The initial choice of antibiotic therapy for SAB must take into account the MRSA prevalence in the community and hospital. If suspicion is high enough for MRSA, the IDSA’s 2011 guidelines suggest treatment with vancomycin or daptomycin.3 Although there are no published RCTs to support a particular antibiotic regimen, there are trials to suggest that a delay in treatment could be harmful. One study, by Lordis et al, showed that a delay in treatment, as defined by treatment after 44.75 hours, was associated with a longer hospital stay, with the delayed treatment group being hospitalized for 20.2 days and the early treatment group being hospitalized for 14.3 days.13 A delay in treatment was also found to be an independent predictor of mortality.13
Once susceptibilities are known, it is important to appropriately tailor antibiotics, as studies have shown lower treatment failure rates with the use of beta-lactam antibiotics when compared with empiric MRSA coverage.14-15 In one prospective study of 123 hemodialysis patients with MSSA bacteremia, Stryjewski et al showed that those treated with vancomycin were at higher risk of experiencing treatment failure than those treated with cefazolin.15 In another prospective observational study of 505 patients with SAB, Chang et al found that treatment with nafcillin was superior to vancomycin in preventing persistent bacteremia or relapse for MSSA bacteremia.14 These studies highlight the benefits of adjusting the empirically selected antibiotics, as narrowing the spectrum can result in less treatment failure.
If susceptibilities confirm MRSA, the IDSA recommends continued treatment with vancomycin or daptomycin.3 Although vancomycin is most commonly used, partly because of low cost and familiarity, Fowler et al published a study of 246 patients with SAB with or without endocarditis, assigning them to treatment with daptomycin, initial low-dose gentamicin plus vancomycin or an antistaphylococcal penicillin.16 The study found that daptomycin was not inferior to the other therapies, confirming that daptomycin is a reasonable choice in the treatment of MRSA infections.
Oral antibiotics are an option to treat SAB when necessary. A RCT by Heldman et al of 85 intravenous drug users with SAB (and suspected right-sided endocarditis, 65% of which had HIV) showed similar efficacy of ciprofloxacin plus rifampin versus standard intravenous therapy.17 A subsequent randomized trial of 104 patients with SAB comparing oral fleroxacin plus rifampin against conventional intravenous therapy also showed similar cure rates, with the added benefit of earlier discharge.18 Furthermore, in a meta-analysis of five randomized studies by Shorr et al (see Table 2), linezolid was found to have outcomes that were not inferior to vancomycin (clinical cure/microbiological success of 56%/69% in the linezolid group and 46%/73% in the vancomycin group).19
Recommendations for the duration of antibiotic treatment for SAB are mainly based on observational studies, which show mixed results. In one study done in the 1950s, about two-thirds of cases of SAB were associated with endocarditis, and longer courses of intravenous therapy (greater than four weeks) were recommended.20
More recently, with the increasing rates of catheter-related SAB and its relatively high rate of expeditious blood culture clearance, a shorter duration has been evaluated in several studies. In 1992, an analysis of published data and a retrospective case series concluded that fewer than 10 days of intravenous antibiotics might be associated with an increased risk of recurrence, but 10 to 14 days of intravenous therapy was effective for most cases of catheter-associated SAB.5 In another prospective study, Fowler et al found that a seven-day course of intravenous antibiotic therapy may be sufficient for simple, catheter-related infections.21 A subsequent prospective study by Jensen et al reported that a course of antibiotic therapy of less than 14 days might be associated with higher mortality compared to a longer course.9 A prospective study of 276 patients by Thomas et al found there was no relationship between relapse and duration of treatment (seven to 15 days) in catheter-related SAB, concluding that more than 14 days of antibiotic therapy was unnecessary.22