Mode of action studies of the novel bisquaternary bisnaphthalimide MT02 against Staphylococcus aureus

Screening of various bisquaternary bisnaphthalimides against a variety of human pathogens revealed one compound, designated MT02, with strong inhibitory effects against gram-positive bacteria. The minimal inhibitory concentrations ranged from 0.31 µg/ml against community-acquired methicillin resistant Staphylococcus aureus (MRSA) strain USA300 to 20 µg/ml against Streptococcus pneumonia. DNA-microarray studies generated a transcriptional signature characterized by a strong increase of genes involved in DNA-metabolism, DNA-replication, SOS-response and transport of positively charged compounds. Radioactive whole cell labeling experiments indicated a strong impact of MT02 on bacterial DNA-replication. Furthermore, surface plasmon resonance and gel retardation experiments demonstrated direct binding of MT02 to DNA in a concentration dependent, reversible and sequence-unspecific manner. The data presented suggest that the bisquaternary bisnaphthalimide MT02 exerts anti-gram-positive activity by binding to DNA and thereby prohibiting appropriate DNA-replication. WT strain exposed to MT02 for 60 minutes in rich medium