Staphylococcal endocarditis. Results of combined antibiotic therapy in fourteen consecutive cases (1956-1959)

Value of the serum bactericidal test in management of patients with bacterial endocarditis

A review is given of the available clinical data on the prognostic value of the serum bactericidal test in the treatment of patients with bacterial endocarditis. It is concluded that the test, even when performed in a standardized manner, does not provide useful information for the majority of patients with bacterial endocarditis. Until further clinical data are available, routine performance of the test in patients with bacterial endocarditis is not recommended.

Multicenter collaborative evaluation of a standardized serum bactericidal test as a prognostic indicator in infective endocarditis

One hundred twenty-nine patients with bacterial endocarditis were evaluated in a multicenter collaborative study to determine whether a standardized serum bactericidal test could predict the outcome of the infection. All centers used a microdilution test method that defined all known test variables, including inoculum size, culture medium, dilution technique, incubation time, method of subculture, and bactericidal endpoint. Peak serum bactericidal titers of 1:64 or more and trough serum bactericidal titers of 1:32 or more predicted bacteriologic cure in all patients. The traditionally recommended serum bactericidal titer of 1:8 had statistically significant predictive accuracy at trough antibiotic levels only. The serum bactericidal test was a poor predictor of bacteriologic failure and ultimate clinical outcome, which depends on many factors. Wider recognition by physicians and clinical microbiologists that this in vitro test of antimicrobial activity can accurately predict bacteriologic success but cannot accurately predict either bacteriologic failure or clinical outcome could lead to a better consensus about its appropriate use. On the basis of the results of this study, peak serum bactericidal titers of 1:64 or more and trough serum bactericidal titers of 1:32 or more are recommended to provide optimal medical therapy for infective endocarditis.

Association between serum inhibitory and bactericidal concentrations and therapeutic outcome in bacterial endocarditis

Several recent reviews on the therapy of bacterial endocarditis have recommended that a serum inhibitory and/or bactericidal concentration (SIC/SBC) of 1:8 or more be achieved to ensure successful therapeutic outcome. We conducted a methodologic and statistical analysis of the available literature on endocarditis to determine the association between SIC/SBC titers of 1:8 or more and therapeutic outcome. We reviewed 17 studies published between 1948 and 1980 in which both SIC/SBC and therapeutic outcome were available. Factors that affect outcome, such as age, duration of symptoms, organism, and valve status, varied widely among the 226 patients. The methods used to measure SIC/SBC differed with respect to the time of obtaining the blood specimen relative to the antibiotic dose, size of the bacterial inoculum, type of broth, and definition of the bactericidal end-point. None of the 17 studies showed a significant association between SIC/SBC titers of 1:8 or more and survival or bacteriologic cure. Fifteen of the 17 also failed to demonstrate a significant association between SIC/SBC titers of 1:8 or more and medical cure. However, each of the studies that failed to demonstrate an association between SIC/SBC titers of 1:8 or more and improved therapeutic outcome had an insufficient sample size to confidently exclude a false-negative result. Analysis of the published data reveals insufficient evidence to demonstrate that SIC and SBC titers are of prognostic value in the therapy of patients with bacterial endocarditis.

Use of the serum bactericidal titer to assess the adequacy of oral antibiotic therapy in the treatment of acute hematogenous osteomyelitis

Serum bactericidal titers against Staphylococcus aureus were measured in 63 children who were receiving mafcillin or methicillin intravenously, or dicloxacillin, penicillin, or cephalexin orally. The SBTs obtained following unit does of 25 mg/kg of dicloxacillin, 35 mg/kg of penicillin, or 25 mg/kg of cephalexin with probenecid were comparable to those seen following intravenous doses of 40 mg/kg nafcillin or methicillin. Twenty-two children with acute hematogenous osteomyelitis proven or presumed to be due to S. aureus were treated intravenously until point tenderness and fever had resolved, and then with oral therapy. The mean duration of intravenous therapy was 14 days. Oral doses were adjusted so that a peak SBT of greater than or equal to 1:16 and a trough SBT of greater than or equal to 1:2 were obtained in most children. No recurrences occurred. The SBT proved to be a practical means of assessing the adequacy of oral therapy in children with infections due to S. aureus.

Medium-dependent variation in bactericidal activity of antibiotics against susceptible Staphylococcus aureus

Staphylococcus aureus resistant to bactericidal activity of antibiotics caused sepsis in three patients. Bacteriological and clinical responses were not achieved until serum and tissue fluid levels of administered antibiotics exceeded the minimum bactericidal concentration (MBC) of the infecting organism. Fifteen clinical isolates of S. aureus were tested in brain heart infusion broth and Mueller-Hinton broth for the MBC of gentamicin, vancomycin, clindamycin, oxacillin, cefazolin, and cephalothin. Results showed significant eightfold or greater broth-dependent differences in the MBC of at least one antibiotic against 87% (13/15) of strains tested. The MBC was unpredictable and varied with the strain, antibiotic, and medium used. No controlled studies are available to indicate the clinical significance of the MBC demonstrated in different media. The necessity for treating serious infection with bactericidal drugs has not yet been established; however, in septicemia such as that caused by bacterial endocarditis, bacteriostatic antibiotics have generally failed to eradicate the infection, whereas bactericidal agents have often been curative. Therefore, in patients unresponsive to usual antistaphylococcal therapy, we suggest that MBC testing be performed in at least two media and that treatment be instituted with antibiotics demonstrating the lowest MBC in all media used.

Gram-negative bacillary endocarditis. Interpretation of the serum bactericial test

Although the serum bactericidal test is commonly used in the management of infective endocarditis, little has been written about its validity or limitations. We report three cases of gram-negative bacillary endocarditis (Pseudomonas aeruginosa, Vibrio fetus and Serratia marcescens) encountered in 1 year at a Veterans Administration hospital. Serum bactericidal titers were considered necessary to identify inadequate antibiotic regimens or to avoid unnecessary drug toxicity. The limitations of the test, particularly those pertaining to gram-negative infections, are reviewed. Misleading results during treatment with aminoglycoside antibiotics could be due to the tendency of serum to become alkaline on standing. A detailed study of the interaction of the complement-dependent bactericidal system of serum with eight antibiotics is presented. In the context of the serum bactericidal test, the interaction was additive or synergistic in 15 of 16 determinations, indicating the need to include a control study of serum sensitivity of the infecting microorganism in each case.

Oral antibiotic cure of staphylococcal endocarditis

There are as yet few clinical reports of successful treatment of bacterial endocarditis with oral antibiotics, or of the use of the rifamycins in non-tuberculous infections. A case of refractory staphylococcal endocarditis successfully treated with oral rifampicin and erythromycin is presented. The availability of effective oral antibiotic therapy avoids the serious difficulties of long term parenteral antibiotics and offers a hopeful alternative to emergency cardiac surgery in this condition.