Nosocomial bloodstream infections caused by Klebsiella pneumoniae: impact of extended-spectrum β-lactamase (ESBL) production on clinical outcome in a hospital with high ESBL prevalence

Background The frequency of ESBL producing Klebsiella pneumoniae bloodstream infections (BSI) is high in Brazilian hospitals, however little is known regarding what role, if any, resistance plays in the expected outcome in hospitals with a high prevalence of these pathogens. Methods From 1996 to 2001, hospital acquired K. pneumoniae BSI were evaluated retrospectively. Each patient was included only once at the time of BSI. ESBL producing strains were identified using the E-test method. The association of variables with the mortality related to bacteremia was included in a stepwise logistic regression model. Results One hundred and eight hospital acquired K. pneumoniae BSI met criteria for inclusion. Fifty two percent were due to ESBL producing strains. The overall in-hospital mortality was 40.8%. Variables independently predicting death by multivariate analysis were the following: mechanical ventilation (p = 0.001), number of comorbidities (p = 0.003), antimicrobials prescribed before bacteremia (p = 0.01) and fatal underlying disease (p = 0.025). Conclusion Bacteremia due to ESBL producing K. pneumoniae strains was not an independent predictor for death in patients with BSI. An increased mortality in hospital-acquired BSI by K. pneumoniae was related to the requirement for mechanical ventilation, more than two comorbidities, the previous use of two or more antibiotics, and the presence of a rapidly fatal disease.


Background
Klebsiella pneumoniae is an important cause of many infections [1]. It ranks among the top ten pathogens that cause bloodstream infection (BSI) in the United States and Canada [2]. In Latin America, it is the third most prevalent pathogen isolated in the respiratory tract of hospitalized patients with pneumonia and corresponds to 12% of all pathogens isolated [3].
Extended-spectrum β-lactamase (ESBL) producing organisms were first isolated in Germany in 1983 [4] and in the United States in 1989 [5]. ESBLs are plasmid-mediated enzymes that hydrolyze broad-spectrum β-lactams [6]. The emergence of ESBL-producing K. pneumoniae has been reported as an important cause of nosocomial infection in the United States and Europe. The prevalence of ESBLproducing K. pneumoniae strains in hospitals, ranges from 5 to 25% in several parts of the world [7,8]. In Brazilian hospitals, the frequency of ESBL-producing K. pneumoniae is higher than those observed in many European and American hospitals, accounting for 45% of K. pneumoniae strains [3].
It has been shown that a poor outcome occurs when patients with serious infections due to ESBL-producing organisms are treated with antibiotics to which the organism is highly resistant [9]. The mortality rate in such patients is significantly higher than those observed in patients treated with antibiotics to which the organism is susceptible. In addition, a suboptimal clinical outcome occurs when cephalosporins are used for the treatment of serious infections due to ESBL-producing organisms, which may appear to be susceptible on the basis of cephalosporin MICs of 2 to 8 µg/mL [7].
Risk factors associated with infections caused by ESBL producing organisms include central venous catheters [10], tracheostomy [11], and cephalosporin use [11], however little is known regarding what role, if any, resistance plays in the expected outcome in hospitals with a high prevalence of these pathogens.
The aim of this study was to evaluate whether ESBL producing K. pneumoniae is associated with a high mortality rate in a hospital with a high ESBL prevalence.

Methods
A retrospective cohort study was carried out at the Universidade Federal de São Paulo, a 624-bed university hospital, located in the state of São Paulo, Brazil. The study was approved by the Hospital Ethics Committee. All patients for whom blood culture results were positive for K. pneumoniae from January 1996 to May 2001 were eligible for inclusion in the study. Each patient was included only once. If multiple blood cultures from the same patient were positive for the above organism, only the first episode was reviewed and recorded. Pseudobacteremia caused by K. pneumoniae, defined as the presence of a positive blood culture without clinical manifestations of sepsis (fever, hypotension, tachycardia, tachypnea and leukocytosis or leukopenia), and cases with incomplete data in the medical record were excluded.

Clinical and laboratory characteristics of patients
Potential risk factors for mortality due to K. pneumoniae infection were ascertained by means of a review of medical charts. Data obtained included age, sex, ward, number of hospital days prior to infection, the presence of a central venous line, haemodyalisis, Swan-Ganz and urinary catheters, draining tubes and mechanical ventilation. The presence of septic shock was defined by a systolic blood pressure ≤ 90 mmHg or a reduction of 70 mmHg in systolic blood pressure in hypertensive patients. The severity of illness at the time of bacteremia was classified by the Simplified Acute Physiology Score (SAPS) [12]. The presence of the following comorbid conditions was documented: cardiovascular diseases, solid or hematologic malignancies, neurological diseases, renal failure (indicated by a creatinine level >2.0 mg/dL or the requirement of dialysis), diabetes mellitus, hepatic dysfunction, chronic obstructive pulmonary disease, intravenous drug use, and HIV infection. Previous antibiotic treatment was defined as an antibiotic prescribed for at least 48 hours during the fifteen-day period prior to the onset of BSI [13].
Underlying diseases were classified according to the McCabe classification [14]. Nosocomial infections and sources of infection were defined according to Centers for Disease Control and Prevention (CDC) criteria [1]. Inadequate empiric antimicrobial treatment was defined as therapy administered within 24 hours after blood cultures were obtained that included the administration of an antimicrobial agent to which the K. pneumoniae isolate was resistant [16]. Antimicrobial agents were considered adequate if the organism was susceptible, except when cephalosporins were used for the treatment of ESBL infections [7]. A group of five infectious disease specialists responsible for BSI surveillance at our hospital assessed the adequacy of antimicrobial therapy (including the dosage and route of administration) for patients infected with K. pneumoniae BSI. We inform doctors (generally residents or fellows) about the blood culture results daily (in two periods, generally in the morning and in the afternoon). We based our adequacy of antimicrobial treatment for ESBLproducing K. pneumoniae on guidelines that were written and followed by these five infectious diseases specialists. Each case was reviewed by all five physicians, independently. Rarely, when there was any discordance among us, the final decision was made after further discussion with the majority deciding on the adequacy of antimicrobial therapy. Mortality was defined as death from any cause within 15 days from the date of the first positive blood culture for K. pneumoniae.

Microbiological methods
Blood cultures (consisting of a pair of blood culture bottles including aerobic and anaerobic resin-containing media) obtained from adult patients were processed using the BACTEC ® 9240 blood culture system (Beckton Dickinson, USA). Organisms were identified according to routine bacteriological procedures. Susceptibility testing was performed by the disk diffusion method, following the National Committee for Clinical Laboratory Standards (NCCLS) recommendations [17]. K. pneumoniae isolates were screened for the ESBL phenotype according to the NCCLS guidelines [18]. The ESBL phenotype was confirmed using the E-test ESBL (AB BIODISK, Solna, Sweden) [19].

Statistical analysis
The association of variables was compared by the use of the χ 2 or Fisher's exact tests as appropriate. Significance probabilities (p values) were defined for entry and removal of variables in the logistic regression model: p value <0.05 (deaths from the univariate analysis) and p value <0.10, respectively. All tests of significance were two tailed.
When colinearity existed between two variables, only the one that had the greatest clinical relevance associated with mortality within 15 days was included in the multivariate analysis. Odds ratios were calculated for independent variables associated with 15-day mortality. The association of independent variables was expressed as odds ratios with 95% confidence intervals. A p value of <0.05 was considered statistically significant. All statistical calculations were performed using SPSS for MS Windows software (SPSS 11.0 for Windows).

Results
During the study, 115 patients with K. pneumoniae BSI isolates were identified, of whom 108 met criteria for inclusion. ESBL-producing K. pneumoniae was detected in 56 of 108 patients (51%). We excluded two patients with pseudobacteremia by K. pneumoniae and five patients were excluded because the data in medical records were incomplete.
The proportions for the different variables in the two groups of patients in stratified by ESBL production are listed in Table 2. There were no age (<1 year or >60 years) or gender differences between the two groups (p = 0.693, p = 0.561 and p = 0.339, respectively). A higher proportion of patients with ESBL-producing strains were in intensive care units and had central venous catheters (p = 0.031 and p = 0.044, respectively). There was also a trend towards more than 10 days of hospitalization in patients   institution, of which 39% were ESBL producers. However, these authors analyzed strains that were found in infections from many sources. Curiously, in our study, the frequency of occurrence of ESBL-producing K. pneumoniae was even higher than that observed by Gales et al. A possible explanation for this fact is that there was a dissemination of ESBL-producing strains over the years in our institution. However, the limitations of our study should be acknowledged. First, we do not have information regarding the specific situation in Brazil, such as genotyping results or type of ESBL. Only case reports were described [20,21]. Second, we performed a retrospective cohort study instead of a case-control study because it was not our objective to establish the risk factors for the acquisition of ESBL infection. Third, we did not observe a statistically significant difference in mortality between the ESBL producing K. pneumoniae and non-ESBL producing K. pneumoniae BSI. The small number of cases could lead to a type II error. And fourth we did not evaluate co-infections or recent infections with other nosocomial bacteria evaluated as predictors of mortality.
The 15-day mortality of K. pneumoniae BSI was 24.1%, and 69.2% in patients with ESBL producing strains. Hansen et al., in Denmark, reported similar mortality rates in Klebsiella bacteremia [22]. In contrast, Menashe et al., in Israel, reported a mortality rate of 43.6% with 50% of isolates producing ESBL [23]. By univariate analysis, we identified other variables associated with mortality, such as ICU stay, urinary catheter, mechanical ventilation, fatal underlying disease, two or more comorbidities, a SAPS score >40, shock, thrombocytopenia (<80,000/mm 3 ) and previous use of two or more antibiotics, which have been demonstrated in previous studies [22][23][24][25]. To characterize the severity of the patients' conditions in this study, in addition to the MacCabe classification, we utilized the SAPS-II classification (Simplified Acute Physiology Score) at the time of bacteremia. We decided to use this classification method instead of the APACHE II system used in intensive care therapy, because it provides better-defined criteria. However, the data are collected in the first twentyfour hours of admission and is applicable only for patients in intensive care. In addition, the data do not present the seriousness of the patient's condition during the hospitalization period. Through a univariate analysis, we found that patients with a SAPS-II score >40 have a higher chance of mortality when compared with patients with a SAPS-II score ≤ 40 (p = 0.014).
Multivariate analysis identified that the most important risk factor for death was the requirement for mechanical ventilation. The respiratory tract was the source of BSI in 38% of patients. Fifty-two percent of the patients required mechanical ventilation. The number of comorbidities (more than two) and the presence of fatal underlying disease were the variables associated with death during the fifteen days following K. pneumoniae BSI, demonstrating the importance of the patient's underlying diseases. The same results were obtained by Garrouste-Orgeas et al. in their study. These authors observed that the outcome was influenced by the severity of the underlying host conditions, particularly with patients requiring mechanical ventilation [26]. The prior use of two or more antibiotics was independently associated with death, suggesting a possible selection of resistant antimicrobial strains, such as ESBL-producing strains [25].
By multivariate analysis, previous studies [23,24,27,28] have reported no increase in the mortality rate of infections caused by this resistant microorganism; these studies identified the clinical implications of extended-spectrum beta-lactamase (ESBL) production, not only for Klebsiella pneumoniae, but also for E. coli. Many studies have demonstrated that inadequate antibiotic therapy is related to an increase in the mortality rate [7,15,16,29]. The adequacy of antibiotic therapy was similar when we compared ESBL and non-ESBL producing strains of K. pneumoniae. We considered as inadequate the use of cephalosporins for all ESBL-producing K. pneumoniae BSIs cases in the hospital [7]. Another important consideration is that our institution has a high rate not only of ESBL-producing K. pneumoniae but also ceftazidime-resistant Pseudomonas aeruginosa and Acinetobacter baumannii. It certainly changed our clinical practice over time and contributed to a better appropriate antimicrobial treatment for this types of infections. We believe that it limits our ability to generalize these findings to hospitals with lower prevalence of ESBL-producing K. pneumoniae. In our institution the empiric therapy for gram-negative infections is carbapenems. Conversely, Kim   difference in mortality was observed between patients who received appropriate empiric antibiotic therapy and those who did not (26.3 vs. 20.8%; P = 0.67). One the other hand, Hyle et al. observed that inadequate initial antimicrobial therapy was an independent risk factor for mortality in ESBL-producing K. pneumoniae BSIs [31].
The mortality rate of BSI remains high, despite adequate antibiotic treatment and intensive care measures. In our study, an increase in mortality in hospital-acquired BSI was related to the need for mechanical ventilation, rapidly fatal disease, more than two comorbidities and the use of more than two antibiotics before infection. Additional studies should be carried out to confirm the relationship between high mortality and ESBL-producing K. pneumoniae.
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