Arginine Regulates the Mucoid Phenotype of HypervirulentKlebsiella pneumoniae
Authors: Ring Brooke E. , Shepard Grace E. , Khadka Saroj , Holmes Caitlyn L. , Bachman Michael A. , Mike Laura A. 
Published: November 2024

ABSTRACTHypervirulentKlebsiella pneumoniaeis associated with severe community-acquired infections. HypervirulentK. pneumoniaecolonies typically exhibit a mucoid phenotype.K. pneumoniaemucoidy is influenced by a complex combination of environmental factors and genetic mechanisms. Mucoidy results from altered capsular polysaccharide chain length, yet the specific environmental cues regulating this phenotype and their impact on pathogenesis remain unclear. This study demonstrates that casamino acids enhance the mucoidy phenotype but do not affect total capsular polysaccharide levels. Through targeted screening of each amino acid present in casamino acids, we identified that arginine is necessary and sufficient to stimulate the mucoid phenotype without altering capsule abundance. Furthermore, arginine activates thermpADCpromoter, increasingrmpDtranscript levels, which in turn modulates capsular polysaccharide chain length and diversity. The arginine regulator, ArgR, plays a pivotal role in this regulatory cascade since deletingargRdecreases mucoidy and increases capsular polysaccharide chain length diversity. Additionally, the ΔargRmutant displays increased macrophage association and has a substantial competitive defect in the lungs of mice, suggesting a link between arginine-dependent gene regulation, immune evasion andin vivofitness. We discovered that arginine-dependent regulation of mucoidy is conserved in four additional hypervirulentK. pneumoniaeisolates likely via a conserved ARG binding box present inrmppromoters. Our findings support a model in which arginine activates ArgR and increases mucoidy in hypervirulentK. pneumoniae.As a result, it is possible that arginine-dependent regulation of mucoidy allows hypervirulentK. pneumoniaeto adapt the cell surface across different niches. This study underscores the significance of arginine as a regulatory signal in bacterial virulence.IMPORTANCEThe rise of hypervirulentKlebsiella pneumoniaeas a global health threat underscores the urgent need to understand its pathogenic mechanisms. Its ability to cause severe infections in healthy individuals and spread beyond the endemic Asia-Pacific region demands a deeper investigation into the mechanisms driving hypervirulence. The hypermucoid phenotype is primarily associated with hypervirulent isolates and is regulated by RmpD, which increases capsular polysaccharide chain length and uniformity. Understanding how environmental and genetic factors influence mucoidy is vital for elucidating the mechanisms by whichK. pneumoniaeadapts and thrives in different ecological and host niches. Our study defines the role of amino acids, particularly arginine, in regulating the bacterial surface by modulatingrmpDexpression. These results reveal that mucoidy is not a constitutive phenotype, but rather a dynamic process finely tuned by nutrient availability. Our findings expand our understanding of how thermplocus is controlled and how changes in arginine availability may optimizeK. pneumoniaeimmune evasion.