Mechanistic insights into the inhibitory effects of eukaryotically expressed Sarcotoxin II 50 on <i>Klebsiella pneumoniae</i> invasion into pulmonary epithelial cells

Hypervirulent (<i>hvKP</i>) and classical (<i>cKP</i>) strains of <i>Klebsiella pneumoniae</i> are major contributors to serious hospital-acquired infections, and the growing issue of antibiotic resistance highlights the urgent need for new antimicrobial approaches. In this study, we investigated the inhibitory effects of Sarcotoxin II 50, an insect-derived antimicrobial peptide, on <i>K. pneumoniae</i>. Our findings show that Sarcotoxin II 50 exerts potent antibacterial activity by impairing capsular polysaccharide (CPS) production and fimbrial formation, which compromises bacterial adhesion and colonization capacity. <i>In vitro</i> experiments demonstrated that Sarcotoxin II 50 effectively suppressed the growth of <i>cKP</i>. Mechanistic analyses revealed that the peptide interferes with CPS biosynthesis and alters the structure and expression of type 1 fimbriae (<i>fimA</i>) and type 3 fimbriae (<i>mrkA</i>, <i>mrkD</i>, <i>mrkH</i>), which are associated with reduced bacterial tissue invasion and attenuated virulence in a murine abscess model. Overall, this study provides a detailed mechanistic understanding of how Sarcotoxin II 50 inhibits <i>K. pneumoniae</i> both <i>in vitro</i> and <i>in vivo</i> under the tested experimental conditions. These results offer a mechanistic basis for the development of novel antimicrobial therapies targeting <i>K. pneumoniae</i> infections and support the further exploration of this peptide as a promising antimicrobial candidate. Characterization of a new insect peptide, Sarcotoxin II 50, with anti-<i>K</i>. <i>pneumoniae</i> activity from the housefly for the first time.IMPORTANCESarcotoxin II 50 inhibited <i>K. pneumoniae</i> invasion into A549 and BEAS-2B lung epithelial cells. Sarcotoxin II 50 inhibits <i>K. pneumoniae</i> invasion into epithelial cells by affecting capsular polysaccharide synthesis, fimbriae formation, and cell division. This manuscript investigates, for the first time, the function and mechanism of the Sacrotoxin II family member in combating <i>K. pneumoniae</i>, 40 years after its initial identification.