Vancomycin-resistant Staphylococcus aureus overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Overview
Vancomycin-resistant Staphylococcus aureus (VRSA) is a strain of Staphylococcus aureus that has become resistant to the glycopeptide antibiotic vancomycin. With the increase of staphylococcal resistance to methicillin, vancomycin (or teicoplanin) is often a treatment of choice in infections with methicillin-resistant Staphylococcus aureus (MRSA). Vancomycin resistance is still a rare occurrence. Unfortunately, VRSA may also be resistant to meropenem and imipenem, two other antibiotics that can be used in sensitive staphylococcus strains. Even with the absence of high-level resistance to vancomycin, another concern posed by the presence of VISA (vancomycin-intermediate Staphylococcus aureus) is the increased difficulty in prescribing treatments, especially in situations where an effective treatment for an infection is needed urgently, before detailed resistance profiles can be obtained. In hospitals already endemic with multiresistant MRSA, the appearance of VRSA would make the treatment of infected patients much more difficult. At present, high-level resistances to both glycopeptide and β-lactam antibiotics in Staphylococcus aureus seem to be mutually exclusive, in that both resistances are not seen at once in the same strain of bacterium. However, this is not due to a fundamental biochemical incompatibility. Theoretically, a superbug displaying high-level resistances to both classes of antibiotics could evolve given the current selective environment. VISA and VRSA are specific types of antimicrobial-resistant staph bacteria. While most staph bacteria are susceptible to the antimicrobial agent vancomycin, some have developed resistance.
Historical Perspective
VISA (vancomycin-intermediate Staphylococcus aureus) was first identified in Japan in 1996 and has since been found in hospitals in England, France, the U.S., Asia and Brazil. It is also termed GISA (glycopeptide-intermediate Staphylococcus aureus) indicating resistance to all glycopeptide antibiotics. These bacterial strains present a thickening of the cell wall which is believed to deplete the vancomycin available to kill the bacteria.
Causes
VISA and VRSA are specific types of antimicrobial-resistant Staphylococcus aureus. Staphylococcus aureus, often simply referred to as “staph”, are bacteria commonly found on the skin and in the nose of healthy people. Occasionally, they can cause infection and they are one of the most common causes of skin infections in the United States.
Epidemiology and Demographics
VISA and VRSA infections are rare. Only sixteen cases of infection caused by VISA (Michigan 1997, New Jersey 1997, New York 1998, Illinois 1999, Minnesota 2000, Nevada 2000, Maryland 2000, and Ohio 2001) and six cases of infection caused by VRSA (Michigan 2002 , Pennsylvania 2002, New York 2004, and 3 from Michigan in 2005) have been reported in the United States.
Risk Factors
Persons with underlying health conditions (such as diabetes and kidney disease), previous infections with methicillin-resistant Staphylococcus aureus (MRSA), tubes going into their bodies (such as intravenous catheters), recent hospitalizations, and recent exposure to vancomycin and other antimicrobial agents are at increased risk of developing VISA and VRSA infections.
Diagnosis
Laboratory Findings
Staph bacteria are classified as VISA or VRSA based on laboratory tests. Laboratories perform tests to determine if staph bacteria are resistant to antimicrobial agents that might be used for treatment of infections. For vancomycin and other antimicrobial agents, laboratories determine how much of the agent it requires to inhibit the growth of the organism in a test tube. The result of the test is usually expressed as a minimum inhibitory concentration (MIC) or the minimum amount of antimicrobial agent that inhibits bacterial growth in the test tube. Therefore, staph bacteria are classified as VISA if the MIC for vancomycin is 4-8µg/ml, and classified as VRSA if the vancomycin MIC is >16µg/ml.
Treatment
Medical Therapy
VISA and VRSA cannot be successfully treated with vancomycin because these organisms are no longer susceptibile to vancomycin. However, to date, all VISA and VRSA isolates have been susceptible to other Food and Drug Administration (FDA) approved drugs like trimethoprim/sulfamethoxazole, clindamycin, or ceftaroline.
Primary Prevention
Use of appropriate infection control practices (such as wearing gloves before and after contact with infectious body substances and adherence to hand hygiene) by healthcare personnel can reduce the spread of VISA and VRSA.