Background Two different types of hypervirulent K. pneumoniae (HvKp), the MLST-11 and serotype K1/K2 strains, have been frequently described in recent studies. Although these two types of strains were described to be HvKp, their virulence was not compared. In this study, in vitro and in vivo approaches were used to assess differences in virulence. Materials and methods A total of twenty-nine isolates, including 6 strains of each of serotype K1 and K2 isolates and 17 strains of ST11 isolates, were selected for this study. Phenotypic tests of virulence were performed by the string test and analysis of the virulent associated genes was detected by PCR. In vitro models of serum resistance and phagocytosis were used as the parameters to assess the virulence. In-frame deletion of virulence-associated genes was performed to study their contributions to virulence. The median lethal dose, i.e., the LD50, in mice was determined following IP injection. Results Although serotype K1 and K2 strains and ST11 isolates had similar virulence gene profiles, the ST11 isolates showed less serum and phagocytic resistance than the serotype K1/K2 isolates. The mouse lethality test revealed that all ST11 isolates were unable to cause lethality, even at > 107 CFU, while serotypes K1 and K2 showed an LD50 at ≤ 103 CFU. Aerobactin or capsule knockout mutants exhibited a lower LD50 than 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 in in vitro and in vivo models, it may be better to use the designation "HvKp" for some strains based on animal studies to avoid confusion. Virulence and non-virulence could be analysed in a relative manner, especially in comparison studies.

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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.