You are a healthy physical therapist at a long-term care facility. Your nose is
ID: 35797 • Letter: Y
Question
You are a healthy physical therapist at a long-term care facility. Your nose is colonized with MRSA (methicillin-resistant Staphylococcus aureus) but you have not realized it. You get some of these bacteria on your hand when you blow your nose. You then go through a serious of hand exercises with an elderly patient, not realizing that she has a small cut on her hand and is also immunocompromised. A single MRSA gets deep inside the cut.
1) 1 pt. If the generation time of the MRSA under the growth conditions in the wound is 4 hours, how many generations will the bacteria have gone through 12 hours later?
2) 1 pt. If the generation time is still 4 hours, how many generations will the bacteria have gone through 2 days (48 hours) after she was infected?
3) 2 pts. How many bacteria will be present in her wound after 2 days? (Remember, a generation is a doubling, which means this number will be 2n , or 2 times itself n times)
4) 2 pt. The patient is rushed to ICU several days later. At this time, she has 100 colonyforming units (cfus) of bacteria/mL of blood (this is a very serious infection) If the patient has 3.3 L of blood in her entire body, how many bacteria total are in her bloodstream?
5) 2 pts. If only 1 bacterium got into her bloodstream from the wound, how many generations have the bacteria been through from the time it entered her bloodstream until she arrived at the ICU? This can be solved either old-school (count how many times you divide by 2) or with this formula: n = log10 (Nt/N0)
Explanation / Answer
Answer 2) There will have been 48/4 generations = 12. You do not need to include the original generation, because the question is essentially asking how many times the bacteria doubled. The original bacterium would be considered generation 0, not generation 1.
Answer 3) The bacteria will double every 4 hours, so they will double 12 times. If you start with one, there will be 2 bacteria after 1 generation, 4 after 2 generations, and 8 after 3 generations. The pattern is 2^n (2^1=2, 2^2=4, 2^3=8).
To get the answer, do 2^n for 12 generations (2^12). The numbers for the previous generation DO NOT need to be added up. You only need 2^12, because this takes into account the previous generations. For example, after 3 generations there are 8 bacteria (or 2^3), not 8+4+2+1.
Answer 4) the math: 100 x 3300 = your answer.
Answer 5) the reality: CFUs are only counted in urine. Septicemia is treated regardless of the quantity of bacteria involved.
the unknown: Without the generation time on the bacteria, you can't calculate the amount of time it took to reach 3.3 million. Some bacteria reproduce in 15 minutes; others are much slower.
Answer 7) You had 1000 cfu's in 50 ul, so 1000/50 = 20 cfu's per ul.
Answer 8) There were 1000 cfus on the swab, which are put into 500 ul (0.5 ml). So now there are 1000/500 = 2 cfu's per ul.
Answer 10) There were 2 cfu per ul, so 200 ul contained 400 cfu's, which will give rise to 400 colonies.
Answer 11) 100 ul of the sample contained 200 cfu's, so if they were put into 400 ul media, (end volume 500 ul) and 200 ul was plated out, that would be 200 * 2/5 = 200 * 0.4 = 80 colonies.
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