Molecular typing and virulence analysis of serotype K1 Klebsiella pneumoniae strains isolated from liver abscess patients and stool samples from noninfectious subjects in Hong Kong, Singapore, and Taiwan

Abstract

Serotype K1 Klebsiella pneumoniae with multilocus sequence type 23 (ST23) has been strongly associated with liver abscess in Taiwan. Few data regarding the strain types and virulence of this serotype from other Asian countries are available. Serotype K1 K. pneumoniae strains isolated from liver abscess and stool samples from subjects hospitalized in Hong Kong, Singapore, and Taiwan hospitals were examined. Forty-seven serotype K1 isolates were identified: 26 from liver abscess samples and 21 from stool samples. MLST revealed 7 sequence types: 85.1% (40 of 47 isolates) belonged to ST23, 1 isolate belonged to ST163 (a single-locus variant of ST23), and 2 isolates were ST249 (a 3-locus variant of ST23). New STs, namely, ST367, ST425, and ST426, were allocated to 3 of 4 isolates from stool samples. The virulence of these strains was determined by neutrophil phagocytosis and mouse infection models. Except for two ST23 isolates, all Klebsiella pneumoniae isolates were resistant to phagocytosis. Resistance to serum killing varied in isolates of ST23, while all non-ST23 strains were susceptible to serum killing except one with ST249 from a liver abscess. All hypervirulent isolates with a 50% lethal dose of <10(2) CFU were from ST23, were resistant to phagocytosis and serum killing, and also carried both virulence-associated genes, rmpA and aerobactin. Multilocus sequence typing genotype 23 was the most prevalent sequence type among serotype K1 K. pneumoniae isolates from both liver abscess and stool samples in the Asia Pacific region. Serotype K1 K. pneumoniae isolates with capsule expression leading to phagocytic resistance and with the aerobactin gene were associated with hypervirulence.

Fig. 1.

Effects of all isolates on phagocytosis according to their STs and body sites and regions of isolation. The phagocytosis rate is calculated as the percentage of neutrophils ingesting FITC-labeled bacteria at 15 min. Strains from liver abscess patients and stool samples from carriers are labeled with open and filled symbols, respectively, according to their region of isolation. The isolates from Hong Kong (HK), Singapore (SG), and Taiwan (TW) are identified by circles, triangles, and squares, respectively. Two ST23 laboratory capsule-deficient strains (ΔΚ1) from a previous study (24) were included as a control for highly phagocytic susceptible strains. Except for two ST23 isolates with a phagocytic rate of over 50% at 15 min, all isolates were highly resistant to phagocytosis by neutrophils.