Background
Infections caused by carbapenemase-producing
Enterobacteriaceae (CPE), which have increased worldwide in number to become a significant clinical problem over the last decade, are associated with high morbidity and mortality [
1]. An early longitudinal study from Asia (2000–2012) revealed the prevalence of CPE was extremely low with average rates between 0.6–0.9% [
2]. However, a survey from the National Antimicrobial Resistance Surveillance, Thailand from 2008 to 2016 revealed that carbapenemase-producing
Klebsiella pneumoniae (CPKP) had increased in prevalence from 0.4% in 2008 to 5.4% in 2016 [
3]. A recent retrospective cohort study from a 1200-bed university hospital in Bangkok reported on an increased incidence of CPE from 3.37 per 100,000 patient-days in 2011 to 32.49 per 100,000 patient-days between 2011 and 2016 [
4]. The resistance mechanism for CPE is attributed to the following Ambler molecular classes of carbapenem-hydrolysing beta-lactamases: class A (KPC), class B (IMP, NDM, VIM), and class D (OXA-48) [
5]. There is insufficient data from Thailand on the distribution of beta-lactamase (
bla) genes [
6,
7]. However, a recent study from a university hospital in Bangkok revealed that
blaNDM was the most common such gene followed by
blaOXA-48-like alleles (e.g.,
blaOXA-48,
blaOXA-181, and
blaOXA-232) [
7]. Among all
Enterobacteriaceae, CPKP is commonly associated with numerous antimicrobial resistance (AMR) genes and virulence determinants [
8]. CPKP with its plasmid-encoded carbapenemases (e.g.,
blaNDM and
blaKPC) and its multiclass antibiotic resistance is associated with hospital-acquired infections and treatment challenges [
9,
10]. Hypervirulent
K. pneumoniae (hvKP) can also cause invasive diseases such as liver abscesses and metastatic infections [
11]. In addition to having the K1 capsular serotype, hvKP encodes other virulence determinants (e.g., yersiniabactin, aerobactin, and salmochelin siderophores), and
rmpA1/
rmpA2 genes, which upregulate capsule expression and are associated with more invasive infections [
8,
12,
13].
Although genetic diversity in carbapenemases has previously been reported in Thailand [
7,
14], the epidemiology and characteristics of CPKP and its virulence determinants are not as well understood. Information is also lacking on the role played by hypervirulent strains of CPKP in hospital-acquired infections and how specific virulence determinants are associated with AMR profiles, disease severity and outcomes among hospitalised patients. The aim of the present study was to investigate the molecular epidemiological features of CPKP and its association with the clinical presentations of CPKP-infected patients. We also aimed to identify virulence determinants in the CPKP strains isolated from patients in this study.
Discussion
To the best of our knowledge, this is the first study to document detailed molecular bacterial isolate information on carbapenem resistance, plasmid replicons, and virulence determinants in relation to the clinical characterisation of hospitalised patients in Thailand. Of the 25 CPE-patients with follow-up rectal swab cultures, the mean time to culture negativity was 37.7 days in our study. This finding is consistent with previous reports that 54% of CPE rectal carriers remain CPE carriers for 30 to 60 days after their initial screening, 28% remain as such after 6 months to 1 year, and 14% remain as such after 1 year [
31,
32]. In our study, approximately 16.7% of the asymptomatic rectal carriers developed a clinical infection with a median duration of 20 days. The incidence of CPE infections in the CPE-colonised patients in our study was as high as that seen previously [
33,
34], and there are several possible explanations for this. One explanation is that we began to observe patients who already had CPE colonisation at sites other than the gut, which might be a risk factor for them developing clinical infections [
35]. Another explanation is that most of the patients had multiple comorbidities (e.g., diabetes mellitus and renal diseases) resulting in prolonged hospitalisation, possibly predisposing them to CPE colonisation and subsequent CPE infection.
Antimicrobial susceptibility testing in our study confirmed resistance to piperacillin/tazobactam, ciprofloxacin and meropenem in all the CPE isolates. Moreover, only 17.8% of the CPE strains isolated from the patients were susceptible to fosfomycin. Colistin is presumably the most active agent against up to 89% of the CPE isolates from our study. The evidence from a cohort study [
26] and systematic review [
13] on antibiotic therapy in CPE infections revealed that combination therapy is probably more effective than monotherapy. Therefore, the antibiotic therapy recommendation for CPE infections at Siriraj Hospital is combination therapy, with colistin acting as the backbone of the regimen.
In our study, the mortality rate was 47.6% for CPE-infected patients and 33.3% for patients colonised with CPE. The difference was not statistically significant. However, attributable mortality is difficult to assess because both groups already had high overall mortality and the sample size was small.
Although KPC-producing
Enterobacteriaceae are reported to have spread rapidly over the last decade, their prevalence in Thailand remains very low [
6,
7]. Notably, the CPE prevalence was 1.4%, and
blaKPC-13 and
blaIMP-14 were the only carbapenemase genes detected among the CPE isolates identified at Siriraj Hospital during 2009 to 2011 [
6]. However, the incidence of CPE bacteraemia has significantly increased from < 1% in 2011 to 3.8% in 2017 [
6,
36]. The main CPE identified herein was CPKP, which carried one carbapenemase gene (
blaNDM-1 or
blaOXA-232) and at least one other
bla gene.
blaOXA-48-like genes were the most common carbapenemase genes, with
blaOXA-232 detected in 78% of the isolates. We also found that 46% of the
blaOXA-232 isolates also carried
blaNDM-1, a finding consistent with that reported previously in Thailand [
7]. This highlights that isolates with
blaOXA-48-like genes continue to be a problem in Thailand.
The previously reported cases of
blaOXA-232 –harbouring
K. pneumoniae were mainly serotypes ST14 and ST231 [
37‐
39], while ST16 and ST231 were the dominant epidemic serotypes in our study. Thus, ST231 may be a high-risk, carbapenem-producing
K. pneumoniae clone actively disseminating across Southeast Asia, with related outbreaks being reported in Switzerland [
40]. We found that all 36
blaOXA-232-harbouring isolates were present on a small ColKP3 plasmid in our dataset of
E. coli and
K. pneumoniae genomes, a finding concordant with that from a previous report [
37]. Interestingly, IncFIB(pQil) plasmids were identified in all ST-231
K. pneumoniae isolates in our study, and both ColKP3 and IncFIB(pQil) are known to carry
blaOXA-232 and
blaTEM-1 [
41,
42]. These findings confirm that both plasmids, IncFIB(pQil) and ColKP3, are often found in clinical isolates and contain multiple AMR genes, as has been previously reported [
30].
Among our CPKP isolates, we found two virulence loci that have been previously associated with invasive diseases:
ybt and
iuc, encoding the siderophores yersiniabactin and aerobactin, respectively [
8,
12,
43].
Ybt was found in almost all of our CPKP isolates (97.4%), and all
ybt loci detected in the CPKP genomes were associated with an ICE
Kp structure located in a chromosomal region [
12]. ICE
Kp, an integrative conjugative element, is self-transmissible and occasionally contains virulence factors such as
ybt and
iro [
12]. Thus, ICE
Kp is considered to be an important mediator of pathogenicity in
K. pneumoniae [
12]. VAP was the most common CPKP infection, and since all isolates from patients with VAP had
ybt, these findings raise the interesting possibility that the yersiniabactin siderophore can promote respiratory tract infection as previous studies [
44,
45]. This is the first identification of
iuc5 in ST231-CPKP isolates in Thailand and Southeast Asia; otherwise,
iuc5 has only been found in ST231-CPKP from India [
46]. Some KL types (e.g., KL1, KL2, KL5 and KL57) are considered to be hypervirulent variants of
K. pneumoniae and are associated with invasive diseases [
47]. Our results show that there were at least seven distinct
Klebsiella capsule genes/loci present among the 39 isolates, from which KL51 was the most common. However, only 5 out of 39 of our isolates were KL2 types and all of them belonged to ST14, a non-hypervirulent clone usually encountered in hospital-acquired infections [
48]. Our results also revealed that
wzi alleles were associated with the expected MLSTs more than with KL types.
Of note, when we integrated the clinical information with the bacterial genomic data, we identified ST231-CPKP as the most common pathogen in CPE-infected patients, 6 out of 11 of which had invasive diseases such as primary bacteraemia and pneumonia. According to our analysis of virulence determinants, ST231-CPKP had the highest virulence score (Fig.
2) and contained
iuc5. The
iuc locus has been increasingly detected in hvKP over the last couple of years and is considered to be one of the most prominent features of invasive isolates [
46,
49]. Second, four out of five of the colistin-resistant CPKP isolates belonged to ST16. ST16-CPKP with colistin resistance was found in CPE-infected patients presenting with VAP and three of these patients died while in hospital. Therefore, ST16-CPKP is considered to be one of the more clinically significant clones in our study, as was also reported elsewhere [
50]. Third, the core SNP-based phylogenetic tree suggests the possibility of a polyclonal outbreak of CPKP, predominantly involving ST231 and ST16 CPKP in Siriraj Hospital between 2015 and 2017. We identified two major subclades of CPKP: ST231 (
n = 15) and ST16 (
n = 14). Lastly, ST101 and ST14 were identified among the CPE-infected patients, something previously reported in South and Southeast Asia [
46,
51].
Several limitations in our study require mentioning. First, the strains examined were all isolated from rectal swabs, and CPEs that caused infections were not characterised. However, we collected the rectal swabs while the patients were infected with CPEs and the antibiograms of CPE isolates at the sites of infection were similar to the antibiograms of CPE isolates from the rectal swabs. Next, it was not possible to identify the risk factors potentially associated with poor outcomes because of the small sample size that was available in this study. We have probably overestimated the true prevalence of CPE colonisation because there was a lack of routine screening for CPE in patients on admission during the study period. Nevertheless, our results suggest some clinical correlations between the clinical outcomes of patients with CPE infections and the genomic analysis of the organisms responsible, and also provide essential epidemiological data that could be used to guide empirical treatment and infection control strategies for CPE patients.
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