Klebsiella pneumoniae biofilm formation predicts its survival in human serum

Klebsiella pneumoniae is a prominent pathogen causing life-threatening bloodstream infections. Although biofilm formation and resistance to human serum are well-recognized virulence traits, their interrelatedness during K. pneumoniae bloodstream infections remains unclear. Here, we hypothesize that biofilm production is related to K. pneumoniae's ability to thrive in human serum and, therefore, may predict the strains' ability for serum survival. We analyzed 57 clinical, genetically diverse classical K. pneumoniae strains and characterized their survival and biofilm-producing ability in human serum. Serum survival patterns revealed three serum resistance categories-Low, Mid, and High. In addition, the biofilm biomass produced by the strains correlated with their serum resistance level (P < 0.001), and 3D biofilm visualization using confocal microscopy further confirmed that biofilm extracellular polysaccharide substances and biomass patterns were consistent with the serum resistance categories. Moreover, we revealed a direct correlation between the level of biofilm formation and the strain's serum survival level (R2 = 0.696), a prerequisite for systemic K. pneumoniae dissemination. As biofilm formation in serum reflects both survival and biofilm-forming ability, we assessed biofilm formation in defined modified basal medium (BM2), to rule out serum-mediated killing, and discovered a strong and significant association between the serum resistance category and BM2 biofilm biomass (P < 0.0001). By applying regression models, we discovered that biofilm formation serves as a significant predictor for bacterial survival in serum. Overall, our findings establish biofilm production in K. pneumoniae as a biomarker of serum survival and may open a new avenue for predicting bloodstream infection risk in clinical settings.IMPORTANCEBloodstream infections caused by Klebsiella pneumoniae are devastating life-threatening infections worldwide. Understanding the survival strategies of K. pneumoniae in the bloodstream is critical for elucidating key aspects of bacterial pathogenicity and developing new diagnostic and therapeutic modalities. Although serum survival is a recognized virulence trait necessary to thrive in the bloodstream, the relationship between serum resistance and biofilm formation, a multicellular organization that may protect bacteria from bloodstream stressors, remains poorly understood. In this article, we demonstrate biofilm production in human serum by clinical classical K. pneumoniae strains for the first time and discovered a direct correlation between the level of biofilm biomass formation and the degree of serum survival in human serum and in defined modified basal medium. These findings offer insights into the importance of biofilm production in K. pneumoniae serum resistance and may be used to develop future therapeutic strategies targeting bloodstream infections.