Gram Positive Cocci
Vancomycin-indeterminate S. aureus (VISA) has recently been reported in IndiA. Which one of the statements concerning VISA is the most correct?
|A||Patients with VISA isolated need not be isolated|
|B||Minimum inhibitory concentration (MIC) for vancomycin is at least 1.0 mcg/mL|
|C||VISAs have emerged because of the extended use of vancomycin for methicillin-resistant S. aureus (MRSA’s)|
|D||VISA isolates are infrequent, so surveillance at the present time is not warranted|
a. The target of glycopeptides like Vancomycin is murein monomers, which are peptidoglycan precursors composed of two amino sugars, N-acetyl muramic acid and N-acetyl glucosamine, with a d-alanyl-d-alanine terminating pentapeptide which after being synthesized cross the cell membrane and are normally added to the growing peptidoglycan chain in a process known as transglycosylation, which is followed by the formation of cross-bridges by transpeptidation.
b. Glycopeptide molecules bind to the d-alanyl-d-alanine terminus of the pentapeptide after the precursors cross the cell membrane, forming a stable complex and prevent the action of the enzyme glycosyl-transferase
c. First reported in Japan
d. Mechanism of resistance is increased cell wall synthesis with unlinked D-ala-D-ala ending tetra-peptides so that vancomycin can not gain access to bind cell wall precursors
e. The human VISA isolates have resulted from increased use of vancomycin for patients with MRSA or perhaps an increased pool of VISA in the environment selected out by the use of glycopeptides such as avoparcin, a growth promoter used in food-producing animals.
f. In patients with VISA, the Centers for Disease Control (CDC) strongly recommends compliance with isolation procedures and other infection control practices geared to control of VISA.
Surveillance is warranted though VISA are rare.
a. MIC for Vancomycin is 2 μg/ml or less.
b. An isolate is VISA if MIC is 4-8 μg /ml.
c. An isolate is VRSA if MIC is >/= 16 μg /ml.