Abstract
BackgroundTwo 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 methodsA 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.ResultsAlthough 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.ConclusionSince 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.
Highlights
Two different types of hypervirulent K. pneumoniae (HvKp), the multi-locus sequencing type (MLST)-11 and serotype K1/K2 strains, have been frequently described in recent studies
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
Serotype K1 or K2a ST11c a Number of publications found in Medline with the term "hypervirulent K. pneumoniae" only b Number of publications found in Medline with the term "hypervirulent K. pneumoniae" with serotype K1 or K2 c Number of publications found in Medline with the term "hypervirulent K. pneumoniae" with ST11 testing cassette and PCR [15, 16]
Summary
Two different types of hypervirulent K. pneumoniae (HvKp), the MLST-11 and serotype K1/K2 strains, have been frequently described in recent studies. HvKp is differentiated from cKp by its ability to develop subsequent metastatic spread or to present in multiple sites of infection [1] Metastasis, such as liver abscess in patients with meningitis or endophthalmitis [2,3,4], has been well described previously. The term HvKp was recently used for nosocomial isolates with a common multi-locus sequencing type (MLST) 11 [2, 7] This ST11 isolate was first identified to contribute to a significant outbreak of K. pneumoniae with carbapenem resistance in a local hospital [7]. We compared these two types of strains named HvKp and investigated whether there is any difference in virulence that could be used to redefine hypervirulence
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