Virulence Differences in Hypervirulent Klebsiella pneumoniae between Isolates with Multi-Locus Sequence Type (MLST)-11 and Serotype K1 or K2 Strains

Abstract Background: Two different types of hypervirulent K. pneumoniae (HvKp), including MLST-11 or serotype K1/K2 strains, have been frequently described in recent studies. Although these two types of strains were described as HvKp, their virulence was not compared. In this study, an in vitro and in vivo approach was used to assess differences in virulence.Materials and Methods: A total of nine isolates, including one strain of serotype K1 isolate, two strains of serotype K2 isolates and six strains of ST11 isolates, were selected for this study. Phenotypic tests of virulence and associated genes were performed by string test, PCR, and in vitro models of serum resistance and phagocytosis.Results: Although serotype K2 strains and ST11 isolates had similar virulence gene profiles, the ST11 isolates showed less serum and phagocytic resistance than did serotype K1/K2 isolates. The mouse lethality test revealed that all 11 ST isolates were unable to cause lethality, even with > 107 CFU, while one serotype K1 and two serotype K2 showed an LD50 of £ 102 CFU. Aerobactin (or capsule knockout mutants) exhibited a decline in LD50 compared to the parental strain, while capsule mutants showed a more significant decrease in LD50.Conclusion: Since there was a significant difference in virulence levels between the two types of HvKp when assessed by in vitro and in vivo animal models, the designation "HvKp" may be a better term based on animal studies and to avoid confusion. Virulence and nonvirulence could be analysed in a relative manner, especially in comparison studies.