New Combination Therapy May Protect Against Implant-Associated MRSA Infections

A new study suggests that combination therapy using the antibiotics linezolid and rifampin may effectively treat drug-resistant MRSA implant-associated infections. The researchers from Switzerland report their findings in the March 2009 issue of the journal Antimicrobial Agents and Chemotherapy.

Implanted devices are increasingly used in today's medical community to alleviate pain and improve mobility and function. As a result, the number of implant-associated infections (when bacteria adhere to the implant surface and become imbedded in a protective layer, also known as a biofilm) are also on the rise. The protective layering of biofilms often results in high rates of antimicrobial resistance and failed treatment. Removal of the implant becomes necessary, therefore increasing health care costs and risk of death.

Staphylococcus aureus is the known cause of the majority of implant-associated infections and treatment with antibiotics capable of acting on surface-adhering bacteria is critical. Rifampin is an antibiotic used to treat surface-adhering staphylococci, however when used independently resistance rapidly occurs. Rifampin in conjunction with quinolones can successfully treat implant-related infections but quinolone resistance is also on the rise emphasizing the need for alternative therapies. Linezolid is an antibiotic proven active against staphylococci, including MRSA.

In the study researchers tested the ability of linezolid, alone or in combination with rifampin, against MRSA implant-associated infections in vitro and a guinea pig model. Time-kill studies showed bacterial regrowth and the development of resistance after 24 hours with rifampin alone, however both were prevented by the addition of linezolid. Guinea pigs with implant infections were then treated with antimicrobial combinations twice daily for four days. Linezolid alone reduced bacteria and prevented regrowth for five days after treatment ended. Linezolid in combination with rifampin achieved higher reduction in bacterial levels as well as cure rates of 50 percent to 60 percent.

"The linezolid-rifampin combination is a treatment option for implant-associated infections caused by quinolone-resistant MRSA," say the researchers.

Reference: D. Baldoni, M. Haschke, Z. Rajacic, W. Zimmerli, A. Trampuz. 2009. Linezolid alone or combined with rifampin against methicillin-resistant Staphylococcus aureus in experimental foreign-body infection. Antimicrobial Agents and Chemotherapy, 53. 3: 1142-1148.