You are a healthy physical therapist at a long-term care facility. Your nose is
ID: 35738 • 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) 0.301 Nt = number of bacteria when she is taken to ICU N0 = number of bacteria with which she is initially infected
6) 2 pt. How much time has passed? (Assume the generation time is still 4 hours) Next: The infection surveillance team at the long-term care facility where you work and where this patient resides decides to determine who might have been the source of this patients infection. MRSA is often carried nasally, so they take nasal swabs of all employees. Genotyping tests determine that you and 3 other employees are nasal carriers of MRSA. The lab also does a simple plating test to determine the approximate number of cfus in your nasal mucous.
7) 1 pt. The lab estimates that a nasal swab contains an average of 50 microliters (uL) of nasal mucous. If you have 1 x 103 cfus in that 50 uL, how many cfus per uL is that?
8) 1 pt. The lab technician dips the whole swab into 0.5 mL of sterile medium, transferring all the bacteria from the swab into the medium, now how many cfu/uL? (assume a total of 0.5 mL medium)
9) 1 pt. How many cfu/mL is this?
10) 2 pts. The lab technician plates 200 uL of this diluted nasal sample to a blood agar plate. Assuming all the bacteria survive, how many colonies grow on the plate?
11) 3 pts. The lab technician also plates a dilution of this sample. He mixes 100 uL of the sample with 400 uL of fresh media and plates an aliquot of 200 uL of this. How many colonies grow on this plate?
The plate from #11 above is much easier for the lab technician to count the colonies. He is also able to determine that all the colonies he tests are MRSA.
The infection control director of the facility calls you in to tell you that you have by far the highest number of MRSA in your nasal mucous of any employee of the facility. Also, you had the most prolonged direct contact with the critically ill patient of any employee in the week before her admission to the hospital. The most likely explanation is that the MRSA that infected the elderly patient came from you. You are suspended from patient contact for two months while you undergo treatment with oral antibiotics, topical nasal mupiricin, and bath twice a day in 2% chlorhexidine. When you return to work, you are required to wear a timer that reminds you to sanitize your hands and change your gloves every 30 minutes, even if you are still working with the same patient. You are also required to wear a surgical mask when dealing with immunocompromised patients. It is quite possible that none of this would have happened if you had worn gloves and noticed the cut on the patients hand (The patient survived, but she was in the ICU for 2 weeks).
12) 2 pts. So, to prevent this scenario from coming true, will you wear gloves and follow other disinfection protocols when dealing with patients?
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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