Effects of Lipopolysaccharide Core Modulation on Outer Membrane Protein Function and Virulence in Pectobacterium carotovorum.

Lipopolysaccharide (LPS) is a critical component of the bacterial outer membrane, which serves as a permeability barrier and site for sensing environmental signals. The structure and functional significance of LPS vary among bacterial species, influencing pathogenic traits. In this study, we aimed to investigate LPS biosynthesis genes in Pectobacterium carotovorum PCC27, the causal agent of bacterial soft rot in vegetable crops, and characterise the effects of their disruptions on virulence. Mutants lacking any LPS component exhibited reduced rotting symptoms on the midrib of kimchi cabbage and antimicrobial resistance, underscoring the importance of an intact LPS for virulence. The absence of O-antigen components did not affect the expression of major virulence factors; however, LPS core defects significantly impaired bacterial multiplication in planta, proteolytic activity and motility, which were progressively suppressed with greater truncations. Complementation analyses revealed that gene overexpression failed to restore these mutant phenotypes to the wild-type levels. Furthermore, western blot-based assembly assays of the type I secretion system demonstrated that LPS core truncation disrupted the outer membrane component PrtF localization. Additionally, the antiterminator factor RfaH was found to activate a large gene cluster directly involved in O-antigen biosynthesis, although it was dispensable for virulence. Together, these findings highlight the critical role of the LPS core in the functional assembly of outer membrane apparatuses, thereby contributing to virulence in P. carotovorum.