Background
Klebsiella pneumoniae (
K. pneumoniae) is a common pathogen in community-acquired and nosocomial infections [
1]. However, a new type of
K. pneumoniae invasive syndrome has been identified in southeast Asia during the last 2 decades [
2]. In the 1980s, a community-dwelling Taiwanese patient presented with a primary liver abscess that involved
K. pneumoniae [
3]. In Japan, a few recent studies have reported pyogenic infections that were caused by
K. pneumoniae [
4,
5]. Extrahepatic complications have also been observed because of bacteremic dissemination, such as endophthalmitis, meningitis, and other diseases. However, infected aneurysms caused by hypermucoviscous
K. pneumoniae (hvKP) are relatively rare. A liver abscess may occur after leakage of
K. pneumoniae from the intestinal epithelium and bacterial translocation into the liver via the portal circulation [
6]. However, hvKP bacteremia involving the prostate via the ascending route is extremely rare. In this context, hypermucoviscous strains are identified using the appearance of colonies that are grown on an agar plate. Furthermore, several studies have indicated that hypermucoviscosity is associated with the mucoviscosity-associated gene A (
magA) and the regulator of mucoid phenotype A (
rmpA) genes [
7,
8].
We report two rare cases of hvKP invasive syndrome that involved strains that were positive for magA and rmpA. The first case involved an 81-year-old man with hvKP invasive syndrome who presented with brain, liver, and prostate abscesses; bacterial endophthalmitis; and an infected aneurysm. The second case involved a 69-year-old man with hvKP invasive syndrome who presented with brain, lung, and prostate abscesses; bacterial endophthalmitis; but no liver abscesses.
Discussion
K. pneumoniae is a common human pathogen that causes pneumonia and urinary tract infections (UTIs). In recent years, an increased incidence of hvKP syndrome has been reported in Taiwan [
11], and this syndrome is associated with bacteremia, primary liver abscess, and metastatic infections [
6]. Extrahepatic complications have also been observed because of bacteremic dissemination, such as endophthalmitis, meningitis, and other diseases. However, to the best of our knowledge, there are few reports of infected aneurysms that were caused by hvKP [
12,
13]. In Case 1 from the present report, the patient reported new-onset left lower quadrant pain, and contrast-enhanced abdominal CT revealed enlargement of an existing left internal iliac artery aneurysm. Although we did not have positive culture results from the aneurysm tissue, the diagnosis of hvKP infection was made based on our clinical and diagnostic findings (new-onset left lower quadrant pain and enlargement of the existing left internal iliac artery aneurysm despite appropriate antibiotic treatment). A previous report has indicated that DM and an age of <65 years were independent predictors of metastatic ocular or central nervous system complications of pyogenic liver abscesses [
14]. Another report has identified a history of alcoholism and DM as significant risk factors for the development of metastatic complications from pyogenic liver abscesses [
15]. However, the patient in Case 1 had no known risk factors, as he was 81 years old and had no history of DM or alcoholism. In addition, a previous report has indicated that glycemic control in patients with DM played an important role in the clinical characteristics of KP liver abscesses, especially in metastatic complications from KP liver abscess [
16]. The patient in Case 2 took vildagliptin (100 mg/day) and his hemoglobin A1c was 8.1%; therefore glycemic control was relatively poor. It is suggested that the poor glycemic control played an essential role in dissenminated KP infection.
The mechanisms for classic
K. pneumoniae entry into extraintestinal sites include ascension from the perineum into the bladder, disruption of the bowel enabling entry of gastrointestinal tract colonizers into the peritoneal cavity, and aspiration of oropharyngeal colonizers into the respiratory tract [
17]. However, the mechanism for hvKP remains unclear, and it is currently speculated that leakage of
K. pneumoniae from a patient's intestinal mucosa, and bacterial translocation into the liver via the portal circulation, results in liver abscesses and bacteremic dissemination [
6]. In Case 2, the patient presented with fever, pollakiuria, and pain on urination, which was indicative of a UTI. Furthermore, contrast-enhanced whole-body CT revealed prostate abscesses, but no liver abscesses. These findings suggested a primary prostate lesion, although the ascending route of infection via the prostate is an extremely rare cause of hvKP bacteremia. Moreover, this patient presented with disseminated lesions secondary to the prostate infection, which is extremely rare. Unlike liver abscesses, the K1 serotype of
K. pneumoniae is considered closely related to hematogenous metastasis, and is rarely detected in the urinary tract [
18]. In addition, clinicians may reach a rapid diagnosis of a UTI based on its typical symptoms, such as micturition pain, constant urge to urinate, and back pain. Therefore, we speculate that there are few reports of UTIs that were caused by hvKP, as antibiotic treatment would typically be started before the development of disseminated abscesses.
MLST has been used as a nucleotide sequence-based method for characterizing microorganisms [
19]. This approach is particularly useful for the typing of microbial pathogens, in order to identify clones with noticeably different virulence characteristics [
20]. Invasive syndrome caused by highly virulent strains with sequence type 23 has become increasingly common in Southeast Asia [
21], and the MLST revealed sequence type 23 in both of the present cases.
The hypermucoviscous phenotype can easily be confirmed using the string test [
6]. In this context, a positive result is defined as bacterial colonies on an agar plate stretching for >5 mm using the inoculation. Thus, both of the present cases exhibited positive results for the hypermucoviscous phenotype, based on the string test results. Furthermore, the
magA and
rmpA genes are related to the expression of the hypermucoviscous phenotype [
7]. In this context, the
magA gene encodes an outer membrane protein that is essential for the formation of a protective exopolysaccharide web, which is related to the bacterial virulence and mucoviscosity of the K1 serotype [
22]. The enzyme encoded by
magA gene functions as a polymerase involved in capsule synthesis, and this function is restricted to the capsular gene cluster of K1 serotype only [
6]. In contrast, the
rmpA gene is a plasmid-borne regulator of extracellular polysaccharide synthesis [
23]. Moreover, the
rmpA gene is strongly associated with abscess formation [
7].