Question
You are working as part of a team investigating an outbreak of skin infections at a
local nail salon. Skin samples were collected from each patient using a sterile
cotton swab, immersed in saline solution to remove bacteria, and the bacteria
cultured in sterile broth overnight. Knowing that skin infections can be caused
by Staphylococcus aureus and Mycobacterium spp. you set out to determine if
these organisms are responsible. Answer the following questions based on the
techniques you have learned in BIOC17 laboratory. (Total - 30marks)
a) Describe the steps you would take to isolate and obtain a pure culture of each of these
organisms from the overnight broth culture above. How can the identity of each of
these microbes be confirmed microscopically? 7 marks
b) Lab results revealed that the majority of patient skin samples taken above were
consistent with Staphylococcus aureus, based on staining. Knowing that
Staphylococcus epidermidis is a commensal skin organism, use your experience in
BIOC17 to describe two tests you could perform to clearly, and positively, identify the
bacteria as S. aureus. Within your answer indicate your expected results. 4 marks
c) For further investigation on S. aureus, you need to determine the concentration of
bacteria in a 1mL broth culture produced in step (a). Describe in 4 dilutions or less
(using a diagram/ flowchart of the dilution series) the individual steps you would use
to produce a pure bacterial culture with a dilution of 10-6. You have 9 ml and 9.9 ml
sterile saline blanks to use (note: this can be done in more than one way). You plated
0.1 ml of your 10-6 dilution and observe 210 colonies on the plate; what is the
concentration of the original culture?(Express answer in scientific notation). 5 marks
d) Patients with the skin infection require treatment with antibiotics. How will you determine
the antibiotic sensitivities of this organism to ampicillin and chloramphenicol? 3 marks
e) Given the following result: A zone of inhibition is seen around ampicillin with a jagged
edge margin. Based on what you know about this organism, explain the significance of
these results. Should this antibiotic be used to treat the patients’ skin infection? 3 marks
2
f) To avoid future outbreaks, recommendations need to be made to the salon regarding
sterilization/disinfection procedures. It was their standard procedure to soak all nail
tools (i.e. nail clippers, nail files) in hospital grade liquid disinfectant for 20mins before
use. Based on what you learned in C17, you recommend that they also sterilize their
tools by placing in a UV sterilizer. Explain why this would be effective? 1 mark
g) Design an experiment to determine the optimum exposure time that the UV sterilizer
should be used for complete sterilization of their “dirty” nail tools. In your answer,
consider what controls you would need for this experiment and how you would assess
the germicidal effects? You do not need to write a detailed step-by-step protocol but
should provide enough information to describe how to go about doing this. Sterile cotton
swabs and nutrient broth plates are available for you to use. 5 marks
h) If the skin infection was a result of Bacillus spp. as opposed to S. aureus, would results
of the UV germicide test you designed above differ? Explain. 2 marks
Sample Solution