The role of the microbiology laboratory in the treatment of infective endocarditis.

The antibiotic era has greatly improved the prognosis for patients with infective endocarditis. The need for bactericidal activity against the infecting organism has created an additional role for the clinical microbiology laboratory. This role involves the selection and performance of the proper in-vitro susceptible tests in order to ensure bactericidal activity. However, there are problems in both the performance and the interpretation of bactericidal tests; these include persisters, the 'paradoxical effect', tolerance and the development of resistance. Technical factors include inoculum size, growth phase of the inoculum, insufficient contact between the test organism and the antimicrobial agent, and the volume of transfer for the count of survivors. Appreciation of these factors is important for the laboratory performing bactericidal tests, which include time-kill curves, minimal bactericidal concentrations and the serum bactericidal test. Of these tests, the serum bactericidal test offers the most logical approach. However, the performance of this test and the interpretation of the results has been controversial. Recent attempts at standardization should allow better utilization. In addition to the serum bactericidal test, the clinical microbiology laboratory can offer time-kill curves and minimal bactericidal activity and can offer serum assays of antibiotics to ensure adequate levels without toxicity. The use of any or all of these methods must be tempered by clinical judgement in each individual case. Application of the proper tests can assist in the optimal therapy of infective endocarditis.