An Increasing Prominent Disease of Klebsiella pneumoniae Liver Abscess: Etiology, Diagnosis, and Treatment

Background. During the past two decades, Klebsiella pneumoniae (K. pneumoniae) had surpassed Escherichia coli (E. coli) as the predominant isolate from patients with pyogenic liver abscess (PLA) in Asian countries, the United States, and Europe, and it tended to spread globally. Since the clinical symptom is atypical, the accurate and effective diagnosis and treatment of K. pneumoniae liver abscesses (KLAs) are very necessary. Methods. Here, we have comprehensively clarified the epidemiology and pathogenesis of KLA, put emphases on the clinical presentations especially the characteristic radiographic findings of KLA, and thoroughly elucidated the most effective antibiotic strategy of KLA. Results. K1 serotype is strongly associated with KLA especially in diabetic patients. Computed tomography (CT) and ultrasound (US) were two main diagnostic methods of KLA in the past. Most of KLAs have solitary, septal lobular abscesses in the right lobe of liver, and they are mainly monomicrobial. Broad-spectrum antibiotics combined with the US-guided percutaneous drainage of liver abscesses can increase their survival rates, but surgical intervention still has its irreplaceable position. Conclusion. The imaging features contribute to the early diagnosis, and the percutaneous intervention combined with an aminoglycoside plus either an extended-spectrum betalactam or a second- or third-generation cephalosporin is a timely and effective treatment of KLA.

In this paper, we comprehensively described the epidemiology and the pathogenesis of KLA. And then, we put emphases on the difference of clinical presentations especially the radiographic findings between KLA and other kinds of PLA (Non-KLA), in order to diagnose KLA early and precisely, and further elucidate the effective therapeutic methods of KLA especially the choice of targeted antibiotics. We hope to enhance the understanding of KLA and contribute to the timely, accurate, and effective diagnosis and treatment of this disease.

Epidemiology
In the recent two decades, K. pneumoniae has become the most common causative pathogen of PLA in Asian countries and areas [1][2][3][4][5][15][16][17][18][19][20][21][22]. A shift from E. coli to K. pneumoniae as the causative pathogen of pyogenic liver abscess may also 2 Gastroenterology Research and Practice have occurred in the United States [6][7][8][9][10][11] and Europe [12][13][14]. However, differing from the much higher incidence of KLA in Asian countries and areas [1][2][3][4][5] especially the highest incidence in Taiwan that ranges from 80% to 90% [16,19], a relatively low incidence of 30%-40% is estimated in the US [6,9]. The exact cause for the increasing prevalence of KLA in Asia is not known, but it may be related to the large population of Asia, the host susceptibility to infection, the difference in carriage rates, and the environmental factors or the emergence of a distinct strain of K. pneumonia, and an increased propensity to cause liver abscesses might be potentially contributing to such a geographical difference in the epidemiology of K. pneumonia infection. For example, Chung et al. had noted that people of Korean ethnicity who had lived in countries other than Korea had a lower proportion of carrying serotype K1 of K. pneumonia strains than those who lived in Korea [3]. These findings indicate a potential role of the environmental factors in the intestinal colonization of these strains.
Some studies manifested that translocation from the gastrointestinal tract maybe the most likely route by which K. pneumonia caused liver abscess [3,20]. Fung et al. demonstrated that gastrointestinal carriage was a predisposing factor for liver abscess [20]. Furthermore, they also found that patients with KLA and healthy carriers had identical pulsed-field gel electrophoresis (PFGE) profiles with the same virulence-associated genes and similar LD50 values. In a recent investigation, Lin et al. reported a fecal carriage rate of K. pneumonia in healthy adults of 75% and high prevalence (23%) of serotype K1/K2 isolates among typical strains in Taiwan [49]. K. pneumonia can colonize the gastrointestinal tract of humans, which suggests that colonization by the K. pneumoniae strains precedes invasion of the intestinal mucosa and portal venous flow or ascending biliary infection, which is followed by the development of liver abscess.
Several studies have found that capsule K1 and K2 were the most two common causes of KLA (Table 2). K1 isolates occur at a significantly higher frequency than those of all other serotypes, especially in patients with diabetes mellitus [15]. A recent case-control study performed by Kim et al. suggested that diabetes mellitus was an important underlying factor that correlates with a high incidence of K1 serotype in KLA [29]. Poor glycemic control plays an important role in impairing the neutrophil phagocytic function of patients with K1/K2-type KLA, whereas it does not significantly affect those of patients with non-K1/K2 KLA [50]. Capsule k1 serotype is found to express the hypermucoviscous phenotypes which can produce vast amounts of extracapsular polysaccharide constituting a mucoviscous web that protects these strains from phagocytosis by neutrophils and from serum killing by complement [2,13,51]. In addition, in vitro serum assays show a significantly higher serum resistance on average for K1 than K2 strains, indicating that K1 and K2 strains have unequal virulence [18]. Recently, most isolates of serotype K1 from KLA patients belong to ST23 which is the most prevalent sequence type among serotype K1 isolates [2,24,26].

Bacterial Genes and Pathogenesis
A number of bacterial genes, which are significantly correlated with the high virulence of the invasive strains [2,24,26,52], have been proposed or suggested to play key roles in the pathogenesis of hepatovirulent KLA ( Table 2).
The rmpA gene (a regulator of the mucoid phenotype), which is a transcriptional activator of the cps genes and functions as a positive regulator of extracapsular polysaccharide synthesis, has a strong association with hypermucoid strains in PLA [18,52,53]. Loss of this regulator will downregulate capsule synthesis, and knockout of the rmpA gene can decrease virulence in mouse lethality tests by 1000 folds [12], leading to the loss of phagocytic resistance and the mucoid phenotype. Hsu et al. found a correlation of rmpA/A2 with six PLA-related capsular types (K1, K2, K5, K54, K57, and KN1). However, the correlation of rmpA/A2 with K1 strains from the West was less obvious than with the strains from Asia [53].
On the other hand, magA, a chromosomal gene which is located in the cps (capsular polysaccharide synthesis) operon, has been recently renamed wzyKpK1 and has been shown to be specific for K1 capsule formation [26]. MagA can contribute to capsular polysaccharide formation, and it is identified as an important virulence gene in invasive K. pneumoniae strains causing primary liver abscess and septic metastatic complications [54]. MagA mutants are also shown to lose the potential to produce this protective mucus and became susceptible to human serum and phagocytosis.
The growth of bacteria in host tissues is limited not only by host defense mechanisms but also by their supply of available iron. Many bacteria attempt to secure their supply of iron in the host by secreting high-affinity iron chelators called siderophores, like aerobactin. Aerobactin, an iron chelator called iron siderophore, can increase virulence in mouse lethality tests by 100 folds [55].
A 20 kb chromosomal region including an iron uptake system (kfu) and a phosphoenolpyruvate sugar phosphotransferase system (PTS) was found to be presented in most of the genomes of the tissue-invasive K. pneumoniae strains 4 Gastroenterology Research and Practice   [18,56]. Iron uptake is critical to pathogenesis as a vital cofactor for many components of microbial antioxidative stress defense. The kfu/PTS region could enrich the ability of bacteria to secure iron, even in the relatively iron-deficient conditions of the human host, and to eventually enhance the virulence of the bacteria. The celB gene, which encodes the putative cellobiose-specific PTS, has been confirmed to play an important role in the virulence of PLA-associated K. pneumonia strains. When deleting the celB, the PTS activity is significantly decreased, the biofilm development is delayed, and the thickness of the biofilm does not further increase [57]. The allS gene that possessed a 22 kb region associated with anaerobic metabolism of allantoin as the sole source of carbon, nitrogen, and energy under either aerobic or anaerobic condition [58] is only found in K1 isolates [52] ( Table 2). The allS region can help bacteria to compete for nitrogen sources via the allantoin-utilizing ability.
KLA is also associated with a higher likelihood of hematogenous spread and the potential for metastatic infection in other parts of the body compared with other kinds of PLA [16,21,27,30,35,38,40,41,50,56]. The high incidence rate of metastatic infection ranges from 10% to 45% [9,16,30,[35][36][37][38], especially in patients with diabetes mellitus [14,15,22,30,35,38,39,50]. Patients with diabetes mellitus are at increased risk for common infection due to impaired host-defense mechanisms. Furthermore, abscess with size of 5-6 cm is proven to be a significant independent predictor of KLA patients with metastatic infections [22,39]. It is suggested that we should not neglect the small-sized liver abscess in diabetic patients in the early course of the disease, for hematogenous dissemination of K. pneumonia can occur, which leads the severe clinical symptoms that can result in earlier detection of liver abscess, and this supported the metastatic infections of small-sized KLA.
In terms of underlying diseases, individuals with KLA have a lower proportion of comorbidity such as malignancy [38], liver cirrhosis, chronic kidney disease, and biliary disease [30] than did Non-KLA ones, a significantly higher proportion of DM on the other hand [29,33]. Other presentations such as bacteremia, septic shock, disseminated intravascular coagulation, acute renal failure, and acute respiratory failure are also reported to be more prevalent in KLA than in Non-KLA patients [38].
On physical presentations, fever and right upper quadrant tenderness are the most common findings. Jaundice is found in the patients with underlying biliary disease. Hepatomegaly is less common in KLA than in Non-KLA patients [38].

Laboratory and Imaging Findings
Anemia, leukocytosis, high erythrocyte sedimentation rate, C-reactive protein, hypoalbuminemia, elevated total bilirubin, and alanine aminotransferase are the common features. A recent study in the USA found an elevated white blood cell count in 68%, a low albumin level in 70.2%, and an elevated alkaline phosphatase level in 67% of PLA [6]. None of the blood tests specifically helps to diagnose a liver abscess, but they can suggest a liver abnormality that leads to imaging studies.
The most essential technology to make the diagnosis of KLA is radiographic imaging. Pulmonary X-ray can reveal right-sided pulmonary infiltrates with pleural effusion [34,60], and plain abdominal X-rays, which are rarely used but can be helpful in some cases, can show air-fluid levels [45,59] or portal venous gas. And now, US and CT are two main diagnostic methods which are both sensitive in the diagnosis of KLA.
The appearance of KLA at US imaging may range from hyperechoic to hypoechoic, and this variation has a close relationship to the pathologic stage of KLA. Hui et al. found that 84% patients predominantly had a solid appearance unlike other PLAs in US imaging and that 52% patients had diabetes mellitus [40]. However, Lee et al. found that KLA predominantly had a solid appearance but did not show much association with diabetes mellitus (the prevalence of diabetes mellitus between KLA and Non-KLA is 61.0% versus 51.4%, resp.) [30]. Overall, KLA appears as hypoechoic nodules and solid in US appearance imaging.
The sensitivity of CT was reported to reach as high as 100% compared with a sensitivity of 96% of US [47]. Recently, various studies were reported to compare the differences of CT imaging between KLA and Non-KLA [1,27,30,38,41] ( Table 3). Most of KLA patients have a solitary abscess in the right lobe of liver due to its size and propensity to receive most of the portal blood flow [1,6,9,27,28,30,35,38,40,41], which does not show much difference from Non-KLA ones [1,27,30,38]. However, some reports found that KLAs were more likely to appear as single abscesses [35,41] and unilobar involvements [41] than Non-KLAs (unilobar: 82.6% versus 61.5%). The majority of liver abscesses in the two groups are not more than 10 cm in diameter, and there are no significant differences between KLA and Non-KLA with respect to the size of the abscess cavity [1,35,38,41]. KLA is predominantly with septations in the abscess (i.e., multilocular), which is similar to that of Non-KLA [30,38,41], whereas multilocular abscesses are more common in the KLA group than in the Non-KLA group [41]. Gas-forming liver abscess had been rarely reported in pyogenic liver abscess in the past; however, with the etiologic shift to K. pneumoniae as the primary causative agent of PLA infections, there is an increased risk of gas-producing liver abscess especially in patients with uncontrolled DM [1,5,22,45,60]. In our previous research, we found that KLA was more associated with gas-formation than Non-KLA [24(32.9%) versus 5 (13.5%)] [1] (Figure 1). It is assumed that, under anaerobic conditions, these facultative anaerobes can produce carbon dioxide by fermentation of glucose in tissue, especially under hyperglycemic conditions. No differences are found between groups regarding the presence of gas bubbles [30,35,38,41] as shown in Table 3; however, KLA also shows a trend toward higher incidence of thrombophlebitis, whereas pneumobilia is more common in the Non-KLA group [41]. Other series do not show much difference between KLA and Non-KLA on account of the incidence of pneumobilia and thrombophlebitis [1,38].
On CT imaging, liver abscesses are of lower attenuation than the surrounding normal liver parenchyma, thin-walled abscess on unenhanced scans [30,34,38]. The abscess wall usually shows a rim-enhancement on contrast-enhanced CT [38].
Although a significantly higher complication rate is found in KLA patients treated with cefazolin than in those treated with an extended-spectrum cephalosporin [63], another research has shown the similar therapeutic effects between patients treated with extended-spectrum cephalosporins and those treated with a combination of first-generation cephalosporins and aminoglycosides; furthermore, the latter treatment is recommended for patients without risk factors such as endophthalmitis and meningitis [62] ( Table 4).
The third-generation cephalosporin is used more in patients with KLA group as compared with that in the Streptococcus milleri (SM) group [38]. The SM group tends to use extended-spectrum penicillin more, whereas 10% of Klebsiella isolates are resistant to penicillin. Initial antibiotic regimens should comprise a second-generation cephalosporin and an aminoglycoside with metronidazole when treating PLA caused by E. coli isolates, whereas the first-generation cephalosporin covers most pathogens found in KLA [27].
The optimal duration intravenous therapy, as well as the duration of subsequent oral therapy, remains unclear. In the study of Taiwan, therapy generally consisted of 3 weeks of intravenous antibiotics followed by 1-2 months of oral therapy [16]. However, a US study in 2004 indicated shorter courses of antibiotic therapy with durations of intravenous therapy (17.5 days), and oral therapy (13.6 days) which were associated with extremely low mortality [6].
The treatment of KLA without metastatic infections includes pigtail catheter drainage by negative-pressure and combination of parenteral cefazolin and gentamicin for two weeks [16]. Gentamicin is discontinued after 2 weeks to avoid nephrotoxicity, but cefazolin is continued for at least 3 weeks and oral cephalosporin for 1-2 months to prevent relapse. In patients with septic endophthalmitis or other distal metastases, the prognosis is bad. Systemic intravenous and intravitreous antibiotics are necessary [35,64,65]. For example, a third-generation cephalosporin, ceftriaxone, is considered to be a useful antibiotic due to its good penetration into the vitreous compartment [15]. Intravitreal vancomycin and ceftazidime are also successfully administrated in some studies [64,65].
Resistance of K. pneumonia to strains that produce extended-spectrum -lactamase (ESBL) had been noted in many parts of the world [6,9,66]. Antibiotics such as -lactam/ for Beta-lactamase inhibitor combinations such as piperacillin/tazobactam (TZP) and ampicillin/sulbactam have replaced extended-spectrum cephalosporins (ESCs) to control ESBL prevalence in Korea [67]. Recently, TZP    resistance among K. pneumonia isolates has been shown as high as 20.9% (50/239) versus 7.6% (13/170) of E. coli, and the mechanisms for TZP resistance might include the presence of AmpC producers, multiple b-lactamases in individual organisms of a given isolate, and possible TEM-1 hyperproducers [66]. Drug-resistant K. pneumonia is more prevalent in the DM group than in the non-DM group [68]. If a patient presents with the risk factors for infection with ESBL-producing organisms, carbapenem antibiotics (e.g., imipenem, meropenem, ertapenem, or doripenem) should be recommended before the culture and isolation results are obtained. Carbapenem (primarily imipenem) has been found to be independently associated with lower mortality than other antibiotics [61].

Interventional Therapy.
Percutaneous drainage was widely used during the past two decades [6,37,46,69], and the obvious advantages are the simplicity of treatment and avoidance of general anesthesia and laparotomy. As for multiple abscesses, percutaneous drainage usually meets with a higher failure rate [47]. However, a recent retrospective study showed that the treatment with percutaneous transhepatic drainage demonstrated similar effectiveness for the patients with multiple abscesses but shorter hospitalization when compared with surgical drainage group, which suggested that percutaneous drainage should always be undertaken before surgery in terms of its lower morbidity and less cost [69]. Percutaneous drainage includes percutaneous aspiration and percutaneous catheter drainage. Yu et al. found that intermittent needle aspiration was probably as effective as continuous catheter drainage for the treatment of PLA [46]. Due to the solid nature of KLA, procedure simplicity, patient comfort, and reduced price, needle aspiration deserves to be considered as a first-line drainage approach. Patients with the following criteria are taken for percutaneous drainage: (1) patients who continued to be febrile even after 48-72 h of adequate medical treatment; (2) liver abscess more than 6 cm in size; and (3) clinical or ultrasonographic features suggesting impending perforation [70].

Surgical Intervention.
Although percutaneous drainage has replaced surgery as the primary treatment of liver abscess [37,69], the surgery still has its irreplaceable position under some conditions. Surgery drainage is carried out in patients falling within the criteria which are as follows: (1) thick pus which could not be aspirated; (2) patients with multiple liver abscess; (3) patients with ongoing sepsis even after antibiotic therapy and percutaneous drainage; (4) patients with underlying diseases such as the biliary tract disease or the liver cirrhosis; (5) multiloculated abscess; (6) abscess in the left lobe; and (7) ruptured abscesses [28,34,36,37,59,60,69,71,72].
Recently, a case with intraperitoneal rupture of pyogenic liver abscess caused by K. pneumonia was successfully treated with hepatectomy combined with antibiotics [34]. The solid nature of the abscess and the complicating DIC preclude percutaneous catheter drainage of the abscess. This indicates that surgical hepatectomy is necessary and useful in KLA treatment. In general, surgical drainage has been reserved for patients who fail to respond to treatment with percutaneous drainage and antibiotics or who have concurrent intraabdominal pathology which requires surgical management [47]. Thus, percutaneous and surgical techniques are not competing methods, but they have different indications, and surgery also represents an option for nonresponders to percutaneous treatment.

Prognosis
Overall, the prognosis is better for patients with KLA than for those with other bacterial liver abscesses insofar as mortality [16,19,35] and disease relapse [16]. Diabetes mellitus and the K1 serotype were common risk factors for recurrent KLA [51,73]. The outlook for patients who develop metastatic infections especially endophthalmitis is grim [13,15,16].

Conclusion
PLA caused by K. pneumonia is an emerging infectious disease in diabetic patients in Asian countries and areas, the United States, and Europe, and it tends to spread globally. Strains of capsule K1 are the most virulent serotypes and are commonly associated with KLA and its complications. Various genes contribute to these hypermucoviscous features, including rmpA, aerobactin, magA, kfu, and allS, which can be exploited as a genetic marker for rapid molecular diagnosis and for treatment of this disease. The characteristics of imaging features contribute to the early diagnosis, and percutaneous intervention combined with an aminoglycoside plus either an extended-spectrum betalactam, such as piperacillin/sulbactam or a second-or third-generation cephalosporin, is a timely and effective treatment of KLA. The prognosis is better for patients with KLA than for those with other bacterial liver abscesses; however, KLAs with metastatic infections especially endophthalmitis have poor outcomes.