Clinical and molecular characteristics, risk factors and outcomes of Carbapenem-resistant Klebsiella pneumoniae bloodstream infections in the intensive care unit

Klebsiella pneumoniae, a member of the family Enterobacteriaceae, is a causative organism of various infections, including serious community-onset infections, such as necrotizing pneumonia, pyogenic liver abscesses, and endogenous endophthalmitis [1, 2]; and nosocomial infections, particularly urinary tract infections (UTIs), respiratory tract infections, and bloodstream infections (BSIs) [1, 3, 4]. Due to abuse of antimicrobial agents in developing countries, the incidence of carbapenem-resistant Enterobacteriaceae (CRE) is of considerable concern. K. pneumoniae is the most prevalent Enterobacteriaceae species, accounting for 71.9% of 242 CRE strains in a retrospective study conducted in a tertiary hospital in Hangzhou, China [5]. Surveillance of antibiotic resistance by the China CHINET showed that 2.9% and 2.8% of Klebsiella spp. were resistant to imipenem and meropenem, respectively, in 2005 compared to 10.5% and 13.4%, respectively, in 2014 [6]. The production of carbapenemases (e.g., KPC, NDM, VIM, OXA-48-like) is the most common mechanism of resistance among K. pneumoniae isolates. Other mechanisms include alterations in outer membrane permeability, mediated by loss of porins and the upregulation of efflux systems [7].

The mortality rate of carbapenem-resistant Klebsiella pneumoniae (CRKp) infections in North America, South America, Europe, and Asia is reportedly 33.24%, 46.71%, 50.06%, and 44.82%, respectively [8]. Similarly, a study done in Shanghai, China showed that the 28-day mortality and in-hospital mortality rates of CRKp BSI patients were significantly higher than those of patients with carbapenem-susceptible K. pneumoniae (CSKp) BSIs (33.3% vs. 16%, p = 0.04; 42.4% vs. 24.6%, p = 0.005, respectively) [9]. Although CRKp is reportedly associated with prolonged hospitalization and mortality [8, 10‐12] because such patients typically receive inappropriate empiric therapy, other studies found no such relationship [13‐15].

There are various risk factors for CRKp BSI. A study conducted in a teaching hospital in Shanghai, China suggested that skin and soft tissue infection (odds ratio [OR] 26.63 and ICU-acquired infection (OR 5.82) was a risk factor for CRKp BSI [9]. multisite colonization (hazard ratio [HR] 13.73), ICU stay (HR 3.14) and previous BSI (HR 6.62) was associated with the development of CRKp BSI in colonized patients [16]. Primary liver disease and hepatitis C virus infection or hepatocellular cancer were significantly associated with development of CRKp in intensive care unit (ICU) patients after orthotopic liver transplantation [17]. Even no exposure independently predicted CRKp BSI in carriers of CRKp [18].

Similarly, several factors are reportedly associated with mortality related to K. pneumoniae BSI. The lung as the probable source of infection (OR 4.23) and a high Sequential Organ Failure Assessment (SOFA) score (OR 1.40) were strong prognostic factors for crude 28-day K. pneumoniae BSI mortality in a teaching hospital in Shanghai, China [9]. Septic shock (HR 3.86), acute respiratory failure (HR 2.32), inadequate initial antimicrobial therapy (HR 1.87) and carbapenem resistance by K. pneumoniae isolates (HR 1.85) were independently associated with mortality in onco-hematological patients [19]. In a univariate analysis, Acute Physiology and Chronic Health Evaluation (APACHE II) score, SOFA score, and CRKp BSI were predictive of ICU mortality after orthotopic liver transplantation [17].

It is generally accepted that CRKp BSIs are associated with high mortality, mostly because of the paucity of antimicrobials active against CRKp and the multiple comorbidities of patients [20]. Severe infection causes organ dysfunction and/or failure via complex mechanisms, including pathogenic microorganisms, an excessive inflammatory response, and immune dysfunction. However, antimicrobial resistance does not always lead to organ dysfunction and/or failure [21]. The immune system plays an important role in disease manifestations, with multiple contributing factors, some of which may not be accounted for by routinely collected data. At present, whether the systemic manifestations of infection and frequency of bacterial virulence genes differ between CRKp and CSKp BSI patients is unclear. Therefore, the objective of our study was to compare the prognosis and clinical characteristics of patients with CRKp and CSKp infections in the ICU, identify risk factors for the development of CRKp BSI and mortality of K. pneumoniae BSI, and assess the frequency of bacterial virulence genes in patients with CRKp and CSKp BSI.

CRKp BSI is a global public health problem that has been increasing in recent times, and is responsible for considerable morbidity [20, 28, 29]. The incidence of K. pneumoniae BSIs in ICU patients exceeds that of Escherichia coli BSIs [30, 31]. The mortality rate of critical patients with K. pneumoniae BSIs in the ICU was reported to rise up to 67.6% [30].

Population-based screening for K. pneumoniae bacteremia was conducted in the Calgary Health Region (population, 1.2 million) from 2000 to 2007. Dialysis, solid-organ transplantation, chronic liver disease, and cancer were risk factors for K. pneumoniae bacteremia [10]. CRE surveillance in Michigan healthcare facilities showed that cardiovascular disease, renal failure, and diabetes mellitus were the most frequently reported comorbidities, and risk factors for CRE included surgery within the previous 90 days, recent infection or colonization with a multidrug-resistant organism, and recent exposure to antimicrobials, particularly third- or fourth-generation cephalosporins [32]. The more frequent hospital contact associated with serious comorbidities may result in exposure to, and possibly infection by, nosocomial microorganisms. Moreover, severe chronic comorbidities were more frequent among patients with CRKp BSIs, but chronic comorbidities were not risk factors for CRKp BSI [33]. Similarly, in this study, comorbidities were not independent risk factors for CRKp BSI.

Glucocorticoids and immunosuppressors were not independent risk factors for CRKp BSI; however, previous studies reported different results. In one study, only prior carbapenem administration (p = 0.003), was significantly associated with CRKp infection, and another study revealed that the type of antibiotic used before infection—such as third-generation cephalosporins, macrolides and quinolones—was an independent risk factor for CRKp (p < 0.05) [11, 12]. Indeed, prior use of macrolides and antibiotic exposure for ≥14 days were the only factors independently associated with nosocomial CRKp bacteremia [33]. In another case-case control study, exposure to quinolones was not associated with CRKp infection, and colonization by CRKp and use of carbapenems were risk factors for infection with CRKp [34]. However, in our study, no antibiotic was a risk factor for CRKp BSI. Different in definitions, the duration of exposure to antibiotics, or different drug-treatment populations among these studies may account for the divergent findings.

In terms of invasive procedures, only an indwelling central venous catheter and urinary catheter were associated with CRKp BSI in univariate analyses. In the multivariate analysis, an indwelling central venous catheter was the only factor independently associated with CRKp BSI, partly consisted with the past studies [35, 36]. However, previous studies indicated that other variables, such as mechanical ventilation and a nasogastric tube, were related to CRKp BSI [36‐38].

Liver and kidney function indices at admission were higher in the CSKp group than in the CRKp group in this study, similar to some previous studies [15, 39]. However, reduced liver and kidney function was not associated with CRKp BSIs.

To improve outcomes, and where there is a need to avoid unnecessary antibiotics so as to reduce CRKp emergence, greater efforts should be made to ensure that initial appropriate antibiotic therapy is delivered to critically ill infected patients, and antibiotic de-escalation should be practiced to avoid unnecessary antibiotic exposure [40]. Moreover, control of the infection source is important for reducing the incidence of BSI. Central line (CL)-associated BSIs in ICUs result in increased morbidity and mortality, and are largely preventable; thus, preventive measures for catheter-related infection are important. Such measures can be applied at central line insertion and maintenance. For example, use of maximal sterile barrier precautions and/or avoiding the femoral vein were applied to reduce the risk of central venous catheter-related bloodstream infection. Moreover, use of central lines should be reduced wherever possible, such as by daily assessment of the need for a CL and timely removal of an unnecessary CL [41].

Infection-related mortality involves a number of factors, including host defense, virulence of the pathogen, source location and control, and the efficacy of available antimicrobials. A recent meta-analysis showed that patients with CRKp had a significantly higher mortality rate than those with CSKp (42.14 vs. 21.12, p < 0.001) [8]; this suggests that antimicrobial resistance is related to mortality. However, severe infections are not necessarily caused by drug-resistant bacteria, i.e., antimicrobial resistance is not linked with infection-induced organ dysfunction or failure, or with mortality, in critically ill patients. Therefore, the difference in clinical features and risk factors between CSKp and CRKp BSIs is intriguing.

In our study, the CSKp BSI patients had more severe clinical characteristics, such as higher APACHE II scores and lower alanine transaminase (AST) levels. Indeed, the serum creatinine level at the time of the positive culture was significantly higher in the CSKp group than in the CRKp group. However, CSKp BIS patients had only a trend towards a higher mortality rate. Therefore, although CS-Kp infection can lead to worse clinical symptoms, the mortality rate is similar between the two groups, despite there being fewer therapeutic options for CRKp BSI.

The risk factors for mortality due to CSKp and CRKp infection were evaluated in this study. The mortality of K. pneumoniae BSI patients was associated with a higher APACHE II score, liver failure, and trachea cannula on the day of bacteremia, but not with carbapenem resistance. In another study, multivariate analyses revealed that carbapenem resistance was not a risk factor for mortality due to K. pneumoniae bacteremia [33]. The mortality rate of CRKp patients was significantly higher than that of CSKp patients in that study, which contradicts our findings and those of a study conducted in Israel in 2012 [42]. The risk factors for mortality due to K. pneumoniae BSI vary among studies, and include bedridden status, chronic liver disease, Charlson comorbidity index ≥5, mechanical ventilation, hemodialysis [33], and Pitt bacteremia score [42].

PFGE and MLST revealed that the isolates had considerable clonality; the 33 isolates were of different origins. No suspected outbreaks occurred during the study period. We assessed the frequency of 10 virulence genes in the CSKp and CRKp BSI isolates. magA is involved in the production of the K1 capsule, which is an important virulence factor [43]. A previous study confirmed local emergence of K. pneumoniae invasive syndrome and implicated magA and rmpA in its pathogenesis [44]. A number of putative virulence factors, including magA and rmpA, are associated with hypermucoviscous K. pneumoniae (hvKP), which can cause serious infections [45, 46]. Alarmingly, multidrug-resistant, including carbapenem-resistant hvKP isolates have emerged [23, 47, 48]; thus, we assessed the frequency rates of various virulence genes in the CRKp and CSKp groups. The frequency rates of allS and magA were higher in the CSKp group than in the CRKp group, despite the small number of subjects. Therefore, further studies of virulence genes, possibly using whole-genome sequencing (which is becoming less costly and more rapid) and involving larger populations, are warranted.

Our study had several limitations. First, relatively few patients were enrolled, which hampered the multivariate analysis and ability to draw firm conclusions. Second, some K. pneumoniae isolates were not stored. Third, the therapeutic regimen for K. pneumoniae BSI was not taken into consideration. Fourth, in the ICU setting, a heterogeneous population can limit the statistical analysis. Fifth, the role of the immune system was not analyzed. Despite these limitations, however, we identified several differences between the CRKp and CSKp groups, and explored the impact of carbapenem resistance and bacterial virulence genes on the outcomes of patients with K. pneumoniae BSIs.These data can lay the groundwork for future research in this field.

An indwelling central venous catheter is a risk factor for CRKp BIS. Liver and kidney function at admission were lower in the CSKp group than in the CRKp group in this study. The mortality rate and frequency of bacterial virulence genes were similar between the CSKp and CRKp groups. Mortality due to K. pneumoniae BSI was not related to carbapenem resistance in univariate analysis. Further study is required to verify the correlation between CRKp-mortality mortality and the virulence genes of K. pneumoniae isolates.