In vitro activity of newer antimicrobials against penicillin non-susceptible strains of Streptococcus pneumoniae Marlene Hipp, Irene BurckhardtDepartment for Infectious Diseases, Microbiology and Hygiene, University Hospital of Heidelberg, Heidelberg, GermanyBackground: Since the first isolation of Streptococcus pneumoniae with low penicillin susceptibility in the 1960s, resistant strains have spread over the globe, causing substantial problems in the treatment of pneumococcal infections. However, in Germany, rates of non-susceptibility are still below 5%.Methods: Since 2009 clinical pneumococcal strains have been collected at the Center for Infectious Diseases, Heidelberg University Hospital, Germany. In this study, 56 of these strains were chosen due to their decreased penicillin susceptibility (minimal inhibitory concentration (MIC)≥0.12 μg/mL). Sixteen of these strains even showed an MIC of ≥2 μg/mL. We examined the in vitro activity of newer antimicrobials known to be active against Gram-positive bacteria. For this purpose MICs of ceftaroline, ceftobiprole, dalbavancin, delafloxacin, eravacycline, tedizolid, and telavancin were determined and evaluated.Results: All of the 7 antimicrobial agents inhibited pneumococcal growth at concentrations of 0.5 μg/mL or lower. Currently, clinical breakpoints are only available for two substances, ceftaroline and ceftobiprole. According to these breakpoints, all MICs were below the susceptibility breakpoint; however, there was a correlation between high penicillin MICs (≥2 μg/mL) and MICs near the ceftaroline and ceftobiprole susceptibility breakpoint. The other agents showed very promising effects against all tested strains with the lowest MIC90 of 0.002 μg/mL for telavancin.Conclusion: Consequently, this study demonstrates the promising in vitro activity of newer antimicrobials against penicillin non-susceptible strains of S. pneumoniae.Keywords: Streptococcus pneumoniae, penicillin resistance, susceptibility testing