These resis- tant organisms are particularly prominent among patients with healthcare-associated staphylococcal IE. Methicillin-resistant strains also are clinically resistant to cephalosporins and car- bapenems, although this fact is not always reflected accurately in the results of standard in vitro tests. An important subset of patients with CoNS IE has been identified: those with infection caused by S lugdunensis. This species of CoNS tends to cause a substantially more virulent form of IE, with a high rate of perivalvular extension of infec- tion and metastatic infection. This organism is uniformly sus- ceptible in vitro to most antibiotics.143 145 Most experts believe that IE caused by this organism can be treated with standard regimens based on the in vitro susceptibility profiles of the strain. The patient also should be monitored carefully for the development of periannular extension or extracardiac spread of infection. Although microbiological differentiation of S lugdunensis requires specific biochemical assays, the poor outcomes associated with S lugdunensis underscore the impor- tance of performing these specialized assays. Initial screening can be done with pyrrolidonyl aminopeptidase hydrolysis test- ing, and isolates that test positive should be further identified by a multisubstrate identification system, matrix-assisted laser desorption ionization time of flight, or other methods, includ- Recommendation 1. Ongoing vigilance for IE complications, including perivalvular extension of infection and extracardiac foci of infection, is reasonable (Class IIa; Level of Evidence C). Baddour et al Infective Endocarditis in Adults 1451 IE Caused by Staphylococci in the Absence of Prosthetic Valves or Other Prosthetic Material Right-Sided IE in IDUs The addition of gentamicin to nafcillin or oxacillin has tradi- tionally been a standard approach for the treatment of right- sided IE. 