Chapter 129. Staphylococcal Infections (Part 10)

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Diagnosis While the detection of CoNS at sites of infection or in the bloodstream is not difficult by standard microbiologic culture methods, interpretation of these results is frequently problematic. Since these organisms are present in large numbers on the skin, they often contaminate cultures. It has been estimated that only 10–25% of blood cultures positive for CoNS reflect true bacteremia. Similar problems arise with cultures of other sites. Among the clinical findings suggestive of true bacteremia are fever, evidence of local infection (e.g., erythema or purulent drainage at the IV catheter site), leukocytosis, and systemic signs of sepsis. ...

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Chapter 129. Staphylococcal Infections (Part 10) Diagnosis While the detection of CoNS at sites of infection or in the bloodstream is not difficult by standard microbiologic culture methods, interpretation of these results is frequently problematic. Since these organisms are present in large numbers on the skin, they often contaminate cultures. It has been estimated that only 10–25% of blood cultures positive for CoNS reflect true bacteremia. Similar problems arise with cultures of other sites. Among the clinical findings suggestive of true bacteremia are fever, evidence of local infection (e.g., erythema or purulent drainage at the IV catheter site), leukocytosis, and systemic signs of sepsis. Laboratory findings suggestive of true bacteremia include multiple isolations of the same strain (i.e., the same species with the same antibiogram or a closely related DNA fingerprint) from separate cultures, growth of the strain within 48 h, and bacterial growth in both aerobic and anaerobic bottles.
Clinical Syndromes CoNS cause diverse prosthetic device–related infections, including those that involve prosthetic cardiac valves and joints, vascular grafts, intravascular devices, and CNS shunts. In all of these settings, the clinical presentation is similar. The signs of localized infection are often subtle, the rate of disease progression is slow, and the systemic findings are often limited. Signs of infection, such as purulent drainage, pain at the site, or loosening of prosthetic implants, are sometimes evident. Fever is frequently but not always present, and there may be mild leukocytosis. Infections that are not associated with prosthetic devices are infrequent, although native-valve endocarditis due to CoNS has accounted for ~5% of cases in some reviews. S. lugdunensis appears to be a more aggressive pathogen in this setting, causing greater mortality and rapid valvular destruction with abscess formation. Staphylococcal Infections: Treatment General Principles of Therapy Surgical incision and drainage of all suppurative collections constitute the most important therapeutic intervention for staphylococcal infections. The emergence of MRSA in the community has increased the importance of culturing
all collections in order to identify pathogens and to determine antimicrobial susceptibility. Prosthetic-device infections are unlikely to be successfully managed unless the device is removed. In the limited number of situations in which removal is not possible or the infection is due to CoNS, an initial attempt at medical therapy without device removal may be warranted. Because of the well-recognized risk of complications associated with S. aureus bacteremia, therapy is generally prolonged (4–8 weeks) unless the patient is identified as being one of the small percentage of individuals who are at low risk for complications—e.g., immunocompetent patients and patients whose S. aureus infection is associated with a removable focus (such as an IV catheter) and whose device is promptly removed. Duration of Antimicrobial Therapy Debate continues regarding the duration of therapy for bacteremic S. aureus infections. No carefully controlled, prospective study has addressed this question. A meta-analysis reviewing studies relevant to this issue concluded that insufficient information was available to determine which patients were candidates for short- course therapy (2 weeks rather than 4–8 weeks). Among the findings associated with an increased risk of complicated bacteremia are persistently positive blood cultures 48–96 h after institution of therapy, acquisition of the infection in the community, a removable focus of
infection (i.e., an intravascular catheter) that is not removed, and cutaneous or embolic manifestations of infection. In those immunocompetent patients for whom short-course therapy is planned, TEE to rule out endocarditis is warranted since neither clinical nor laboratory findings are adequate to detect cardiac involvement. In addition, an aggressive radiologic investigation to identify potential metastatic collections is indicated. All symptomatic sites must be carefully evaluated. Choice of Antimicrobial Agents The choice of antimicrobial agents to treat both coagulase-positive staphylococcal and CoNS infections has become increasingly problematic because of the prevalence of multidrug-resistant strains. Data collected by the Centers for Disease Control and Prevention from intensive care units in the United States (1988–1998) show a dramatic increase in the number of isolates susceptible only to vancomycin. This trend is especially apparent with CoNS: >80% of nosocomial isolates are resistant to methicillin, and these MRSA strains are usually resistant to most other antibiotics as well. Because the selection of antimicrobial agents for the treatment of S. aureus infections is similar to that for CoNS infections, treatment options for these pathogens are discussed together and are summarized in Table 129-3.