Background: In January 2008, the Clinical and Laboratory Standards Institute (CLSI) published revised penicillin breakpoints for Streptococcus pneumoniae according to clinical presentation and the route of penicillin administration. The aim of this study was to evaluate the impacts of the new penicillin breakpoints on the susceptibility rates of S. pneumoniae isolated from blood. 

Methods: A total of 156 non-duplicated S. pneumoniae strains recovered from blood of hospitalized patients were collected between January 2003 and December 2008. Penicillin and cefotaxime susceptibility tests were performed using an E-test (AB Biodisk, Solna, Sweden). Results of the penicillin susceptibility tests were analyzed using the former and new CLSI guidelines. 

Results: Of the 156 S. pneumoniae strains isolated from blood, penicillin susceptibility under the former CLSI guidelines resulted in 42.3% susceptible, 42.3% intermediate, and 15.4% resistant states. According to the new CLSI guidelines (nonmeningitis, parenteral), 87.8% of isolates were susceptible, 9.6% were intermediate, and 2.6% were resistant to penicillin. 

Conclusion: When the new CLSI guidelines are applied, the penicillin susceptibility rate of S. pneumoniae strains isolated from blood is considerably increased. This suggests that penicillin should still be useful for the treatment of nonmeningeal pneumococcal infections and that the use of broad-spectrum antimicrobials should not replace this treatment.  (Korean J Clin Microbiol 2010;13:68-72)