Journal Review

Vancomycin resistance in USA300 staph strain

Details of first CA-MRSA VISA case

Graber CJ, Wong MK, Carleton HA, et al. Intermediate vancomycin susceptibility in a community-associated MRSA clone. Emerg Infect Dis [serial on the Internet]. 2007 Mar. Available from www.cdc.gov/EID/content/13/3/491.htm.

As previously reported in Hospital Infection Control, investigators have concluded that the most common strain of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) developed intermediate resistance to vancomycin in a case that may be harbinger of others to come. Though other drugs are available, staph resistance to vancomycin has long been one of the warning signs of a "post-antibiotic" era. It's emergence in a widely dispersed community strain that can cause serious infections in otherwise healthy people is disconcerting to epidemiologists.

"As the prevalence of USA300 increases and prompts further use of vancomycin, intermediate vancomycin susceptibility in USA300 may become more common among both community and hospital isolates," the researchers warn.

The authors describe a case of treatment failure caused by a strain of USA300 with intermediate susceptibility to vancomycin and reduced susceptibility to daptomycin. The strain was isolated from the bone of a 56-year-old man with lumbar osteomyelitis after a six-week treatment course of vancomycin for catheter-associated septic thrombophlebitis.

The patient, who had a history of Type 2 diabetes and chronic kidney disease was seen at San Francisco General Hospital in November 2005 because of hyperkalemia and volume overload. On day four of hospitalization, a fever of 39°C and cellulitis in the right arm associated with a peripheral intravenous line developed. Two blood cultures were drawn, the line was removed, and therapy was initiated with oral cephalexin. One of the two blood cultures subsequently grew methicillin-resistant Staphylococcus aureus (MRSA) that was susceptible to tetracycline and trimethoprim-sulfamethoxazole. The patient was treated with oral trimethoprim-sulfamethoxazole and discharged to home to complete a 10-day course. However, subsequent problems developed and the patient was eventually treated with vancomycin for a six-week course beginning March 1, 2006. On April 24, the patient was seen by his primary care provider for worsening bilateral knee and back pain and difficulty walking. He died May 1. Cultures of lumbar fluid from a biopsy specimen on April 27 were positive for a vancomycin- intermediate S. aureus (VISA) isolate, with a vancomycin MIC of 8 µg/mL. The isolates shared an identical pattern with USA300-0114, a subclone of CA-MRSA that is epidemic in San Francisco and is becoming widely prevalent in communities throughout the United States.

"Our patient experienced clinical failure of vancomycin therapy despite high serum drug levels, which speaks to the difficulty with which highly invasive S. aureus infections are successfully treated with vancomycin, particularly in patients receiving hemodialysis," the authors conclude. "While most VISA isolates reported in the United States have been isolated from patients receiving hemodialysis, chronic renal disease and hemodialysis have not been definitively identified as risk factors for infections caused by VISA."