The a-hemolytic streptococci are the most common pathogens isolated. Although frequently sensitive to penicillin, a proportion display tolerance; the minimum bactericidal concentration of penicillin exceeds the MIC by a factor of 32 or more. The American Heart Association does not recommend determination of the minimum bactericidal concentration (Wilson e.L§L 1995).
For streptococci (including Streptococcus bovis) which are highly susceptible to penicillin (MIC 0.1 mg/l), treatment with penicillin G is curative in almost all patients
(Table 1). Penicillin can be given in six equal doses or as a continuous infusion. Ceftriaxone is effective and is more convenient if treatment is eventually completed as an out-patient. Penicillin or ceftriaxone alone is to be preferred in the elderly patient or those at high risk of ototoxicity or nephrotoxicity with aminoglycosides. Gentamicin shows synergism with penicillin in bactericidal activity, but addition of gentamicin has not been demonstrated to improve cure rates significantly. However, some recommend an initial 2-week course of gentamicin (S.immo.0s,...eLal 1985). Serum concentrations of gentamicin need only be 3 to 4 mg/l at peak and less than 1
mg/l at trough. Infections of prosthetic valves should be treated for 6 weeks with penicillin combined with gentamicin for the first 2 weeks ( Wilson eL§L 1995). Patients allergic to b-lactam antibiotics can be treated with ceftriaxone if there is no history of immediate type hypersensitivity or, alternatively, vancomycin or teicoplanin for 4 weeks. Vancomycin must be infused over 1 h to avoid 'red man' syndrome and hypotension, and is usually assayed. Teicoplanin (not available in the United States) does not suffer these disadvantages but fever can be an adverse effect.
Table 1 Medical treatment of infective endocarditis
Combination treatment should be used for treating infections caused by a-hemolytic streptococci which are not fully susceptible to penicillin (MIC > 0.1 mg/l) and in patients with a mycotic aneurysm, with symptoms that have been present for more than 3 months, who have relapsed, or who are on long-term penicillin. If the MIC is less than 0.5 mg/l, gentamicin can be given for the first 2 weeks of treatment with penicillin. If the MIC is greater than 0.5 mg/l or the streptococci are nutritionally deficient variants, gentamicin should be given for 4 weeks at a dose just sufficient to keep peak levels of 3 mg/l and trough levels below 1 mg/l. Serum assays two or three times weekly are essential to minimize the risk of cumulative toxicity. Vancomycin or teicoplanin can be used as alternatives without an aminoglycoside.
The b-hemolytic streptococci and Streptococcus pneumoniae are infrequent causes of endocarditis. In some areas, pneumococci increasingly have a reduced susceptibility to penicillin (MIC > 0.1 mg/l) or are resistant (MIC > 2 mg/l). Treatment with penicillin G or a glycopeptide should be decided by laboratory results. Group A streptococci should be treated with penicillin G, but other b-hemolytic streptococci may require addition of gentamicin for the first 2 weeks if susceptibility is reduced.
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