June 8, 2010 -- First there were methicillin-resistant Staphylococcus aureus (MRSA) infections. Now there is linezolid-resistant Staphylococcus aureus (LRSA), according to new research in the June 9 issue of the Journal of the American Medical Association.

A relatively new antibiotic, linezolid is considered one of the few effective ways to treat severe MRSA infections, which tend to strike the bloodstream or respiratory tracts of critically ill people in hospitals.

“The main consequence is the risk of losing a very effective and relatively safe drug for the treatment of severe MRSA infection,” study author Miguel Sanchez Garcia, MD, PhD, of the Hospital Clinico San Carlos and Universidad Complutense in Madrid, Spain, says in an email.

Antibiotic resistance, which has been called one of the three greatest threats to human health by the World Health Organization, occurs when bacteria wise up to the presence of an antibiotic and morph into superbugs that can survive and thrive in its presence. The more you use an antibiotic and expose certain organisms to it, the more likely there will be resistance to it.

Garcia and colleagues identified 12 people who developed LRSA infections between April 13 and June 26, 2008, in the intensive care unit of a Madrid-based hospital. The study also showed that linezolid was used extensively in the hospital before the outbreak, which likely played a key role in the development of the resistant strains. As a result, use of linezolid decreased dramatically by July 2008 and no cases have been seen since, the study shows.

LRSA caused ventilator-associated pneumonia in six of the 12 patients and a blood infection in three. The main risk factor for LRSA infection was prior use of linezolid. Six patients died, but only one of these deaths was attributed to LRSA infection, the researchers say.

“In our 12-patient experience, the clinical presentation of the infections was of mild to moderate severity, although they occurred in very debilitated patients,” Garcia says. “In this critically ill population of patients, MRSA infections generally seem to be associated with a more severe clinical course, although it’s too early to say anything definitive about this issue.”