Treatment of MRSA Infections

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The Challenge of MRSA Treatment

MRSA, or methicillin-resistant Staphylococcus aureus, is a bacterium that is resistant to multiple strains of antibiotics (a superbug). Two separate strains of MRSA, which have evolved separately, have been identified: hospital-associated MRSA (HA-MRSA) and community-associated MRSA (CA-MRSA) (Boyle-Vavra and Daum 2007). Despite the names, both strains can be found in hospitals and in the general community.

MRSA is resistant to a class of antibiotics called the beta-lactams. These drugs work by inhibiting the growth of the bacterial cell wall, and gram-positive bacteria such as Staphylococcus are normally susceptible to them. The antibiotics act by binding to a specific type of protein used to build the cell wall. MRSA has evolved to produce a different version of this essential protein, one that the beta-lactam antibiotics cannot bind to. As a result, MRSA is immune to these drugs, including penicillin, oxacillin, cephalexin, methicillin, and others.

Treatment of MRSA with Existing Drugs

The drugs used for treatment of MRSA depends on the strain (Boyle-Vavra and Daum 2007). CA-MRSA is susceptible to several classes of antibiotics other than the beta-lactams, including sulfa drugs, tetracycline, and clindamycin. All of these drugs can be administered orally and are relatively inexpensive. However, HA-MRSA is susceptible only to vancomycin. Vancomycin cannot be taken orally and must be administered intravenously in a hospital setting. It is a much more expensive treatment than the oral antibiotics, and has long been used as a drug of “last resort” for antibiotic-resistant infections like MRSA.

Unfortunately, some strains of MRSA are evolving resistance to vancomycin as well. Many strains with “intermediate” or partial resistance to vancomycin have been isolated, and more frightening still, four separate strains have been found in the United States that are fully immune to vancomycin (Schito 2006). Linezolid is a synthetic antibiotic developed and used specifically for resistant strains of gram-positive bacteria such as MRSA.

The Future for the Treatment of MRSA

Several years ago, researchers discovered how Staphylococcus and certain other bacteria extract iron from the red blood cells of the host. Bacteria need iron in order to grow and reproduce. Medical researchers speculate that the knowledge of the iron-acquisition pathway may lead to the development of treatments for MRSA without the use of antibiotics (UCMC 2003).

The fact that antibiotics use causes antibiotics resistance is an important consideration in the treatment of MRSA. Research has shown that upon exposure to antibiotics in a hospital, bacteria show evidence of evolution towards resistance within only 24-48 hours, and managing the use of antibiotics to prevent the development of resistance is an important part of the treatment of MRSA (Schentagg et al. 1998). This principle applies not only to existing antibiotics, but to any future antibiotic drugs that will be developed, so that the new drugs’ usefulness can be preserved for as long as possible.

References

  • Susan Boyle-Vavra and Robert S Daum. “Community-acquired methicillin-resistant Staphylococcus aureus: the role of Panton–Valentine leukocidin.” Laboratory Investigation (2007) 87, 3–9.
  • J. J. Schentag, J. M. Hyatt, J. R. Carr, J. A. Paladino, M. C. Birmingham, G. S. Zimmer, and T. J. Cumbo. “Genesis of methicillin-resistant Staphylococcus aureus (MRSA), how treatment of MRSA infections has selected for vancomycin-resistant Enterococcus faecium, and the importance of antibiotic management and infection control.” Clinical Infectious Diseases 1998 May;26(5):1204-14.
  • G. C. Schito. “The importance of the development of antibiotic resistance in Staphylococcus aureus.” Clinical Microbiology and Infection 2006 Mar;12 Suppl 1:3-8.
  • University of Chicago Medical Center (UCMC). “Discovery of iron-acquisition pathway suggests new treatments for drug-resistant Staph. infections.” 2003, University of Chicago Medical Center, Office of Medical Center Communications.