Infectious diseases caused by microbial pathogens, are one of the major causes of morbidity and mortality worldwide. However, they represent a great challenge for antimicrobial therapies not only due to the threat of spreading antimicrobial resistances but also due to the fact that some major bacterial pathogens can also adopt an intracellular lifestyle. Thus, intracellular pathogens are widely protected against the usual antimicrobial therapeutics and they constitute a niche for recurrence and reinfection. Commonly used antimicrobial agents are characterized by poor cellular uptake. They cannot efficiently reach the intracellular target, resulting in low antimicrobial activity against intracellular pathogens. Therefore, there is a need to develop and improve strategies for the treatment of such infectious diseases. In this regard, Cell-penetrating peptides (CPPs), also called protein transduction domains (PTDs), are small peptides that are able to cross the plasma membrane autonomously and can mediate the internalization of biologically active molecules such as antibiotics into the cytoplasm.

In this regard, we showed that CPP-gentamicin conjugates were able to penetrate different endothelial and epithelial cell lines and importantly, we confirmed that these conjugates were able to target and efficiently kill intracellular Gram-negative pathogenic bacteria in infected cells. Furthermore, we could show that Bacteria-derived peptidoglycan-degrading proteins fused to a CPP can be used to kill pathogenic Gram pos. and Gram neg. bacteria, as well. Interestingly, these fusion constructs also showed a high effectivity against biofilm formation and might represent novel therapeutic tools to treat infectious diseases.