Introduction: Streptococcus pneumoniae is known to be the causative agent of pneumonia, otitis media and even systemic infections. Increasing resistance of S. pneumoniae towards commonly used antibiotics makes the treatment of these infections very challenging. Thus, the WHO identified S. pneumoniae as a priority pathogen highlighting the urgent need for new antibiotic agents against S. pneumoniae. Our lab previously showed that the human MEK inhibitor Zapnometinib reduces growth of several gram-positive bacteria.

Goals: In the current project, we investigate the potential of Zapnometinib as a new antibiotic agent against S. pneumoniae. We characterized the effects of Zapnometinib on S. pneumoniae growth behavior and morphology with the ultimate goal to identify the mechanism of action.

Methods: Atomic force microscopy, Fluorescence microscopy, growth reduction studies, growth kinetics, EC₅₀ determination

Results: Treatment of S. pneumoniae with Zapnometinib resulted in a dose-dependent growth reduction, with the EC₅₀ being in the low micromolar range. Growth kinetic experiments revealed a bacteriostatic mode of action of Zapnometinib in S. pneumoniae. Further characterization of the morphological changes induced by the treatment showed that Zapnometinb interfered with the process of pneumococcal cell division which resulted in significantly elongated cell chains. Atomic force microscopy confirmed that the formation of septa was impaired upon treatment with Zapnometinib. Fluorescence microscopy showed that this was accompanied by a dysregulated incorporation of peptidoglycan and defective cell pole maturation.

Summary: Our data confirm that Zapnometinib harbors antibacterial properties against S. pneumoniae. Zapnometinib treatment resulted in defective cell division, which was characterized by elongated cell chains, impaired septum formation, dysregulated peptidoglycan incorporation as well as cell pole maturation defects.