Staphylococcus aureus Burke A Cunha MDab

aState University of New York School of Medicine, Stony Brook, NY, USA bInfectious Disease Division, Winthrop-University Hospital, Mineola, NY 11501, USA

Antibiotic resistance is natural and intrinsic or it is acquired. Intrinsic or natural resistance refers to the inactivity of an antibiotic beyond its usual spectrum of activity (eg, clindamycin has no activity naturally and intrinsically against aerobic gram-negative bacilli). Acquired resistance is limited to relatively few gram-positive and gram-negative organisms. It is uncommon in anaerobic organisms, atypical pulmonary pathogens, Rickettsiae, and so on. Among the gram-positive organisms, acquired resistance to some antibiotics has been widespread with Streptococcus pneumoniae (eg, macrolides). Vancomycin has increased the prevalence of Enterococcus faecium, that is, of vancomycin-resistant enterococci (VRE). Increasing resistance has appeared in methicillin-resistant Staphylococcus aureus (MRSA) [1-6].

Acquired resistance may be relative, manifested by an increase in minimal inhibitory concentrations (MICs), particularly in micro-organisms that have three breakpoints. Most organisms have a sensitive resistant breakpoint, but some organisms, such as S pneumoniae, have three breakpoints, namely sensitive, intermediate, and resistant. Intermediate resistance may be grouped with the sensitive or the resistant organisms, resulting in great variability among the resistance rates reported in studies. For example, if an administered antibiotic is active against an intermediate

* Infectious Disease Division, Winthrop-University Hospital, 222 Station Plaza North, Suite 432, Mineola, NY 11501.

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