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Escherichia coli is one of the most common bacteremia-causing pathogens; it has several virulence factors associated with bloodstream invasion and infection [12]. Of these, the papG class II gene is thought to play a more important role in the development of E. coli bacteremia in patients with an upper urinary tract infection (UTI) than in patients with acute cholangitis [2]. However, the role of E. coli virulence factors in the pathogenesis of bloodstream infections remains unclear. Among its virulence factors, E. coli can harbor genomic pks islands that code for a polyketide-peptide genotoxin known as colibactin. E. coli strains carrying pks islands induce DNA damage and trigger genetic instability [3], and pks islands were significantly associated with bacteremia [4]. Therefore, we investigated the molecular epidemiology of bacteremic E. coli isolates and the prevalence of bacteremia-causing E. coli carrying pks islands in Korea.
A total of 146 E. coli isolates (one isolate per patient) were collected from blood samples consecutively at a tertiary-care hospital from January 2015 to December 2016. Blood cultures were incubated in the BacT/ALERT system (bioMérieux, Marcy-l'Etoile, France). Each isolate was identified using the MicroScan Walkaway (Beckman Coulter, Brea, CA, USA) or Bruker Biotyper (Bruker Daltonics, Bremen, Germany) matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry systems. Antimicrobial susceptibility testing (AST) was performed using the MicroScan Walkaway system (Beckman Coulter). The AST results were interpreted based on the CLSI guidelines [5]. Phylogenetic groups were determined by multiplex PCR using a combination of three genes (chuA, yjaA, and TSPE4.C2), as previously described; the isolates clustered into four main phylogenetic groups, A, B1, B2, and D [6]. All isolates were screened by PCR for sequence type 131 (ST131)-associated single-nucleotide polymorphisms (SNPs) in mdh and gyrB [7]. For detection of pks islands, we performed PCR for the clbB and clbN genes as colibactin system markers, as previously described [4]. The primary source of bacteremia was determined according to clinical presentation and/or evidence of an identical strain cultured near or on the same date as the onset of bacteremia. If the source of bacteremia could not be identified, it was classified as unknown origin.
Of the 146 bacteremic E. coli isolates, 107 (73.3%) were isolated from the bloodstream of patients with UTIs, and 13 (8.9%) were isolated from the bloodstream of patients with biliary tract infections (Table 1). An association with the source of infection was found for the resulting phylogenetic E. coli groups. Groups B2 and D were detected exclusively in UTIs (98/107, 91.6%), whereas group A was predominantly implicated in biliary tract infections (5/13, 38.5%). Twenty-eight isolates (group A=6, B2=14, and D=8) were recovered from patients with cancer.
Phylogenetic group B2 was the most common, accounting for 54.1% (N=79) of the isolates, followed by group D with 29.5% (N=43), group A with 11.6% (N=17), and group B1 with 4.8% (N=7; Fig. 1). Many strains belonging to group B2 are known to cause extraintestinal infections [8]. Phylogenetic analysis in a previous Korean study revealed that the majority of strains responsible for UTIs belonged to phylogenetic groups B2 (79%) and D (15%) [9]. Group B2 was also the dominant group causing bacteremia in the present study.
E. coli ST131 represents a recently emerging, globally disseminated cause of multidrug-resistant extraintestinal infections [10]. In this study, ST131 strains accounted for 24.7% (36/146) of all isolates tested. ST131 was found in groups B2 (N=32) and D (N=4), but not in groups A and B1. ST131 strains comprised 29.5% (36/122) of group B2 and D isolates.
Of the 146 bacterial isolates, 17.8% (26/146) carried pks islands, all of which belonged to group B2. Only three ST131 strains carrying pks islands were identified (2.1%). A number of studies have reported that pks islands are confined to group B2 strains [48]. Similarly, in this study, the clbB and clbN genes were detected only in group B2 isolates. This suggests that clbB is an excellent marker for group B2. Johnson et al [4] have suggested that group B2 E. coli isolates carrying pks islands increase the likelihood of bacteremia. They found that these isolates were present in 58% of blood samples, but only in 32% of fecal samples. In our study, 32.9% of the group B2 isolates from blood contained the clbB and clbN genes. Thus, the overall pks prevalence was lower than that reported in the previous study (17.8% vs 58%) [4]. We found that only three ST131 isolates in group B2 carried the clbB and clbN genes, whereas the other 23 ST131 isolates did not. These results suggest that the pks gene might not be associated with ST131 strains.
Our results describe the molecular characteristics of E. coli isolated from bloodstream infections in a Korean hospital. Approximately 54.1% of bacteremic E. coli isolates belonged to phylogenetic group B2. Of the group B2 isolates, 40.5% were ST131 strains and 32.9% carried pks islands.
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