Abstract: The present invention relates to a novel peptide extracted from guava (Psidium guajava) seeds, that provides bactericide activity, Preferentially against Gram-negative bacteria which are known to cause urinary, hospital, and intestinal tract infections (Proteus sp. And Klebsiella sp.). The peptide, that has the amino acid sequence RESPSSRMEC YEQAERYGYG GYGGGRYGGG YGSGRGQPVG QGVERSHDDN RNQPR, belongs to the class of glycine rich proteins and has approximately 5 kDa of molecular weight. The invention also relates to antibiotic compositions for human, veterinary and plant treatments. Alternatively, the peptide, or a functionally similar derivative, subjects of the present invention, can be used for transforming organisms aiming pathogen resistance, other adaptive advantages, as well as various properties, specially for plants and animals.