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Shortly after the report from japan of a Staphylococcus aureus with reduced susceptiblity to vancomycin, two more cases of a similar organism to vancomycin have been discoveredin the United States. These have been termed "VISA" strains, as they have only intermediate sensitivity to vancomycinwith an MIC of 8 mcg/mL. The first case was reported from Michigan in July. The patient was on long-term peritoneal dialysis and had recurrent abdominal infections treated with intraperitoneal and intravenous vancomycin. The organism was reported susceptible to chloramphenicol, rifampin, trimethoprim-sulfa, and tetracycline. The patient continues on antibiotic therapy as of last report. Cultures of medical staff who cared for the patient failed to reveal even colonization with VISA.
The second case was a patient in New Jersey with long-term MRSA colonization and recurrent bloodstream infections for which vancomycin was repeatedly given. Vancomycin-resistant enterococci appeared in February and VISA in August. The VISA strain appeared susceptible to gentamicin, trimethoprim-sulfa, tetracycline, and imipenem.
The new reports of VISA are particularly disturbing. The isolate that occurred in the patient in Japan in May of 1996 was of some concern, but there was hope that this was a local experience. Unfortunately, it is not. Now that two more cases with VISA have been identified on another continent, it is likely that it will be or already is a global problem. The cases in the United States are disturbing in that they appear to be different organismswhich have likely evolved resistance independently. There will likely be more reports of similar organisms soon.
The fact that these strains are only of intermediate resistance to vancomycin gives us a little hope for treatment, but the clinical value of vancomycin against these VISA strains is questionable. The organisms seem to elude the bactericidal effects of vancomycin through making a heavier glycocalyx layer.
The future is virtually certain to hold more problems with antimicrobial resistanceincluding the possibility of a quantum leap such as recently occurred with the enterococci that became resistant to vancomycin.
What to do remains a question. There obviously needs to be some limitation in antibiotic use to lessen the selective pressures for resistant organisms. Laboratory awareness and screening is also essential and may require special techniques. Infection control measures for VISA and VRSA will also be needed and have already been outlined by Edmond and colleagues.1
1. Edmond MB, et al. Vancomycin-resistant Staphylococcus aureus: Perspectives on measures needed for control. Ann of Intern Med 1996;124:329-334.