The activity of a mammalian proline-rich peptide against Gram-negative bacteria, including drug-resistant strains, relies on a nonmembranolytic mode of action Tecla Ciociola,1 Laura Giovati,1 Angela Giovannelli,2 Stefania Conti,1 Massimo Castagnola,2,3 Alberto Vitali3 1Department of Medicine and Surgery, University of Parma, Parma, 2Institute of Biochemistry and Clinical Biochemistry, Catholic University, Rome, 3Institute for the Chemistry of Molecular Recognition, C.N.R., c/o Institute of Biochemistry and Clinical Biochemistry, Catholic University, Rome, Italy Background: A peptide of 2,733 Da named SP-E, previously isolated from pig saliva and already described for its antifungal activity and absence of toxicity against mammalian cells, is characterized by a high content of proline residues (70% of entire sequence), that confer structural features probably related to peptide activity. Purpose: The aim of this study was to evaluate the activity of SP-E against Gram-negative bacteria, including drug-resistant clinical isolates. Methods: SP-E and shorter fragments of the same peptide were tested in vitro against the selected bacteria by colony forming unit assays. Scanning electron microscopy and confocal microscopy were also applied. SP-E potential therapeutic activity was evaluated in vivo in a Galleria mellonella model of bacterial infection. Results: SP-E proved to be active against the tested bacteria with EC50 values in the micromolar range. Though maintaining antibacterial properties, the shorter peptides showed lower activity in respect to the parental molecule. Kinetics of killing action and nonmembranolytic internalization within Escherichia coli and Pseudomonas aeruginosa cells strongly suggested a cytosolic mechanism of action involving one or more intracellular molecular targets. A single injection of SP-E exerted a therapeutic effect in G. mellonella larvae infected with P. aeruginosa. Conclusion: The biological properties of SP-E strongly back this peptide as a new promising multitasking antimicrobial molecule. Keywords: antimicrobial peptide, proline-rich peptides, drug-resistant bacteria, confocal microscopy, scanning electron microscopy, Galleria mellonella model