Klebsiella pneumoniae is a Gram-negative bacterium that belongs to the gamma
subdivision of the class Proteobacteria and exhibits relatively close genetic relatedness to the other genera of Enterobacteriaceae, including Escherichia, Salmonella, Shigella, and Yersinia (20). Klebsiella spp. are ubiquitous in nature and
characterized as rod-shaped, non-motile, usually encapsulated bacteria that can live in water, soil, and plants, and infect humans and animals (243, 244). In humans, K.
pneumoniae behaves like a commensal mainly in the nasopharyngeal and intestinal
mucosae (244). In this respect, the genus Klebsiella is like Enterobacter and Citrobacter but unlike Shigella spp. or E. coli, which are common in humans but not in the environment (244).
1.1.1. K. pneumoniae infections
K. pneumoniae is an opportunistic pathogen frequently involved in severe
nosocomial infections in immunocompromised individuals who are hospitalized and suffer from severe underlying diseases, such as diabetes mellitus or chronic pulmonary obstruction (244). K. pneumoniae is responsible for a variety of diseases including suppurative lesions, bacteriemia, urinary tract infections, pneumonia, and sometimes life-threatening septic shock (23, 149, 165, 236, 244). The clinical pattern
of K. pneumoniae infection in humans has changed since this organism was discovered in the 1880s (97, 98). Until the 1960s, K. pneumoniae was an important cause of community-acquired pneumonia (42), however, the incidence of this type of infection has dropped, and hospital-acquired K. pneumoniae infection now predominates (105, 244, 303). Since 1980s, K. pneumoniae is emerging as an important pathogen both in the community and the hospital setting (161). In the hospital environment with the extensive use of antibiotics, multiple drug resistance has been increasingly observed in K. pneumoniae, especially the extended-spectrum β-lactamase (ESBL)-producing strains (93, 96, 161, 208, 212). Carbapenems are considered to be the preferred agents for the treatment of serious infections caused by ESBL-producing K. pneumoniae because of their high stability to β-lactamase hydrolysis and observed retained susceptibility of ESBL producers (61). However, K.
pneumoniae isolates resistant to carbapenems have been reported worldwide since
2000s (128, 169, 224, 329). The emergence of carbapenem-resistant enterobacteria is worrisome because of the option for antimicrobial treatment is further restricted.
The emergence of an invasive form of the community-acquired K. pneumoniae infection, which presents as primary bacteremic liver abscesses, endophthalmitis, and meningitis (50, 89, 90, 189, 246, 315), has been reported almost exclusively in Asia (100), especially in Taiwan (100, 302, 315). Although these invasive and highly
encapsulated K. pneumoniae strains are universally resistant to ampicillin, they are unable to produce ESBL and susceptible to most antibiotics (178, 179). In addition, approximately 50 to 75% of the patients with K. pneumoniae liver abscess also presented with diabetes mellitus (177, 217, 300). Although the preponderance of this severe invasive K. pneumoniae infection remains unknown, the involvement of both host and microbial factors during pathogenesis could be anticipated.
1.1.2. K. pneumoniae virulence factors
A number of bacterial factors that contribute to K. pneumoniae pathogenicity have been identified, which include capsular polysaccharide (CPS), lipopolysaccharide (LPS), iron acquisition systems, and adherence factors (244).
Clinically isolated K. pneumoniae usually produced large amount of CPS and therefore forms large glistering colonies with viscid consistency. The abundant CPS that typically surrounds K. pneumoniae protects against the bactericidal action of serum and impairs phagocytosis (11, 63), and may be regarded as the most important virulence determinant of K. pneumoniae. Among the 77 described capsular (K) types of the serotyping scheme, serotypes K1, K2, K4 and K5 are highly virulent in experimental infection in mice and are often associated with severe infections in humans and animals (216, 220, 229, 281). Furthermore, the K1 and K2 serotypes
were found to be the most prevalent capsular serotypes in liver abscess-causing K.
pneumoniae (100, 178).
Genetic determinants for K1 and K2 CPS biosynthesis and regulation have been reported (15, 58). In K. pneumoniae NTUH-K2044 of serotype K1, deletion of the mucoviscosity associated gene A (magA) abolishes the CPS biosynthesis and thus reduces the bacterial virulence (88). The gene magA is only contained in the K1 cps gene cluster and hence could be applied to rapidly detect K. pneumoniae strains of serotype K1 (328). A PCR analysis for the K2 capsule-associated gene A (k2A) has also been used to identify K. pneumoniae strains of serotype K2 (74, 330). The presence of rmpA (regulator of the mucoid phenotype A) gene correlated with abscess formation in patients with community-acquired K. pneumoniae bacteremia and attributed to be a risk factor for metastatic infection in patients with K. pneumoniae liver abscess (180, 331). The rmpA together with rmpA2 gene both located on the large virulence plasmid pLVPK (48, 295) are able to enhance the CPS biosynthesis thereby confer K. pneumoniae a hypermucoviscosity phenotype (53, 171).
LPS comprising three parts, lipid A, core, and O antigen, is responsible for the resistance to complement-mediated killing as well as antimicrobial peptides attack, and the establishment of septic shock (7, 52, 87, 211). Antimicrobial peptides, such as
polymyxin B, are bactericidal agents that exert their effects by interacting with the LPS of Gram-negative bacteria. The polycationic peptide ring on polymyxin competes for and substitutes the calcium and magnesium bridges that stabilized LPS, thus disrupting the integrity of the outermembrane leading to cell death (118, 332). In our previous study, the genetic determinants for LPS modification and CPS level have been shown to involve in polymyxin B resistance of K. pneumoniae CG43 (52).
Iron starvation is one of the major barriers that virulent bacteria must overcome in order to proliferate in the host. Multiple iron-acquisition systems have been described in K. pneumoniae (136). Analysis of the genomic sequence of K.
pneumoniae NTUH-K2044 revealed 10 putative iron-acquisition systems, whereas K.
pneumoniae strain MGH78578 and CG43 possess only 6 and 8 of these systems,
respectively (136, 185). Prevalence study and animal experiment have been performed to assess the role of iron-acquisition systems in K. pneumoniae pathogenicity (136, 220, 295). Adherence factors possessed by K. pnuemoniae including type 1 and type 3 fimbriae, which play crucial roles in adhesion to host cells, persistence, and biofilm formation, are focused in this thesis and introduced in detail below. Other K. pneumoniae virulence determinants involved in acid resistance (135), oxidative stress response (123), and allantoin metabolism (195) have also been reported.
Complete genome sequence of K. pneumoniae strains MGH78578 (230) and NTUH-K2044 (325), which are human pathogens respectively isolated from sputum and liver abscess, and a nitrogen-fixing endophyte strain 342 (95) have been determined. Genomic and phenotypic analyses have also been performed among 235 K. pneumoniae strains in order to identify the evolutionary emergence of virulent clones (37).