Antimicrobial susceptibility of blood culture isolates of viridans streptococci: relationship to a change in empirical antibiotic therapy in febrile neutropenia.

This study investigated the antibiotic susceptibilities of 67 isolates of viridans streptococci from 61 cases of bacteraemia in immunocompromised paediatric patients with malignancy. The majority of patients (87%) had received prior courses of empirical antibiotic therapy, which consisted of ceftazidime plus amikacin during period 1 and piperacillin/tazobactam plus amikacin during period 2. Susceptibility to vancomycin and quinupristin/dalfopristin was 100%. Susceptibility to beta-lactam antibiotics varied. For period 1, the geometric mean MICs of all beta-lactams tested against blood culture isolates (n = 31) exceeded those against isolates (n = 36) collected from blood after the change in empirical therapy (by 3.3-fold for ceftazidime, 2.8-fold for piperacillin/tazobactam and 1.6-fold for penicillin). The selection of a beta-lactam antibiotic for empirical therapy must be made with care, as repeated courses of certain agents may be more likely to select for viridans streptococci with diminished susceptibility.