ABSTRACT
Vancomycin has been a key antibiotic agent for the treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections. However, little is known about the relationship between vancomycin MIC values at the higher end of the susceptibility range and clinical outcomes. The aim of this study was to determine the impact of MRSA bacteremia on clinical outcomes in patients with a vancomycin MIC near the upper limit of the susceptible range. Patients with MRSA bacteremia were divided into a high-vancomycin-MIC group (2 μg/ml) and a low-vancomycin-MIC group (≤1.0 μg/ml). We examined the relationship between MIC, genotype, primary source of bacteremia, and mortality. Ninety-four patients with MRSA bacteremia, including 31 with a high vancomycin MIC and 63 with a low MIC were analyzed. There was no significant difference between the presence of agr dysfunction and SCCmec type between the two groups. A higher vancomycin MIC was not found to be associated with mortality. In contrast, high-risk bloodstream infection sources (hazard ratio [HR], 4.63; 95% confidence interval [CI] = 1.24 to 17.33) and bacterial eradication after treatment (HR, 0.06; 95% CI = 0.02 to 0.17), irrespective of vancomycin MIC, were predictors of all-cause 30-day mortality. Our study suggests that a high-risk source of bacteremia is likely to be associated with unfavorable clinical outcomes, but a high vancomycin MIC in a susceptible range, as well as genotype characteristics, are not associated with mortality.
INTRODUCTION
Staphylococcus aureus has been recognized as a major human pathogen since it was first isolated from a surgical abscess in 1880 (1). S. aureus is one of the principal causes of bacteremia, and S. aureus bacteremia (SAB) still remains a potentially fatal infection with reported mortality rates of between 13 and 34% (2, 3). During the past decade, methicillin-resistant Staphylococcus aureus (MRSA) has become a worldwide problem for clinicians, including community-associated MRSA and health care-associated MRSA disease (4).
Vancomycin, which was first used in 1958, has been a key antibiotic agent for treatment of MRSA infections. However, reduced vancomycin susceptibility in clinical isolates was reported in Japan in 1997 and considered to be associated with poor clinical outcomes in patients with MRSA bacteremia (5–7). The Clinical and Laboratory Standards Institute (CLSI) reduced the breakpoints for vancomycin against S. aureus from 4 to 2 μg/ml in 2006 (8). Although the increased vancomycin MIC is included within the susceptible range, previous reports (3, 6) have led to increased concerns about the clinical relevance of reduced vancomycin susceptibility in S. aureus. As a reflection of these concerns, some studies have focused on vancomycin MIC values at the higher end of the susceptibility range (3, 6, 7), but the relationship between isolate MIC and clinical outcomes in patients with SAB remains uncertain.
The aim of the present study was to determine the impact of MRSA bacteremia on clinical outcomes in patients with a vancomycin MIC near the upper limit of the susceptible range. The incidence, clinical characteristics, and genotypic characteristics were investigated in patients with a higher MIC and compared to a lower-MIC patient group. To further investigate the relationship between a high vancomycin MIC and the primary source of the bacteremia, we compared patients with high-risk and low-risk bacteremia sources (7).
The accessory gene regulator (agr) locus of S. aureus encodes a quorum-sensing system that coordinates the expression of virulence and other accessory genes (8). There have been some reports suggesting that agr dysfunction is associated with a decreased susceptibility of MRSA to vancomycin (9–11). To determine whether there is any link between the MIC of vancomycin and the genetic background of the patients, staphylococcal cassette chromosome mec (SCCmec) type and agr dysfunction were also examined in MRSA bloodstream isolates.
MATERIALS AND METHODS
Patients and data collection.All adult patients with an S. aureus bloodstream infection (BSI) who were hospitalized in a 2,700-bed tertiary care hospital in Seoul, South Korea, were eligible for enrollment in the present study. Consecutive SAB were collected prospectively when SAB was identified in the daily reports from the clinical microbiology unit. Corresponding clinical data, including demographic and microbiological data, were collected using computerized records. Patients with SAB were evaluated for complications associated with this condition, length of hospital stay, discharge status, and cause of death. Furthermore, clinical factors such as age, gender, underlying disease, Charlson score, and the initial manifestation of bacteremia on the calendar date of the initial blood culture were evaluated. Patients were excluded from the present study if they were outpatients or had polymicrobial infection. Only the first episode was analyzed if a patient had a recurrent episode of SAB during the study period. The study was approved by our hospital Ethical Committee.
Definitions.The duration of bacteremia was defined as the number of days between the first positive culture being taken and the last positive blood culture (12, 13). True SAB was defined in accordance with the Centers for Disease Control and Prevention criteria (14). In our institute, it has been recommended that follow-up blood culture is conducted 48 to 72 h after the initial bacteremia, with repeat blood cultures to be performed every 48 to 72 h until a negative result was obtained. The patients with MRSA bacteremia were divided into the following two groups: vancomycin MIC at the high end of the CLSI susceptibility range (MIC = 2 μg/ml) and a vancomycin MIC of <1.0 μg/ml (high- and low-MIC groups, respectively). The primary source of bacteremia was divided into the three categories described elsewhere (7). A low-risk source was defined by a related mortality rate of <10% and includes intravenous catheter, urinary tract infection, gynecologic sources, and the ear, nose, or larynx. A mortality rate of 10 to 20% was used to define an intermediate-risk source, which included osteoarticular sources, soft tissue sources, and unknown sources. High-risk sources, associated with a >20% mortality rate, included endovascular sources, the lower respiratory tract, abdominal sources, and central nervous system foci. The primary foci of infections, including catheter-related bacteremia (15) and the presence of pneumonia (16, 17), were defined according to generally accepted criteria.
When the source of infection was not determined by clinical and microbiological criteria, it was considered unknown (6). Bloodstream infections were considered to be health care associated when there was hospitalization or health care contact in the 30 days prior to the onset of bacteremia. Prior antibiotic therapy was defined as the use of any antibiotics for more than 48 h within 3 months before the occurrence of the bacteremic episode. Prior vancomycin therapy was defined as the use of vancomycin for more than 48 h during the month prior to the occurrence of the bacteremia. Empirical vancomycin therapy was defined as the vancomycin administered within 48 h after blood culture. A 30-day mortality outcome was defined as death within 30 days of an index S. aureus blood culture.
Microbiological tests.Strain identification and antimicrobial susceptibilities of S. aureus were determined using the MicroScan PBC 28 panel (Siemens, West Sacramento, CA), which contained six vancomycin wells with different concentrations ranging from 0.5 to 16 μg/ml. The function of the agr operon was measured through an analysis of delta-hemolysin production on blood agar plates using S. aureus RN4220, as described elsewhere (18). The presence of synergistic hemolysis made by the interaction of the beta-hemolysin indicates the delta-hemolysin production by the test organism that means the existence of a functional agr locus (19, 20). agr dysfunction was defined as the complete absence of enhancement by delta-hemolysin within the beta-hemolytic zone (8–11). Eight loci selected on the basis of previously described mec element sequences and an internal positive control of the mecA gene were included in multiplex PCR experiments to determine SCCmec type for MRSA isolates (21). All tests were performed in the absence of any information regarding the S. aureus bacteremia.
Statistical analysis.All statistical analyses were performed using SAS software (version 9.1; SAS Institute, Inc., Cary, NC). Categorical variables were compared using the Fisher exact test or the Pearson χ2 test, as appropriate, and continuous variables were compared using the Kruskal-Wallis test. To evaluate the effects of isolates with different vancomycin MIC levels on mortality, relative risks, and 95% confidence intervals (CI) were calculated as hazard ratios (HRs) derived from the Cox proportional-hazards regression model. Multivariable models were fitted using available clinical covariates, and analysis using a backward elimination model was then performed. P values of ≤0.05 were considered to be statistically significant.
RESULTS
Study population.A total of 190 adult patients in our institution were identified with S. aureus bacteremia between May 2010 and June 2012. Of these patients, 96 cases were excluded from our current analysis because they had methicillin-susceptible S. aureus bacteremia (MSSA) or polymicrobial infections. The final cohort included 94 patients with MRSA bacteremia, of which 31 (33%) were classified with MRSA bacteremia with a vancomycin MIC near to the upper limit of the susceptible range (MIC = 2 μg/ml). The remaining 63 (67%) patients were assigned to the low-MIC group (MIC ≤ 1.0 μg/ml). Table 1 lists the baseline characteristics of these subject patients. The two study groups were similar in baseline characteristics. In addition, there was no difference of the mean duration of follow-up days after the initial bacteremia between the high- and low-MIC groups (median 2.8 days versus 3.0 days, respectively, P = 0.88). The proportions of patients with high-risk (23% versus 21%), intermediate-risk (45% versus 41%), or low-risk (32% versus 38%) sources of MRSA were similar between the high- and low-MIC groups. The frequency of prior vancomycin exposure did not differ between the two groups (P = 0.08). However, the overall frequency of prior antibiotic exposure was higher for patients in the high-vancomycin-MIC group than in the low-MIC group (87% versus 59%, P = 0.006).
Baseline characteristics and clinical courses of patients with S. aureus bacteremia
Clinical outcomes between the high- and low-MIC groups.There was no significant difference found in the total duration of antibiotics therapy between the high- and low-MIC groups (median 19 days versus 14 days, P = 0.74). Twenty-two (71%) isolates with a high MIC and 48 (76%) with a low MIC showed negative results in a follow-up culture. The 30-day mortality for the whole group was 22.3% (21/94), but there were no significant differences in attributable mortality or 30-day mortality between the two groups.
Genotype characteristics.Table 2 reports the genotypic characteristics of MRSA isolates from the high-MIC or low-MIC groups. Most isolates were found to be associated with SCCmec type II in both groups (84 and 68%, respectively). Genotype analysis of the agr locus showed that 77% of the high-MIC patients and 67% of the low-MIC patients belonged to agr group II. A total of 23% of the isolates in the high-MIC group (n = 7) and 35% in the low-MIC group (n = 22) had an agr dysfunction (P = 0.22).
Genotypic characteristics of S. aureus bacteremia
Clinical characteristics and outcomes according to the bacteremia source.The main clinical characteristics of the MRSA bacteremia cases in our study population according to the primary source of their bacteremia are indicated in Table 3. Thirty-four patients had a low-risk source of bacteremia, 40 patients had an intermediate-risk source, and 20 patients had a high-risk source. The clinical characteristics of patients were similar between patient groups. The percentage of isolates in patients with a vancomycin MIC of 2 μg/ml also did not differ among the three groups (29% in the low-risk group, 35% in the intermediate-risk group, and 35% in the high-risk group). A total of 29 isolates had an agr dysfunction: 8 isolates (24%) in the low-risk group, 17 isolates (43%) in the intermediate-risk group, and 4 isolates (20%) in the high-risk group.
Clinical demographic and courses of patients with MRSA bacteremia, by source of the bacteremia
There were no differences found among the three groups in terms of the total duration of antibiotic therapy or the length of hospital stay after the onset of bacteremia. Patients with a low-risk MRSA bloodstream infection source showed increased bacterial eradication rates (82%) compared to patients having intermediate (70%) or high-risk sources (70%), but this was found not to be statistically significant. The all-cause and MRSA-associated mortality rates were both found to be higher in patients with a high-risk source of bacteremia compared to other groups; a total of 13 patients died due to MRSA bloodstream infection—6 patients (30%) in the high-risk source group, 5 patients (13%) in the intermediate-risk group, and 2 patients (6%) in the low-risk source group (P = 0.004).
Predictors of mortality.By univariate analysis, a history of hospitalization, intensive care unit admission at the time of the initial bacteremia, initial septic shock, the source of bacteremia, and bacterial eradication after treatment were shown to be associated with overall mortality (Table 4). By multivariate analysis, bacterial eradication after treatment (hazard ratio [HR], 0.06; 95% CI = 0.02 to 0.17), and a high-risk source of bacteremia (HR, 4.63; 95% CI = 1.24 to 17.33) were found to be independently associated with overall mortality, whereas a vancomycin MIC of 2 μg/ml was dropped out the model. Factors associated with attributable mortality identified by univariate analysis—initial septic shock, a high-risk source of bacteremia, empirical vancomycin therapy, and bacterial eradication after treatment—were found to be associated with attributable 30-day mortality (HR, 0.04; 95% CI = 0.01 to 0.15 [P < 0.001]). In multivariate analysis, the association between bacterial eradication (HR, 0.04; 95% CI = 0.01 to 0.15 [P < 0.001]) and an attributable 30-day mortality remained significant.
Factors associated with crude 30-day mortality by Cox proportional hazard regression with univariate and multivariate modelsa
DISCUSSION
Infections caused by MRSA remain a major disease burden in community and hospital patient populations. Vancomycin has been the treatment cornerstone for MRSA infections, but there are increasing concerns regarding its clinical usefulness in the treatment of serious MRSA infections. In present analyses, we found that MRSA bacteremia cases with a high vancomycin MIC did not show a higher mortality rate. High-risk bloodstream infection sources of MRSA were found to be a main predictor of overall mortality outcomes, irrespective of the vancomycin MIC level.
Understanding the impact of MRSA infections on outcomes in patients with a higher vancomycin MIC within the susceptible range is important to the treating clinicians. Thus, a number of interesting studies on this issue have been reported in recent years (3, 6, 7, 21–24). However, controversy remains regarding the effects of vancomycin on MRSA infection in patients with reduced vancomycin susceptibility.
In a study by Holmes et al. (3), a vancomycin MIC of >1.5 μg/ml was found to be associated with a higher mortality, but this relationship was not found to be related to the antibiotic treatment received (flucloxacillin or vancomycin). Lodise et al. observed that MRSA bloodstream infections in patients with a higher vancomycin MIC (≥1.5 mg/liter) had a higher likelihood of treatment failure, including persistent bacteremia and a recurrence of MRSA bacteremia (6).
However, in a previous study by Neuner et al., a high vancomycin MIC (2 μg/ml) was found to be associated with persistent bacteremia but with no increase in hospital mortality (24). Interestingly, there are similarities between our current study findings and those of Honda et al. (22), which indicated a relationship between reduced vancomycin susceptibility and 28-day mortality among a cohort of patients with MRSA bacteremia. The Honda study reported that a vancomycin MIC ≥2 μg/ml is not associated with mortality (HR, 1.57; 95% CI = 0.73 to 3.37) after adjusting for underlying patient factors.
Several possible explanations exist for the discordant findings among these previous studies. First, there may be geographical variations in pathogen-specific factors, including polymorphisms at the agr locus. Molecular studies can help to uncover the impact of clonal differences on treatment failure in MRSA infections with reference to the vancomycin MIC. We have examined some of the genetic characteristics of all isolates from MRSA cases at our institution, while most previous studies have not evaluated the molecular characteristics of such infections (21).
The relationship between specific microbiological factors and MRSA isolates in cases where isolates showed a reduced vancomycin susceptibility, and the impact of this on clinical outcomes still remains unclear. Han et al. (25) recently reported that MRSA isolates characterized by a reduced vancomycin susceptibility (MIC > 1.0 mg/liter) are significantly associated with SCCmec type II but not with agr dysfunction. Furthermore, the data presented did not demonstrate vancomycin failure or persistent bacteremia. In our present study, a high vancomycin MIC was not found to be associated with specific SCCmec type or agr dysfunction. Furthermore, we did not find that the SCCmec type or agr dysfunction is associated with increased mortality in MRSA bacteremia. Recently, Nienaber et al. demonstrated that clonal complex 30 genotype was associated with an increased risk for S. aureus infective endocarditis (26). Thus, further research into organism factors, including the molecular epidemiology of MRSA as well as genome comparison, will be needed to elucidate virulence factors and determine the optimal treatments for this condition.
A second explanation for the discordant results may be due to methodological differences between prior studies. Some studies included patients that did not have bacteremia but had other diseases, such as pneumonia (27), or that had MSSA bacteremia (3, 28) and applied different separation criteria for the vancomycin MIC levels (i.e., including an MIC ≥ 2 μg/ml) (22). Furthermore, the lack of information concerning the severity of infection and source control has also produced conflicting results in the past (29). It is thus likely that not only pathogen-specific factors but also patient factors are responsible for differing clinical outcomes of MRSA infections. Further studies are needed to examine these parameters.
It has remained unclear to date whether a low-risk BSI has comparable results to a high-risk BSI (21). In our present study, we decided to evaluate the impact of the primary source of the bacteremia on mortality outcomes in patients with a high vancomycin MIC since no previous study has stratified MIC data in this way. Our findings demonstrate that a high-risk source of bacteremia, such as endocarditis, pneumonia, and meningitis, is predictive of an overall 30-day mortality associated with MRSA BSI (HR, 4.63; 95% CI = 1.24 to 17.33, P = 0.02). Our results further suggest that earlier identification of the primary source of bacteremia is important in reducing the mortality associated with MRSA.
Our study has some limitations. The MRSA data were obtained from patients in a single center and thus will not reflect possible institutional differences. This may limit their applicability to other hospitals. There was no assessment of heteroresistant vancomycin-intermediate S. aureus (hVISA) infections in our analyses. However, in a study by Musta et al. (30), the hVISA frequency was MIC dependent, and the status of hVISA was not associated with mortality or persistent bacteremia in a multivariate analysis. Similarly, previous studies, including those performed at our center (31–35), have shown that hVISA is not associated with higher mortality following vancomycin therapy. The diverse prevalence of hVISA reported in different articles (from 0% to >50%) also makes it difficult to evaluate the importance of hVISA in clinical settings (7). Finally, MicroScan was used as an MIC testing methodology in this study. Etest is generally used in analysis for MRSA bacteremia, but some authors have shown that the MicroScan can be an appropriate method for testing S. aureus susceptibility (36, 37). In the present study, we essentially tested the impact of a vancomycin MIC near the upper range of susceptibility on outcomes in the clinical setting. Because there is a paucity of available data using MicroScan, studies such as ours will be needed to help in important clinical decision making.
Recently, guidelines from the Infectious Diseases Society of America have recommended that the patient's clinical response to treatment should determine the continued use of vancomycin, independent of the MIC, when isolates show an vancomycin MIC of ≤2 μg/ml (38). Our current data accord with this MRSA recommendation. By extension, these findings imply that the choice of antibiotics for the treatment of MRSA bacteremia might be based on the degree of risk in terms of the primary source, especially in patients who have a high-risk source of MRSA. This will be valuable information for the treating physician, because there are currently no satisfactory alternatives to vancomycin for the treatment of MRSA bacteremia. Although concerns regarding increasing vancomycin MIC levels have led to a preference among clinicians for alternative therapies such as daptomycin, linezolid, and telavancin, there are currently no head-to-head trials under way to elucidate the effectiveness of these antibiotics against MRSA. Furthermore, treatment failures for daptomycin, based on a cross-resistance of MRSA isolates with a high vancomycin MIC to this drug, have been reported (39, 40). Our current findings have implications for clinicians in countries where daptomycin is not approved, including South Korea. Further data are needed to determine whether the optimization of therapies in accordance with the primary sources of MRSA infection can improve clinical outcomes in affected patients.
In conclusion, we found in our current analysis that a high vancomycin MIC within the susceptible range is not associated with significant differences in the overall 30-day mortality or attributable mortality outcomes in patients with MRSA bacteremia. Our data indicate that a high-risk source of bacteremia is likely to be associated with an unfavorable clinical outcome, but that genotype characteristics such as the SCCmec type and agr dysfunction are not associated with mortality. A randomized trial is needed to assess the role of primary sources of MRSA in patient outcomes, especially in cases with a high vancomycin MIC, and to determine effective treatment options.
ACKNOWLEDGMENTS
We have no potential conflicts of interest, and there is no financial support to report.
FOOTNOTES
- Received 29 May 2013.
- Returned for modification 21 July 2013.
- Accepted 18 August 2013.
- Accepted manuscript posted online 26 August 2013.
- Copyright © 2013, American Society for Microbiology. All Rights Reserved.
