Characteristics of Hypervirulent Klebsiella pneumoniae: Does Low Expression of rmpA Contribute to the Absence of Hypervirulence?

Multidrug-resistant hypervirulent Klebsiella pneumoniae (MDR-hvKP) has been increasingly reported and is now recognized as a significant threat to public health; however, characterization of MDR-hvKP has not been systematically investigated. In the present study, 124 of 428 (28.92%) K. pneumoniae clinical isolates collected from January 2010 to December 2016 were identified with aerobactin and defined as hvKP; these included 94 non-MDR-KP, 20 extended-spectrum β-lactamase-producing K. pneumoniae (ESBL-KP), and 10 carbapenem-resistant K. pneumoniae (CR-KP) isolates. The remaining 304 isolates without presence of virulence factor aerobactin were defined as classic K. pneumoniae (cKP). The antimicrobial resistance rate of cKP was significantly higher than that of the hvKP isolates in the non-MDR-KP group, but showed no significant differences in the ESBL-KP and CR-KP groups. The detection frequencies of capsular serotype K1 (magA), hypermucoviscosity, sequence type 23 (ST23), and the virulence gene rmpA were significantly higher in the hvKP than cKP isolates in all three groups (P < 0.05). Most of the hypervirulent ESBL-KP and CR-KP isolates were K non-typeable (16/30) and harbored at least one gene for virulence (26/30). The hypervirulent ESBL-KP isolates primarily carried bla CTX-M (12/20, 60%) genes, and the hypervirulent CR-KP isolates mainly carried bla NDM- 1 (8/10, 80%) genes. Moreover, three hypervirulent ESBL-KP and two hypervirulent CR-KP isolates showed resistance to tigecycline but were sensitive to colistin. The transcriptional levels of rmpA in cKP were much lower than that in hvKP isolates in all three groups. Furthermore, overexpression of rmpA in the rmpA-low-expression cKP isolates could enhance bacterial virulence in the mouse infection experiment. In conclusion, our data suggest that the capsular serotype K1 (magA), rmpA, hypermucoviscosity, and ST23 were strongly associated with hvKP in non-MDR-KP, ESBL-KP, and CR-KP groups, and low rmpA expression levels contributed to the absence of hypervirulent phenotype.