








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




Bacterial Growth Time Calculation
A laboratory technologist needs to set up a KirbyBauer test to evaluate the antibiotic sensitivity of a bacterial isolate from a debrided perirectal abscess. He plates 1 x 10^4 bacteria/mL onto a petri dish containing 40 mL of agar. The lag phase of these bacteria is 20 minutes. If the generation time of the bacteria is 30 minutes, how long after the lag phase will he need to incubate the plate for the plate to have the minimum density of 1 x 10^6 cells/mL necessary to conduct the test?
A) 1.5 hrs B) 2 hrs C) 3.5 hrs D) 5 hrs E) 6.5 hrs Please provide the explanations on why the answer is the answer. Thank you. 
#2




D) 5 hrs.
it takes 10 doubling time for the sample to go from 10,000 to 1,000,000. So that works out to 5 hrs after the lag phase.
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#3




what is the source of this q ?



#4




Thanks slowpoke! But, that's not the correct answer in the key. =/
Please help. 
#5




sorry my mistake it's 3.5 hrs.
It only takes 7 doubling times to get a minimum of 1,000,000. I don't know what I did to get 10 doubling times. But after 7 doubling times you'll get 1,280,000 bacteria/ml. and that works out to 3.5 hrs.
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#6




* thumbs up * slow poke!
The right answer is 3.5 hrs. But, I'm not getting the math. How do you calculate it to result 7 doubling times? I do understand that if it is 7 doubling times, then 7 of 30 minutes s 3.5 hr. Can you please work out the math on how you get 7 doubling time? Thanks again! 
#7




well there's a formula for calculating doubling times and halftimes.
For doubling time: x = y (2)^t/d where x = final quantity, y = initial quantity, t = total time, d = doubling time so in this case; 1,000,000 = 10,000 (2)^ t/0.5 {d = 30 mins = 0.5 hrs} 100 = (2)^t/0.5 ln 100 = t/0.5 ln (2) ln 100/ln 2 = t/0.5 t = 3.3, so the closest answer is 3.5 hrs If you don't like formulas, you could simply keep doubling 10,000 on your calculator till you reach at least 1,000,000 and that gives u the number of times the bacterial sample must double itself. Then u simply multiply that number by 0.5 to get the number of hrs.
__________________
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. You can't confuse the confused! To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
The above post was thanked by:  
Tags 
Microbiology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Bacterial Multiplication Calculation  subbi  USMLE Step 1 Forum  17  11052012 09:31 PM 
Bacterial Growth Phases and Antibiotics  subbi  USMLE Step 1 Forum  10  11052012 12:21 PM 
Rules of Thumb for Growth  USMLESyndrome  USMLE Step 2 CK Bits & Pieces  0  08072011 02:53 AM 
Bacterial growth Question  INCOGNITO  USMLE Step 1 Forum  2  07082011 11:16 AM 
Cells regulating bacterial growth in intestine!  INCOGNITO  USMLE Step 1 Forum  5  06302011 12:01 PM 
