 1,612,500 CFU's per ml of theoriginal solution Hope this helps : )

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