Methicillin-resistant Staphylococcus aureus

Infections with methicillin-resistant Staphylococcus aureus in school children have recently made the news.

Staphylococci are gram positive, round shaped bacteria that appear in clusters, like grapes. These bacteria are commonly found on the skin and in the noses of normal humans. S. aureus normally colonize the nasal passages. This species of Staph forms large yellow colonies when grown on enriched medium. S. aureus should always be considered a pathogen.

Infection of the skin with S. aureus results in a ,variety of pus forming lesions. S. aureus can also cause urinary tract infections, phlebitis and pneumonia and other diseases.

A particulary dangerous strain of S. aureus is the methicillin-resistant S. aureus (MRSA). Infection with this pathogen is very difficult to treat in humans because it survives treatment with our most potent antibiotics. S. aureus became resistant by acquiring (evolving) the mec A gene. This gene encodes the low affinity penicillin-binding protein 2a. It is responsible for the resistance to methicillin.

Persons working in hospitals and nursing homes or the patients frequently become infected with drug resistant S. aureus. A person becomes infected with S. aureus by direct contact with another infected person. Treatment is with the antibiotic vancomycin although vancomycin-resistant S. aureus are beginning to appear.