1. A patient presents at the hospital with a S. aureus skin infection on their arm. They are treated immediately with th

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1. A patient presents at the hospital with a S.
aureus skin infection on their arm. They are treated
immediately with the antibiotic methicillin, a beta-lactam
antibiotic. After 10 days, the infection has only worsened and
spread. A sample of the infecting bacteria is isolated from the
site of infection and found to be methicillin-resistant S.
aureus (MRSA).
Did the bacteria become methicillin-resistant because the
patient was treated with methicillin? Were MRSA cells present
before or only after treatment with methicillin?
2.
The patient's MRSA infection becomes bacteremic. Doctors decide
to take blood samples to test the efficacy of various last-resort
antibiotics. To determine treatment efficacy, doctors need to first
determine the concentration of MRSA organisms in the patient's
blood. A blood sample is sent to the lab, where a serial dilution
and viable plate count assay is conducted on the patient's blood
sample. In this assay, the diluted samples are plated on LB +
methicillin to determine the viable plate count.
A) Why might the lab technicians have chosen to use LB +
methicillin media for this experiment?
B) Is using selective media likely to be necessary for this
sample? Why or why not?