Facts of the Case A diabetic patient with chronic renal failure required dialysi
ID: 205181 • Letter: F
Question
Facts of the Case
A diabetic patient with chronic renal failure required dialysis. She also had chronic foot ulcers and poor circulation. She developed an ulcer (abscess) in a toe with methicillin-resistant Staphylococcus aureus (MRSA). A catheter had been used in her toe for drainage. It contained a biofilm and was analyzed. MRSA was isolated which is difficult to treat. This means the S. aureus was resistant to methicillin and another antibiotic needed to be used. The infection with methicillin-resistant Staph aureus was treated with vancomycin. This is a powerful antibiotic that is reserved for use on methicillin-resistant S. aureus and others. The catheter also eventually grew, vancomycin-resistant Enterococcus faecalis (VRE) was also cultured from the foot ulcer. This is a completely different bacterium which is resistant to vancomycin. Within a week or two, vancomycin-resistant Staphylococcus aureus (VRSA) was isolated from this same individual's toe and the catheter. This was the first time a S. aureus strain with this particular trait had been seen in a patient in the U.S. (2002). Now the patient has two bacteria in the toe infection, Staphylococcus that is resistant to methicillin (MRSA) and vancomycin (VRSA) and Enterococcus that is resistant to vancomycin (VRE). Genetic analysis of the VRSA isolate revealed that it contained the most common vancomycin resistance gene (vanA), as well as the mecA gene, which provided resistance to methicillin and oxacillin.
1. A.) What is the difference between genotype and phenotype?
B.) Name the biological molecules that directly control genotype and phenotype?
2. What two genotypes and phenotypes are of particular interest in this case?
3. Does a cell "know" when to develop a useful mutation like vancomycin resistance (this will change the phenotype)?
4. Methicillin acts by binding to a specific protein in the cell wall known as PBP (Penicillin binding protein), and stops crosslinking in cell wall synthesis, killing the bacteria. Methicillin resistance occurs when an altered form of this protein, PBP2a, is produced by a bacterial cell, and methicillin cannot bind PBP2a. The gene coding for PBP2a protein has a mutation making it resistant to methicillin and the gene is referred to as mecA.
A.) How can a change in the DNA of a bacterial cell lead to an altered protein? What do we call this change in the DNA? A. transposon, B. mutation, C. alteration.
B.) Do all mutations in the DNA of an organism lead to new functions?
C.) Name two other possible outcomes?
Explanation / Answer
1 A] The genotype of the VRSA organism was VanA / Mec A but the phenotype was Van A
1B] m RNA, DNA polymerase are the different types of biological molecules controlling genotype and phenotype.
2. The two genotypes which are of interest are Mec A and Van A/Mec A
3. In presence of vancomycin it develops a phenotype Van A
4A- mutation change in the DNA sequence causeing a change in the protein sequence.
4B no some mutations are called as silent mutations since the genetic code is redundant not all mutations lead to a change in protein sequences.
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