Phage Modulates Virulence and Biofilm in Klebsiella pneumoniae: A Trade-Off Between Immune Evasion and Environmental Adhesion

Capsular polysaccharides (CPS) and type 3 fimbriae are critical virulence factors of Klebsiella pneumoniae, yet their potential interplay and the mechanisms by which bacteriophages regulate these two factors remain poorly understood. In this study, we isolated a novel capsule-specific bacteriophage phiA25 targeting a hypervirulent K. pneumoniae (hvKpn) strain A25, which is of the K2 capsular type and exhibits weak biofilm formation. The bacteriophage phiA25 harbors both a K30 depolymerase and a newly identified K2-specific depolymerase, reported here for the first time. Treatment with the depolymerase Dop? reduced the virulence of strain A25 while significantly enhancing its biofilm formation capacity. Additionally, we identified an uncharacterized protein, T9, which suppresses biofilm formation in strain A25 by downregulating the expression of mrkA, a key subunit of type 3 fimbriae. Further investigation revealed a trade-off between capsular polysaccharide-driven immune evasion and type 3 fimbriae-dependent environmental adhesion in K. pneumoniae: CPS suppresses the expression of type 3 fimbriae, and in the absence of CPS, type 3 fimbriae become prominent virulence factors, accompanied by increased biofilm formation. Our findings provide novel insights into the regulatory mechanisms of bacteriophage-mediated modulation of CPS and type 3 fimbriae in K. pneumoniae and shed light on the dynamic interplay between these two critical virulence factors.