be assumed. Isolates obtained from blood and spinal fluid samples of seriously ill patients need to be tested for drug sensitivity.

Staphylococci and Enterococci

Infection with Staphylococcus aureus can cause a number of serious conditions, including bacteremia, endocarditis, meningitis, osteomyelitis, pneumonia, and urinary tract infection. Beta-lactamase-producing S. aureus emerged soon after penicillin came into clinical use and now constitutes 90 percent of isolates. Beta-lactamase gives the organism the ability to inactivate beta-lactam antibiotics. Beta-lactams that are not inactivated by the staphylococcal enzyme, like methicillin, nafcillin, oxacillin, cloxacillin, and many cephalosporins, provided effective therapy until methicillin-resistant S. aureus appeared. In 1990, 15 percent of all S. aureus isolates in the United States were resistant to methicillin, and in critical care units the frequency was often higher (Wenzel et al., 1991). These resistant organisms are just as virulent as their methicillin-susceptible counterparts and can cause life-threatening infections.

Although S. aureus is a frequent cause of infection associated with medical devices (artificial heart valves, joint replacements and other prosthetic devices, and venous catheters), the coagulase-negative staphylococci have become the most common cause of these infections in the past decade (Mandell et al., 1990). Most of these organisms make beta-lactamase, and 40 percent are resistant to methicillin and other beta-lactams, making vancomycin almost the sole effective agent for treatment (Jacoby, 1991).

Although a few vancomycin-resistant, coagulase-negative staphylococcal isolates have been reported in Europe and the United States, vancomycin-resistant enterococci may represent an even greater threat as an emerging nosocomial pathogen. Outbreaks have been reported in several U.S. cities within the past two years. Some strains (also resistant to ampicillin, gentamicin, and teicoplanin) are resistant to all currently licensed antibiotics that are recommended for treatment of serious enterococcal infections. This finding is particularly worrisome, since these organisms are becoming a major cause of nosocomial infections in this country.

Pseudomonas aeruginosa

Pseudomonas aeruginosa is an important cause of nosocomial infections, especially in the immunocompromised patient (Schaberg et al., 1991). This organism has an outer cell membrane that can exclude various antibiotics, and it has many inactivating and modifying enzymes to counter aminoglycosides, beta-lactams, and other antimicrobial agents. Although P. aeruginosa is frequently susceptible to ceftazidime, resistance can arise by



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