Heterogeneous vancomycin-intermediate susceptibility phenotype in bloodstream methicillin-resistant Staphylococcus aureus isolates from an international cohort of patients with infective endocarditis: prevalence, genotype, and clinical significance

Is It Useful to Repeat Blood Cultures in Endocarditis Patients? A Critical Appraisal

BACKGROUND

Previous guidelines for endocarditis have suggested repeating blood cultures until they become negative, with limited evidence.

METHODS

Literature reviews were conducted (1) on the incidence of persistent bacteremia and association with outcome and (2) on timing of valve culture negativization to examine the claim for prolongation of antibiotic therapy starting from negative blood cultures.

RESULTS

Persistent bacteremia and fever may be present in the first 3 days of endocarditis, despite treatment, and are more common in Staphylococcus (especially MRSA) and Enterococcus species. Persistent bacteremia (48-72 h), persistent infection (day 7), and new onset septic shock are related and predict in-hospital mortality. It is, however, persistent infection at day 7 and septic shock that primarily determine the infectious course of endocarditis, and not persistent bacteremia. Valve cultures at surgery become negative in most cases (>85-90%) after 14-21 days of antibiotic therapy, with no calculated benefit for prolonging therapy after 21 days.

CONCLUSIONS

Persistent infection at 7 days after appropriate antibiotic therapy is a better key event for prognosis then positive or negative blood cultures at 48-72 h. Therapy prolongation from the day of negative blood cultures is not reasonable. There is no need to survey blood cultures in endocarditis patients after starting therapy.

Staphylococcus aureus response and adaptation to vancomycin

Antibiotic resistance is an increasing challenge for the human pathogen Staphylococcus aureus. Methicillin-resistant S. aureus (MRSA) clones have spread globally, and a growing number display decreased susceptibility to vancomycin, the favoured antibiotic for treatment of MRSA infections. These vancomycin-intermediate S. aureus (VISA) or heterogeneous vancomycin-intermediate S. aureus (hVISA) strains arise from accumulation of a variety of point mutations, leading to cell wall thickening and reduced vancomycin binding to the cell wall building block, Lipid II, at the septum. They display only minor changes in vancomycin susceptibility, with varying tolerance between cells in a population, and therefore, they can be difficult to detect. In this review, we summarize current knowledge of VISA and hVISA. We discuss the role of genetic strain background or epistasis for VISA development and the possibility of strains being 'transient' VISA with gene expression changes mediated by, for example, VraTSR, GraXSR, or WalRK signal transduction systems, leading to temporary vancomycin tolerance. Additionally, we address collateral susceptibility to other antibiotics than vancomycin. Specifically, we estimate how mutations in rpoB, encoding the β-subunit of the RNA polymerase, affect overall protein structure and compare changes with rifampicin resistance. Ultimately, such in-depth analysis of VISA and hVISA strains in terms of genetic and transcriptional changes, as well as changes in protein structures, may pave the way for improved detection and guide antibiotic therapy by revealing strains at risk of VISA development. Such tools will be valuable for keeping vancomycin an asset also in the future.

Global distribution of heterogeneous vancomycin-intermediate Staphylococcus aureus strains (1997-2021): a systematic review and meta-analysis

BACKGROUND

Heterogeneous vancomycin-intermediate Staphylococcus aureus is considered one of the main causes in treatment failure of vancomycin, which leads to poor clinical outcomes. Herein, we comprehensively evaluated characteristics such as global prevalence, trend, and genetic backgrounds of these strains.

METHODS

In this study, we conducted a meta-analysis based on PRISMA checklist 2020. In the beginning, global databases were searched to achieve the studies related to the prevalence of hVISA in clinical isolates of methicillin-resistant Staphylococcus aureus. After retrieving the eligible English studies, the prevalence of hVISA isolates and their trend changes were assessed using event rate with 95% confidence intervals.

RESULTS

In the present study, the prevalence of 114 801 MRSA isolates (of 124 studies) was 64%. According to our results, although the frequency of infection with hVISA is increasing in recent years, there is not a significant difference between Asian countries and Europe/America (6.1% vs. 6.8%). In addition, infection with hVISA bacteria was higher in bacteraemic patients than other infections (9.4% vs. 5.5%), which increases hospitalization, treatment costs, and mortality in these patients. Isolates harbouring SCCmec types II and III are most common genotypes in hVISA strains.

CONCLUSIONS

The prevalence of hVISA is increasing, which will reduce the effectiveness of vancomycin treatment in the coming years. The presence of hVISA stains in blood samples was higher than the other samples, which is threatening for bacteraemic patients. The results of the current study indicate a universal program to identify and control the spread of such strains in nosocomial infections.

AgrA directly binds to the promoter of vraSR and downregulates its expression in Staphylococcus aureus

Staphylococcus aureus is an important human pathogen and vancomycin is widely used for the treatment of S. aureus infections. The global regulator agr is known as a well-described virulence regulator. Previous studies have found that agr-dysfunction strains are more likely to develop into vancomycin-resistant strains, but the mechanism for this phenomenon remains unknown. VraSR is a two-component regulatory system related to vancomycin resistance. In this study, we found that the expression levels of vraR were higher in agr-dysfunction clinical strains than in the agr-functional strains. We knocked out agr in a clinical strain, and quantitative reverse transcription PCR and β-galactosidase activity assays revealed that agr repressed transcription of vraR. After vancomycin exposures, population analysis revealed larger subpopulations displaying reduced susceptibility in agr knockout strain compared with wild-type strain, and this pattern was also observed in agr-dysfunction clinical strains compared with the agr-functional strains. Electrophoretic mobility experiment demonstrated binding of purified AgrA to the promoter region of vraR. In conclusion, our results indicated that the loss of agr function in S. aureus may contribute to the evolution of reduced vancomycin susceptibility through the downregulation of vraSR.

Prevalence and molecular characteristics of heterogeneous vancomycin intermediate Staphylococcus aureus in a tertiary care center of northern China

The use of glycopeptide medications may decline in line with the annual decline in methicillin-resistant Staphylococcus aureus (MRSA) detection rates in China. The rate of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA)detection may be impacted by this. However, there is currently a dearth of information on the incidence of hVISA in China. This study aims to analyze the recent epidemiology and molecular characteristics of hVISA strains in Beijing, China. A total of 175 non-duplicate MRSA strains from various infection sites were collected from a medical center between January 2018 and May 2023 and underwent molecular typing and susceptibility testing (Vitek2). Vancomycin and teicoplanin MICs were also evaluated by standard broth microdilution method and agar dilution method, respectively. Isolates growing on screening agar (BHIV4 and BHIT5, brain heart infusion agar containing 4 μg/ml vancomycin and 5 μg/ml teicoplanin, respectively) were characterized further by analysis of macro-Etest (MET) and population analysis profiling with area under the curve (PAP-AUC). The proportion of hVISA among MRSA isolates was 8.6 %. BHIT5 could select all hVISA strains while BHIV4 and MET only selected two hVISA strains. Compared with vancomycin- susceptible Staphylococcus aureus (VSSA), hVISA isolates were less susceptible to erythromycin and clindamycin. In addition, hVISA frequency was MIC-independent despite using different detection methods. In total, 11 types of STs, 28 types of spa typing, four types of SCCmec typing, and two types of agr typing were identified and the predominant type in both MRSA and hVISA isolates was ST239-t030-SCCmecIII-agr I. The analysis of biofilm formation, growth, and virulence genes in hVISA strains revealed sparse information. The dataset presented in this study provided the prevalence and molecular characteristics of hVISA in hospital settings and the combination of BHIT5 and PAP-AUC may identify hVISA efficiently. The result of genotyping suggested the genotype of hVISA was mainly consistent with that of local MRSA. Additional studies on the characteristics of hVISA strains were necessary.

Persistent Methicillin-Resistant Staphylococcus aureus Bacteremia: Host, Pathogen, and Treatment

Methicillin-resistant Staphylococcus aureus (MRSA) is a devastating pathogen responsible for a variety of life-threatening infections. A distinctive characteristic of this pathogen is its ability to persist in the bloodstream for several days despite seemingly appropriate antibiotics. Persistent MRSA bacteremia is common and is associated with poor clinical outcomes. The etiology of persistent MRSA bacteremia is a result of the complex interplay between the host, the pathogen, and the antibiotic used to treat the infection. In this review, we explore the factors related to each component of the host-pathogen interaction and discuss the clinical relevance of each element. Next, we discuss the treatment options and diagnostic approaches for the management of persistent MRSA bacteremia.

Impact of vancomycin use trend change due to the availability of alternative antibiotics on the prevalence of Staphylococcus aureus with reduced vancomycin susceptibility: a 14-year retrospective study

BACKGROUND

We investigated the trend change in vancomycin-intermediate Staphylococcus aureus (VISA)/heterogeneous VISA (hVISA) prevalence among methicillin-resistant S. aureus (MRSA) bacteremia strains and antistaphylococcal antibiotic use together with mutation studies of vancomycin resistance-related gene loci to evaluate the impact of changes in antibiotic use after new antistaphylococcal antibiotics became available.

METHODS

Among 850 healthcare-associated MRSA isolates from 2006 to 2019 at a tertiary hospital in South Korea, hVISA/VISA was determined by modified PAP/AUC analysis, and the identified hVISA/VISA strains were genotyped. Gene mutations at vraSR, graSR, walKR, and rpoB were studied by full-length sequencing. Antistaphylococcal antibiotic use in 2005-2018 was analyzed.

RESULTS

Two VISA and 23 hVISA strains were identified. The prevalence rate ratio of hVISA/VISA carrying mutations at the two-component regulatory systems among MRSA was 0.668 (95% CI 0.531-0.841; P = 0.001), and the prevalence rate ratio of hVISA/VISA carrying rpoB gene mutations was 1.293 (95% CI 0.981-1.702; 174 P = 0.068). Annual vancomycin use density analyzed by days of therapy (DOT) per 1,000 patient-days did not decrease significantly, however the annual average length of time analyzed by the number of days vancomycin was administered for each case showed a significantly decreasing trend.

CONCLUSIONS

During the 14-year period when the average length of vancomycin therapy decreased every year with the availability of alternative antibiotics, the prevalence of hVISA/VISA did not decrease significantly. This seems to be because the resistant strains carrying the rpoB mutations increased despite the decrease in the strains carrying the mutations at the two-component regulatory systems.

Infective endocarditis: Role of molecular techniques in early diagnosis

Infective endocarditis (IE), a heart valve infection primarily caused by bacteria such as streptococci or staphylococci, causes significant morbidity and mortality. Despite the long-term use of broad-spectrum antimicrobials, the infection is often difficult to manage. The latest diagnostic modalities for IE are discussed in this study. Blood culture use in pathogen identification can lead to loss of precious time as well as generation of false negative reports. The first steps in diagnosis are blood cultures and echocardiography, but molecular techniques can be extremely useful and may be used for an accurate and early diagnosis.

Clinical and Microbiological Characteristics of Hospital-Acquired Methicillin-Resistant Staphylococcus aureus Bacteremia Caused by a Community-Associated PVL-Negative Strain

Background

ST72-SCCmecIV, a community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strain in Korea, originated in the community and has been spreading in health care settings. Herein, we describe the clinical and microbiological characteristics of patients with hospital-acquired MRSA bacteremia (MRSAB) caused by community-associated strains.

Methods

We analyzed hospital-acquired MRSAB cases caused by ST72-SCCmecIV using a prospective cohort of patients with SAB in a tertiary hospital in Korea from July 2008 to December 2018. We compared the clinical and microbiological characteristics of ST72-SCCmecIV with ST5-SCCmecII, a representative hospital-associated genotype strain.

Results

Of the 1782 S. aureus bacteremia (SAB) cases, 628 (35.2%) were hospital-acquired MRSAB. Of the 628 isolates, 431 (68.6%) were ST5-SCCmecII and 152 (24.2%) were ST72-SCCmecIV. Patients with ST72-SCCmecIV were younger than those with ST5-SCCmecII and less likely to have a history of recent surgery, antibiotic treatment, nasal MRSA colonization, and central venous catheter placement. Compared with ST5-SCCmecII, ST72-SCCmecIV isolates were more likely to have vancomycin MICs ≤1.0 mg/L (P < .001). Osteoarticular infection as the site of infection (7.2% [11/152] vs 1.4% [6/431]) was more common in patients with ST72-SCCmecIV. There were no significant differences in the rate of recurrence (≤90 days), persistent bacteremia (≥7 days), or 30- and 90-day mortality rates between the 2 groups.

Conclusions

Osteoarticular infections were more prevalent in ST72-SCCmecIV MRSAB. Mortality rates between the ST72-SCCmecIV and ST5-SCCmecII groups were not significantly different.

Changing characteristics of S. aureus bacteremia caused by PVL-negative, MRSA strain over 11 years

Community-acquired methicillin-resistant Staphylococcus aureus (MRSA) has emerged as an important cause of infection. We conducted a longitudinal study to evaluate changes in clinical and microbiological characteristics as well as outcomes of sequence type (ST) 72 MRSA bacteremia. We reviewed adult patients enrolled in a prospective cohort with ST72 MRSA bacteremia from August 2008 to December 2018 at Asan Medical Center, Seoul, South Korea. Changes in clinical characteristics, outcomes, and microbiological characteristics of patients over time were evaluated. Generalized linear and linear regression models were used to evaluate changes. Of the 1,760 isolates, 915 (62%) were MRSA bacteremia and 292 (31.9%) were ST72 MRSA. During the study period, the relative risk (RR) of MRSA bacteremia decreased annually by 3.7%; however, among MRSA bacteremia, RR of ST72 MRSA increased annually by 8.5%. Vancomycin minimum inhibitory concentration (MIC) decreased over the study period. Metastatic infection, persistent bacteremia, and recurrence of bacteremia within 12 weeks decreased significantly. There were no significant changes in 30-d and 12-week mortality. Antibiotic susceptibility of ST72 MRSA was evaluated, and the resistance rate to erythromycin decreased significantly. ST72 MRSA incidence increased annually; its vancomycin MIC and erythromycin resistance rate decreased over the 11 years.

Point-Counterpoint: Should Clinical Microbiology Laboratories Report Vancomycin MICs?

INTRODUCTIONWith numerous reported challenges to reporting MICs for vancomycin, clinical laboratories are attempting to identify accurate methods for MIC testing. However, the issues of poor reproducibility, accuracy, and clinical utility remain a challenge. In this Point-Counterpoint, Dr. Sara Revolinski discusses the pros of reporting MICs for vancomycin, while Dr. Christopher Doern argues for the use of caution.

Management of Staphylococcus aureus Bloodstream Infections

Staphylococcus aureus bloodstream infections are associated with a high morbidity and mortality. Nevertheless, significance of a positive blood culture with this pathogen is often underestimated or findings are misinterpreted as contamination, which can result in inadequate diagnostic and therapeutic consequences. We here review and discuss current diagnostic and therapeutic key elements and open questions for the management of Staphylococcus aureus bloodstream infections.

Calculated parenteral initial treatment of bacterial infections: Microbiology

This is the second chapter of the guideline "Calculated initial parenteral treatment of bacterial infections in adults - update 2018" in the 2nd updated version. The German guideline by the Paul-Ehrlich-Gesellschaft für Chemotherapie e.V. (PEG) has been translated to address an international audience. Preliminary microbiological findings regarding the patient and their immediate environment are crucial for the calculation of treatment with antibiotics in each case, as well as the resistance situation of the ward on which the patient is being cared for. If such data is not available, regional or supra-regional data can be used as a fallback. This chapter describes the methods of susceptibility testing, informs about the resistance situation in Germany and describes the main resistance mechanisms of bacterial pathogens against antibiotics. Further, the chapter informs about collateral damage of antibiotics as well as medical measures against increasing resistance.

Relationship between Vancomycin MIC and Virulence Gene Expression in Clonal Complexes of Methicillin-Susceptible Staphylococcus aureus Strains Isolated from Left-Sided Endocarditis

Higher vancomycin MICs have been associated with more complicated courses and higher mortality rates in patients with Staphylococcus aureus bacteremia and infective endocarditis (IE). The aim of this study was to investigate whether the strains belonging to the cohort of 93 patients from a previously published study in which patients with strains with vancomycin MICs of ≥1.5 μg/ml presented higher mortality rates and systemic emboli than patients with strains with vancomycin MICs of <1.5 μg/ml had specific patterns of virulence factors, clonal complex (CC) types, or the ability to form biofilms. Vancomycin MICs were determined by Etest, and the isolates underwent spa typing to infer the CC, biofilm studies, a thrombin-induced platelet microbicidal assay, and multiplex PCR for the presence of virulence genes. We found no differences in genes encoding adhesins, toxins, or other putative virulence genes according to the vancomycin MIC group. CC30, CC34, and CC45 represented nearly half of the isolates, and there was no association with the vancomycin MIC. agr subgroups I and III predominated, with no association with the vancomycin MIC. Isolates with higher vancomycin MICs exhibited a poorer ability to form biofilms with and without the presence of vancomycin (2.03 versus 2.48 [P < 0.001], respectively, for isolates with higher vancomycin MICs and 2.60 versus 2.87 [P = 0.022], respectively, for isolates with lower vancomycin MICs). In the multivariable analysis, efb and V8 were risk factors for major emboli (adjusted odds ratio [aOR] = 7.5 and 95% confidence interval [CI] = 1.2 to 46.6 for efb, and aOR = 3.9 and 95% CI = 1.1 to 14.1 for V8), whereas no genotypic predictors of in-hospital mortality were found. No clear associations between genes encoding virulence factors, agr type, clonal complexes, mortality, and major embolic events according to vancomycin MIC group were found.

Antimicrobial resistance in nephrology

The prevalence of antimicrobial resistance among many common bacterial pathogens is increasing. The emergence and global dissemination of these antibiotic-resistant bacteria (ARB) is fuelled by antibiotic selection pressure, inter-organism transmission of resistance determinants, suboptimal infection prevention practices and increasing ease and frequency of international travel, among other factors. Patients with chronic kidney disease, particularly those with end-stage renal disease who require dialysis and/or kidney transplantation, have some of the highest rates of colonization and infection with ARB worldwide. These ARB include methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus spp. and several multidrug-resistant Gram-negative organisms. Antimicrobial resistance limits treatment options and increases the risk of infection-related morbidity and mortality. Several new antibiotic agents with activity against some of the most common ARB have been developed, but resistance to these agents is already emerging and highlights the dire need for new treatment options as well as consistent implementation and improvement of basic infection prevention practices. Clinicians involved in the care of patients with renal disease must be familiar with the local epidemiology of ARB, remain vigilant for the emergence of novel resistance patterns and adhere strictly to practices proven to prevent transmission of ARB and other pathogens.

Effect of the Initial Strategy for Active Endocarditis Complicated With Acute Heart Failure

BACKGROUND

Early surgery for infective endocarditis (IE) with acute heart failure (AHF) is recommended, despite clinical results being unclear. We investigated the effect of initial treatment in such patients. Methods and Results: Outcomes for 470 patients with active IE who underwent valvular surgery during 2009-2016 were reviewed. Of them, 177 had symptomatic AHF when diagnosed with IE (excluding those with cardiogenic shock or intubated for AHF). They were divided into 2 groups based on initial treatment: Group S (underwent valvular surgery immediately; n=74) and Group M (received initial medical treatment for infection and HF; n=103). The median (interquartile range) waiting period from diagnosis to surgery in Groups S and M was 1 (1-3) and 15 (8-33) days, respectively (P<0.001). The 5-year survival rate was higher in Group S than Group M (80% vs. 64%; P=0.108). Group M was divided into Group P (initial medical treatment was effective and elective surgery was performed; n=62) and Group E (emergency surgery was necessary during medical treatment; n=41); overall 5-year survival was significantly worse in Group E than Group P (42% vs. 79%; P<0.012). In Group M, multivariate analysis indicated that Staphylococcus aureus infection (odds ratio 3.82; 95% confidence interval 1.19-13.3; P=0.024) was a significant risk factor for conversion to emergency surgery.

CONCLUSIONS

Considering poor outcomes of emergency surgery for medically refractory HF, early surgery may be a reasonable option for IE patients, especially those with S. aureus infection.

β-Lactam Combinations with Vancomycin Show Synergistic Activity against Vancomycin-Susceptible Staphylococcus aureus, Vancomycin-Intermediate S. aureus (VISA), and Heterogeneous VISA

Increasing utilization of vancomycin due to the high prevalence of methicillin-resistant Staphylococcus aureus (MRSA) infections has led to the emergence of vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA) strains. In vitro data suggest the potential for potent synergy between several beta-lactams and vancomycin. The objective of this study is to evaluate the synergy between beta-lactams and vancomycin against MRSA that is vancomycin susceptible, vancomycin-susceptible Staphylococcus aureus (VSSA), hVISA, and VISA. Fifty randomly selected clinical MRSA strains with various susceptibility levels to vancomycin were evaluated for vancomycin alone and vancomycin in combination with various concentrations of cefazolin (CFZ), cefepime (FEP), ceftaroline (CPT), and nafcillin (NAF). The potential for synergy was assessed by 24-h time-kill studies. Beta-lactams reduced vancomycin MIC values against all strains (4- to 16-fold reduction). In time-kill studies against MRSA, CFZ, FEP, CPT, and NAF all demonstrated similar degrees of killing at 24 h, and all showed synergistic activity with vancomycin against VSSA, hVISA, and VISA. Each of these combinations was also superior to any single agent against isolates of all three phenotypes, and each was bactericidal (P < 0.001 for all comparisons). All single-agent exposures demonstrated no activity at 24 h. The combination of vancomycin and beta-lactams significantly improved antibacterial activity against VSSA, hVISA, and VISA strains compared to the activity of any agent alone, supporting the potential use of vancomycin-beta-lactam combination therapy in infections caused by MRSA. Further clinical research is warranted to investigate the synergy of vancomycin against these Staphylococcus strains.

Rapid and easy detection of low-level resistance to vancomycin in methicillin-resistant Staphylococcus aureus by matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Vancomycin-intermediately resistant Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA) are associated with treatment failure. hVISA contains only a subpopulation of cells with increased minimal inhibitory concentrations, and its detection is problematic because it is classified as vancomycin-susceptible by standard susceptibility testing and the gold-standard method for its detection is impractical in clinical microbiology laboratories. Recently, a research group developed a machine-learning classifier to distinguish VISA and hVISA from vancomycin-susceptible S. aureus (VSSA) according to matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) data. Nonetheless, the sensitivity of hVISA classification was found to be 76%, and the program was not completely automated with a graphical user interface. Here, we developed a more accurate machine-learning classifier for discrimination of hVISA from VSSA and VISA among MRSA isolates in Japanese hospitals by means of MALDI-TOF MS data. The classifier showed 99% sensitivity of hVISA classification. Furthermore, we clarified the procedures for preparing samples and obtaining MALDI-TOF MS data and developed all-in-one software, hVISA Classifier, with a graphical user interface that automates the classification and is easy for medical workers to use; it is publicly available at https://github.com/bioprojects/hVISAclassifier. This system is useful and practical for screening MRSA isolates for the hVISA phenotype in clinical microbiology laboratories and thus should improve treatment of MRSA infections.

Role of Vancomycin Minimum Inhibitory Concentrations by Modified Population Analysis Profile Method and Clinical Outcomes in High Inoculum Methicillin-Resistant Staphylococcus aureus Infections

INTRODUCTION

Vancomycin remains the standard of care for invasive methicillin-resistant Staphylococcus aureus (MRSA) infections. Treatment failures from heteroresistant vancomycin-intermediate subpopulations (hVISA) are challenging to detect. Minimum inhibitory concentrations (MIC) identified by modified population analysis profile (PAP) is an alternative testing method. The aim of this study was to evaluate the role of PAP MIC on vancomycin failures in two high inoculum infections: MRSA infective endocarditis and pneumonia.

METHODS

Retrospective, observational study at Detroit Medical Center from 2008 to 2016. Adults ≥ 18 years with ≥ 1 positive MRSA blood culture from IE or pneumonia source and received ≥ 48 h vancomycin were included. The primary outcome was composite failure: MRSA bacteremia ≥ 7 days or 30-day all-cause mortality.

RESULTS

A total of 191 patients were included; 47.6% IE and 52.4% pneumonia. About 19% were hVISA isolates, median vancomycin PAP MIC of 3 (2, 3). More than half (54.5%) experienced composite failure with a larger proportion of PAP MIC ≥ 4 mg/L in this group (25 vs. 15%, p = 0.086). Patients with IE experienced prolonged bacteremia whereas patients with pneumonia experienced higher 30-day mortality. On logistic regression analysis, age [adjusted odds ratio (aOR), 1.026; 95% confidence interval (CI), 1.005-1.047; p = 0.014] and APACHE II score (aOR 1.039; 95% CI, 1.004-1.076; p = 0.029) independently predicted composite failure.

CONCLUSION

Vancomycin PAP MIC may be a more relevant predictor of patient outcomes in persistent bacteremic MRSA infections (e.g., IE). This susceptibility method is less applicable in other high inoculum infections with shorter bacteremia durations and higher mortality rates (e.g., pneumonia).

Outcomes of Vancomycin plus a β-Lactam versus Vancomycin Only for Treatment of Methicillin-Resistant Staphylococcus aureus Bacteremia

The objective of this retrospective study was to compare the rates of treatment failure, which was a composite of clinical and microbiologic failure, of patients receiving vancomycin and a β-lactam to those receiving vancomycin only for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. Patients 16 to 89 years of age with MRSA bacteremia admitted to a university-affiliated hospital from 1 January 2014 to 31 December 2016 were screened for study inclusion. Patients were eligible if they received >48 h of vancomycin and a β-lactam (combination group) or vancomycin only (standard group) within 48 h after bacteremia onset. A total of 182 patients were screened: 47 were included in the standard group, and 63 were in the combination group. The combination group had a higher baseline body mass index (29.2 ± 8.0 kg/m² versus 25.8 ± 7.1 kg/m², P = 0.022), acute physiologic assessment and chronic health evaluation-II (APACHE-II) score (median [interquartile range], 21 [15 to 26] versus 16 [10 to 22], P = 0.003), and incidence of septic shock (31.8% versus 14.9%, P = 0.047). Using multivariate analysis, combination therapy was the only variable that decreased treatment failures (odds ratio [95% confidence interval], 0.337 [0.142 to 0.997]), while vancomycin MIC > 1 mg/liter and male gender increased treatment failures (4.018 [1.297 to 12.444] and 2.971 [1.040 to 8.488], respectively). The 30-day mortality rates (15.0% versus 14.9%, P = 1.000) and the incidence of adverse drug events (19.1% versus 23.4%, P = 0.816) were not statistically different between the combination and standard groups. Combination therapy of vancomycin with a β-lactam led to significantly fewer treatment failures than vancomycin monotherapy for MRSA bacteremia.

Identification of Staphylococcus aureus Cellular Pathways Affected by the Stilbenoid Lead Drug SK-03-92 Using a Microarray

The mechanism of action for a new lead stilbene compound coded SK-03-92 with bactericidal activity against methicillin-resistant Staphylococcus aureus (MRSA) is unknown. To gain insight into the killing process, transcriptional profiling was performed on SK-03-92 treated vs. untreated S. aureus. Fourteen genes were upregulated and 38 genes downregulated by SK-03-92 treatment. Genes involved in sortase A production, protein metabolism, and transcriptional regulation were upregulated, whereas genes encoding transporters, purine synthesis proteins, and a putative two-component system (SACOL2360 (MW2284) and SACOL2361 (MW2285)) were downregulated by SK-03-92 treatment. Quantitative real-time polymerase chain reaction analyses validated upregulation of srtA and tdk as well as downregulation of the MW2284/MW2285 and purine biosynthesis genes in the drug-treated population. A quantitative real-time polymerase chain reaction analysis of MW2284 and MW2285 mutants compared to wild-type cells demonstrated that the srtA gene was upregulated by both putative two-component regulatory gene mutants compared to the wild-type strain. Using a transcription profiling technique, we have identified several cellular pathways regulated by SK-03-92 treatment, including a putative two-component system that may regulate srtA and other genes that could be tied to the SK-03-92 mechanism of action, biofilm formation, and drug persisters.

Prevalence and Microbiological Characteristics of qacA/B-Positive Methicillin-Resistant Staphylococcus aureus Isolates in a Surgical Intensive Care Unit

The increasing use of chlorhexidine for methicillin-resistant Staphylococcus aureus (MRSA) decolonization has raised concerns about the emergence of resistance to or tolerance of this antiseptic. We examined the frequency and characteristics of qacA/B chlorhexidine tolerance genes among MRSA isolates in a surgical intensive care unit (ICU) where MRSA-colonized patients are decolonized by chlorhexidine bathing. The MRSA isolates were evaluated for chlorhexidine susceptibility, mupirocin resistance, molecular typing, agr functionality, and the heterogeneous vancomycin-intermediate S. aureus (hVISA) phenotype according to the presence of the qacA/B genes. Overall, 119 MRSA isolates were obtained from active surveillance cultures (93, 78.2%) and clinical cultures (26, 21.8%) between 2012 and 2014. Among these isolates, 39 (32.8%) carried the qacA/B genes, and 23 (19.3%) exhibited mupirocin resistance. Most qacA/B-positive isolates (36/39, 92.3%) were identified as ST5-SCCmecII (69.2%) and ST239-SCCmecIII (23.1%), which are common healthcare-associated (HA)-MRSA strains in Korea. Multivariate analysis found that qacA/B-positive MRSA isolates were associated with agr dysfunction (OR, 4.87; 95% CI, 1.71-13.87) and the hVISA phenotype (OR, 4.09; 95% CI, 1.48-11.34). In conclusion, our study showed that qacA/B carriage was common among MRSA isolates in an ICU where chlorhexidine is commonly used for decolonization. qacA/B carriage was significantly associated with agr dysfunction and the hVISA phenotype. These features may confer a selective advantage on HA-MRSA strains, including ST5-SCCmecII and ST239-SCCmecIII, in the ICU setting.

Mechanisms of Antibiotic Resistance

Emergence of resistance among the most important bacterial pathogens is recognized as a major public health threat affecting humans worldwide. Multidrug-resistant organisms have not only emerged in the hospital environment but are now often identified in community settings, suggesting that reservoirs of antibiotic-resistant bacteria are present outside the hospital. The bacterial response to the antibiotic "attack" is the prime example of bacterial adaptation and the pinnacle of evolution. "Survival of the fittest" is a consequence of an immense genetic plasticity of bacterial pathogens that trigger specific responses that result in mutational adaptations, acquisition of genetic material, or alteration of gene expression producing resistance to virtually all antibiotics currently available in clinical practice. Therefore, understanding the biochemical and genetic basis of resistance is of paramount importance to design strategies to curtail the emergence and spread of resistance and to devise innovative therapeutic approaches against multidrug-resistant organisms. In this chapter, we will describe in detail the major mechanisms of antibiotic resistance encountered in clinical practice, providing specific examples in relevant bacterial pathogens.

Impact of Vancomycin MIC on Treatment Outcomes in Invasive Staphylococcus aureus Infections

There are conflicting data on the association of vancomycin MIC (VAN-MIC) with treatment outcomes in Staphylococcus aureus infections. We investigated the relationship between high VAN-MIC and 30-day mortality and identified the risk factors for mortality in a large cohort of patients with invasive S. aureus (ISA) infections, defined as the isolation of S. aureus from a normally sterile site. Over a 2-year period, 1,027 adult patients with ISA infections were enrolled in 10 hospitals, including 673 (66%) patients with methicillin-resistant S. aureus (MRSA) infections. There were 200 (19.5%) isolates with high VAN-MIC (≥1.5 mg/liter) by Etest and 87 (8.5%) by broth microdilution (BMD). The all-cause 30-day mortality rate was 27.4%. High VAN-MIC by either method was not associated with all-cause 30-day mortality, and this finding was consistent across MIC methodologies and methicillin susceptibilities. We conclude that high VAN-MIC is not associated with increased risk of all-cause 30-day mortality in ISA infections. Our data support the view that VAN-MIC alone is not sufficient evidence to change current clinical practice.

Influence of vancomycin minimum inhibitory concentration on the outcome of methicillin-susceptible Staphylococcus aureus left-sided infective endocarditis treated with antistaphylococcal β-lactam antibiotics: a prospective cohort study by the International Collaboration on Endocarditis

OBJECTIVES

Left-sided methicillin-susceptible Staphylococcus aureus (MSSA) endocarditis treated with cloxacillin has a poorer prognosis when the vancomycin minimum inhibitory concentration (MIC) is ≥1.5 mg/L. We aimed to validate this using the International Collaboration on Endocarditis cohort and to analyse whether specific genetic characteristics were associated with a high vancomycin MIC (≥1.5 mg/L) phenotype.

METHODS

All patients with left-sided MSSA infective endocarditis treated with antistaphylococcal β-lactam antibiotics between 2000 and 2006 with available isolates were included. Vancomycin MIC was determined by Etest as either high (≥1.5 mg/L) or low (<1.5 mg/L). Isolates underwent spa typing to infer clonal complexes and multiplex PCR for identifying virulence genes. Univariate analysis was performed to evaluate the association between in-hospital and 1-year mortality, and vancomycin MIC phenotype.

RESULTS

Sixty-two cases met the inclusion criteria. Vancomycin MIC was low in 28 cases (45%) and high in 34 cases (55%). No significant differences in patient demographic data or characteristics of infection were observed between patients with infective endocarditis due to high and low vancomycin MIC isolates. Isolates with high and low vancomycin MIC had similar distributions of virulence genes and clonal lineages. In-hospital and 1-year mortality did not differ significantly between the two groups (32% (9/28) vs. 27% (9/34), p 0.780; and 43% (12/28) vs. 29% (10/34), p 0.298, for low and high vancomycin MIC respectively).

CONCLUSIONS

In this international cohort of patients with left-sided MSSA endocarditis treated with antistaphylococcal β-lactams, vancomycin MIC phenotype was not associated with patient demographics, clinical outcome or virulence gene repertoire.

Innate Immune Memory Contributes to Host Defense against Recurrent Skin and Skin Structure Infections Caused by Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus is the leading cause of skin and skin structure infections (SSSI). The high frequency of recurring SSSI due to S. aureus, including methicillin-resistant S. aureus (MRSA) strains, despite high titers of specific antibodies and circulating T cells, implies that traditional adaptive immunity imparts incomplete protection. We hypothesized that innate immune memory contributes to the protective host defense against recurring MRSA infection. To test this hypothesis, SSSI was induced in wild-type and rag1^-/- mice in the BALB/c and C57BL/6 backgrounds. Prior infection (priming) of wild-type and rag1^-/- mice of either background afforded protection against repeat infection, as evidenced by reduced abscess severities and decreased CFU densities compared to those in naive controls. Interestingly, protection was greater on the previously infected flank than on the naive flank for wild-type and rag1^-/- mice. For wild-type mice, protective efficacy corresponded to increased infiltration of neutrophils (polymorphonuclear leukocytes [PMN]), macrophages (MΦ), Langerin⁺ dendritic cells (LDC), and natural killer (NK) cells. Protection was associated with the induction of interleukin-17A (IL-17A), IL-22, and gamma interferon (IFN-γ) as well as the antimicrobial peptides CRAMP and mβD-3. Priming also protected rag1^-/- mice against recurring SSSI, with increased MΦ and LDC infiltration and induction of IL-22, CRAMP, and mβD-3. These findings suggest that innate immune memory, mediated by specific cellular and molecular programs, likely contributes to the localized host defense in recurrent MRSA SSSI. These insights support the development of targeted immunotherapeutic strategies to address the challenge of MRSA infection.

Prevalence and Clinical Impact of Heterogeneous Vancomycin-Intermediate Staphylococcus aureus Isolated From Hospitalized Patients

Background

We estimated the prevalence and clinical impact of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA). The concordance between macromethod and glycopeptide resistance detection (GRD) E tests was determined. In addition, predictors of clinical outcomes in hospitalized patients with S. aureus bacteremia (SAB) or pneumonia (SAP) were evaluated.

Methods

We obtained 229 consecutive S. aureus isolates from all hospitalized patients at two university hospitals located in Busan and Yangsan, Korea. Standard, macromethod, and GRD E tests were performed. Additionally, we reviewed the medical records of all patients. Among the 229 patients, predictors of clinical outcomes were analyzed for 107 patients with SAB and 39 with SAP.

Results

Among the 229 isolates, 34.5% of S. aureus isolates and 50.7% of methicillin-resistant S. aureus isolates exhibited the hVISA phenotype based on the macromethod E test. hVISA was nearly associated with treatment failure in patients with SAB (P=0.054) and was significantly associated with treatment failure in patients with SAP (P=0.014). However, hVISA was not associated with 30-day mortality in patients with SAB or SAP. The concordance between the macromethod and GRD E tests was 84.2%.

Conclusions

hVISA is quite common in the southeastern part of Korea. hVISA is associated with treatment failure in patients with SAP.

Determining vancomycin Etest MICs in patients with MRSA bloodstream infection does not support switching antimicrobials

OBJECTIVES

Elevated vancomycin minimum inhibitory concentrations (MIC) have been reported to adversely affect clinical outcome in methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infection (BSI). We therefore examined the association between vancomycin MIC and outcome considering various potential confounders.

METHODS

Clinical data and bacterial isolates were prospectively collected from patients with MRSA BSI from 2006 to 2012 as part of the Invasive Staphylococcus aureus Infection Cohort (INSTINCT) study. Antimicrobial susceptibility was assessed by Etest, broth microdilution (BMD) and VITEK 2. Bacterial genotypes were determined by spa typing. Using univariate and Cox regression analyses, we investigated the impact of low (≤1.0 mg/L) and high (≥1.5 mg/L) vancomycin Etest MIC on clinical outcomes.

RESULTS

Ninety-one MRSA BSI episodes were included, of which 79 (86.8%) were caused by spa types t003, t032 and t045. High vancomycin MICs were seen only if using Etest but not confirmed using standard reference BMD. When episodes were stratified into low and high vancomycin Etest MIC groups, 30-day overall mortality was 34.5% and 27.3%, respectively (P = 0.64, OR 0.71; 95% confidence interval [CI] 0.27-1.79). Variables significantly associated with all-cause mortality in the Cox model were age (P = 0.003), acute physiology score (P = 0.0006), and Charlson comorbidity index (P = 0.018).

CONCLUSIONS

Vancomycin MICs may vary dependent on testing methodologies and local MRSA epidemiology. The patients' underlying disease and individual comorbidities rather than elevated vancomycin MICs determine adverse clinical outcomes in MRSA BSI. Routine Etest MIC testing of MRSA isolates is of limited value for treatment decisions.

Methicillin-resistant Staphylococcus aureus infections: A review of the currently available treatment options

This review is the result of discussions that took place at the 5th MRSA Working Group Consensus Meeting and explores the possible treatment options available for different types of infections due to methicillin-resistant Staphylococcus aureus (MRSA), focusing on those antibiotics that could represent a valid alternative to vancomycin. In fact, whilst vancomycin remains a viable option, its therapy is moving towards individualised dosing. Other drugs, such as the new lipoglycopeptides (oritavancin, dalbavancin and telavancin) and fifth-generation cephalosporins (ceftaroline and ceftobiprole), are showing good in vitro potency and in vivo efficacy, especially for patients infected with micro-organisms with higher vancomycin minimum inhibitory concentrations (MICs). Tedizolid is an attractive agent for use both in hospital and community settings, but the post-marketing data will better clarify its potential. Daptomycin and linezolid have shown non-inferiority to vancomycin in the treatment of MRSA bacteraemia and non-inferiority/superiority to vancomycin in the treatment of hospital-acquired pneumonia. Thus, several options are available, but more data from clinical practice, especially for invasive infections, are needed to assign specific roles to each antibiotic and to definitely include them in the new antibacterial armamentarium.

Vancomycin MIC Does Not Predict 90-Day Mortality, Readmission, or Recurrence in a Prospective Cohort of Adults with Staphylococcus aureus Bacteremia

Staphylococcus aureus bacteremia (SAB) is a tremendous health burden. Previous studies examining the association of vancomycin MIC and outcomes in patients with SAB have been inconclusive. This study evaluated the association between vancomycin MICs and 30- or 90-day mortality in individuals with SAB. This was a prospective cohort study of adults presenting from 2008 to 2013 with a first episode of SAB. Subjects were identified by an infection surveillance system. The main predictor was vancomycin MIC by MicroScan. The primary outcomes were death at 30 and 90 days, and secondary outcomes included recurrence, readmission, or a composite of death, recurrence, and readmission at 30 and 90 days. Covariates included methicillin susceptibility, demographics, illness severity, comorbidities, infectious source, and antibiotic use. Cox proportional-hazards models with propensity score adjustment were used to estimate 30- and 90-day outcomes. Of 429 unique first episodes of SAB, 11 were excluded, leaving 418 individuals for analysis. Eighty-three (19.9%) participants had a vancomycin MIC of 2 μg/ml. In the propensity-adjusted Cox model, a vancomycin MIC of 2 μg/ml compared to <2 μg/ml was not associated with a greater hazard of mortality or composite outcome of mortality, readmission, and recurrence at either 30 days (hazard ratios [HRs] of 0.86 [95% confidence interval {CI}, 0.41, 1.80] [P = 0.70] and 0.94 [95% CI, 0.55, 1.58] [P = 0.80], respectively) or 90 days (HRs of 0.91 [95% CI, 0.49, 1.69] [P = 0.77] and 0.69 [95% CI, 0.46, 1.04] [P = 0.08], respectively) after SAB diagnosis. In a prospective cohort of patients with SAB, vancomycin MIC was not associated with 30- or 90-day mortality or a composite of mortality, disease recurrence, or hospital readmission.

Evolving resistance among Gram-positive pathogens

Antimicrobial therapy is a key component of modern medical practice and a cornerstone for the development of complex clinical interventions in critically ill patients. Unfortunately, the increasing problem of antimicrobial resistance is now recognized as a major public health threat jeopardizing the care of thousands of patients worldwide. Gram-positive pathogens exhibit an immense genetic repertoire to adapt and develop resistance to virtually all antimicrobials clinically available. As more molecules become available to treat resistant gram-positive infections, resistance emerges as an evolutionary response. Thus, antimicrobial resistance has to be envisaged as an evolving phenomenon that demands constant surveillance and continuous efforts to identify emerging mechanisms of resistance to optimize the use of antibiotics and create strategies to circumvent this problem. Here, we will provide a broad perspective on the clinical aspects of antibiotic resistance in relevant gram-positive pathogens with emphasis on the mechanistic strategies used by these organisms to avoid being killed by commonly used antimicrobial agents.

Safety of treatment with high-dose daptomycin in 102 patients with infective endocarditis

Daptomycin is commonly used at doses >6 mg/kg/day for various indications, including infective endocarditis (IE). A systematic assessment of skeletal muscle, renal, haematological, hepatic and pulmonary toxicity of high-dose daptomycin (HDD) in IE is lacking. A total of 102 IE patients treated with HDD were included in this non-comparative, observational, single-centre cohort study conducted from 2007 to 2014. The incidence, timing, severity and evolution of adverse events (AEs) were assessed. Patients had a median age of 61.5 years and a high prevalence of co-morbidities. Staphylococci were cultured in 87.2% of cases (62.2% meticillin-resistant). The median daptomycin dose was 8.2 mg/kg/day for a median of 20 days (range, 1-60 days). HDD was withdrawn due to AEs in 12 patients (11.8%). On-treatment death occurred in 4 cases (3.9%, none HDD-related). Muscle toxicity occurred in 15 patients in a median of 15 days after HDD starts, which was largely mild and reversible with ongoing HDD use. Mild renal toxicity was observed in 9 patients (8.8%) after a median of 12 days of HDD (RIFLE-Risk in 8, Injury in 1). A rise of peripheral blood eosinophils occurred in 16 patients (15.7%). There were three cases of eosinophilic interstitial pneumonia. Four patients (3.9%) had mild allergic or idiosyncratic reactions. No other hepatic or haematological AEs were observed. Our current experience with 102 patients suggests that HDD is safe in significantly ill IE patients with multiple co-morbidities. Muscle toxicity was clinically negligible. Most importantly, there was no significant renal toxicity. Eosinophils should be carefully monitored.

New developments in the treatment of acute bacterial skin and skin structure infections: considerations for the effective use of dalbavancin

Dalbavancin, an intravenous glycopeptide, was approved by the US Food and Drug Administration in May 2014 for use in adult patients with acute bacterial skin and skin structure infections. The recommended dosing regimen for effective use of dalbavancin is 1,000 mg followed by a 500 mg dose after 1 week. Two multinational, identically designed, non-inferiority trials, DISCOVER 1 and 2, demonstrated similar early clinical success with dalbavancin compared to vancomycin with an option to switch to oral linezolid. In a recently published non-inferiority trial, a single-dose regimen of dalbavancin was compared to the traditional two-dose administration and was found to have a non-inferior clinical response. In the aforementioned trials, dalbavancin was well tolerated, with patients experiencing transient adverse events of mild to moderate severity. The prolonged half-life, excellent skin and soft tissue penetration, bactericidal activity against Gram-positive bacteria including methicillin-resistant Staphylococcus aureus, and convenient dosing make dalbavancin a reasonable option for the treatment of acute bacterial skin and skin structure infections in adult patients who have tried and failed other therapies.

Pneumonia Caused by Methicillin-Resistant Staphylococcus aureus: Does Vancomycin Heteroresistance Matter?

Vancomycin remains the mainstay treatment for methicillin-resistant Staphylococcus aureus (MRSA) infections, including pneumonia. There is concern regarding the emergence of vancomycin tolerance, caused by heterogeneous vancomycin-intermediate S. aureus (hVISA), and subsequent vancomycin treatment failure. Pneumonia is associated with high morbidity and mortality, especially with delays in appropriate therapy. This study evaluated the clinical outcomes of patients with hVISA pneumonia compared to those with vancomycin-susceptible S. aureus (VSSA) pneumonia. A retrospective cohort of patients with MRSA pneumonia from 2005 to 2014 was matched at a ratio of 2:1 VSSA to hVISA infections to compare patient characteristics, treatments, and outcomes. hVISA was determined by the 48-h population analysis profile area under the curve. Characteristics between VSSA and hVISA infections were compared by univariate analysis and multivariable logistic regression analysis to determine independent risk factors of inpatient mortality. Eighty-seven patients were included, representing 29 hVISA and 58 VSSA cases of pneumonia. There were no significant differences in demographics or baseline characteristics. Sequential organ failure assessment (SOFA) scores were a median of 7 (interquartile ratio [IQR], 5 to 8) in hVISA patients and 5 (IQR, 3 to 8) in VSSA (P = 0.092) patients. Inpatient mortality was significantly higher in hVISA patients (44.8% versus 24.1%; P = 0.049). Predictors of inpatient mortality upon multivariable regression were SOFA score (adjusted odds ratio [aOR], 1.36; 95% confidence interval [CI], 1.08 to 1.70), Panton-Valentine leukocidin (PVL) positivity (aOR, 6.63; 95% CI, 1.79 to 24.64), and hVISA phenotype (aOR, 3.95; 95% CI, 1.18 to 13.21). Patients with hVISA pneumonia experienced significantly higher inpatient mortality than those with VSSA pneumonia. There is a need to consider the presence of vancomycin heteroresistance in pneumonia caused by MRSA in order to potentially improve clinical outcomes.

Nonredundant Roles of Interleukin-17A (IL-17A) and IL-22 in Murine Host Defense against Cutaneous and Hematogenous Infection Due to Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus is the leading cause of skin and skin structure infections (SSSI) in humans. Moreover, the high frequency of recurring SSSI due to S. aureus, particularly methicillin-resistant S. aureus (MRSA) strains, suggests that infection induces suboptimal anamnestic defenses. The present study addresses the hypothesis that interleukin-17A (IL-17A) and IL-22 play distinct roles in immunity to cutaneous and invasive MRSA infection in a mouse model of SSSI. Mice were treated with specific neutralizing antibodies against IL-17A and/or IL-22 and infected with MRSA, after which the severity of infection and host immune response were determined. Neutralization of either IL-17A or IL-22 reduced T cell and neutrophil infiltration and host defense peptide elaboration in lesions. These events corresponded with increased abscess severity, MRSA viability, and CFU density in skin. Interestingly, combined inhibition of IL-17A and IL-22 did not worsen abscesses but did increase gamma interferon (IFN-γ) expression at these sites. The inhibition of IL-22 led to a reduction in IL-17A expression, but not vice versa. These results suggest that the expression of IL-17A is at least partially dependent on IL-22 in this model. Inhibition of IL-17A but not IL-22 led to hematogenous dissemination to kidneys, which correlated with decreased T cell infiltration in renal tissue. Collectively, these findings indicate that IL-17A and IL-22 have complementary but nonredundant roles in host defense against cutaneous versus hematogenous infection. These insights may support targeted immune enhancement or other novel approaches to address the challenge of MRSA infection.

Clinical MRSA isolates from skin and soft tissue infections show increased in vitro production of phenol soluble modulins

BACKGROUND

Phenol-soluble modulins (PSMs) are amphipathic, pro-inflammatory proteins secreted by most Staphylococcus aureus isolates. This study tested the hypothesis that in vitro PSM production levels are associated with specific clinical phenotypes.

METHODS

177 methicillin-resistant S. aureus (MRSA) isolates from infective endocarditis (IE), skin and soft tissue infection (SSTI), and hospital-acquired/ventilator-associated pneumonia (HAP) were matched by geographic origin, then genotyped using spa-typing. In vitro PSM production was measured by high performance liquid chromatography/mass spectrometry. Statistical analysis was performed using Chi-squared or Kruskal-Wallis tests as appropriate.

RESULTS

Spa type 1 was significantly more common in SSTI isolates (62.7% SSTI; 1.7% IE; 16.9% HAP; p < 0.0001) while HAP and IE isolates were more commonly spa type 2 (0% SSTI; 37.3% IE; 40.7% HAP; p < 0.0001). USA300 isolates produced the highest levels of PSMs in vitro. SSTI isolates produced significantly higher quantities of PSMα1-4, PSMβ1, and δ-toxin than other isolates (p < 0.001). These findings persisted when USA300 isolates were excluded from analysis.

CONCLUSIONS

Increased in vitro production of PSMs is associated with an SSTI clinical source. This significant association persisted after exclusion of USA300 genotype isolates from analysis, suggesting that PSMs play a particularly important role in the pathogenesis of SSTI as compared to other infection types.

Innovative qPCR using interfacial effects to enable low threshold cycle detection and inhibition relief

Molecular diagnostics offers quick access to information but fails to operate at a speed required for clinical decision-making. Our novel methodology, droplet-on-thermocouple silhouette real-time polymerase chain reaction (DOTS qPCR), uses interfacial effects for droplet actuation, inhibition relief, and amplification sensing. DOTS qPCR has sample-to-answer times as short as 3 min 30 s. In infective endocarditis diagnosis, DOTS qPCR demonstrates reproducibility, differentiation of antibiotic susceptibility, subpicogram limit of detection, and thermocycling speeds of up to 28 s/cycle in the presence of tissue contaminants. Langmuir and Gibbs adsorption isotherms are used to describe the decreasing interfacial tension upon amplification. Moreover, a log-linear relationship with low threshold cycles is presented for real-time quantification by imaging the droplet-on-thermocouple silhouette with a smartphone. DOTS qPCR resolves several limitations of commercially available real-time PCR systems, which rely on fluorescence detection, have substantially higher threshold cycles, and require expensive optical components and extensive sample preparation. Due to the advantages of low threshold cycle detection, we anticipate extending this technology to biological research applications such as single cell, single nucleus, and single DNA molecule analyses. Our work is the first demonstrated use of interfacial effects for sensing reaction progress, and it will enable point-of-care molecular diagnosis of infections.

Molecular epidemiology of heteroresistant vancomycin-intermediate Staphylococcus aureus in Brazil

To determine the epidemiological and molecular characteristics of 12 Staphylococcus aureus isolates presenting heteroresistance to vancomycin in laboratories of two cities in Santa Catarina, southern Brazil. Epidemiological data, including the city of isolation, health institution, and date of isolation were considered, as well as the associated clinical specimen. For molecular characterization, we analyzed the staphylococcal cassette chromosome types, the erm gene presence, and the genomic diversity of isolates using pulsed-field gel electrophoresis. The 12 isolates of S. aureus were previously confirmed as heteroresistance to vancomycin using the population analysis profile–area under curve. Regarding genetic variability, two clones were detected: the main one (clone A) composed of four isolates and the clones B, with two isolates. For clone A, two isolates presented identical band patterns and were related to the same hospital, with an interval of 57 days between their isolation. The other isolates of this clone showed no epidemiological link between them because they were isolated in different hospitals and had no temporal relationship. The other clone showed no detectable epidemiological relationship. The heteroresistance to vancomycin recovered in Santa Catarina State from 2009 to 2012 had, in general, heterogeneous genomic patterns based on pulsed-field gel electrophoresis results, which is in accordance with the fact that these isolates had little or no epidemiological relationship among them. Due to the characteristic phenotypic instability and often prolonged vancomycin therapy for selection, clonal spread is not as common as for other resistance mechanisms disseminated through horizontal gene transfer.

Prevalence of vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous VISA among methicillin-resistant S. aureus with high vancomycin minimal inhibitory concentrations in Taiwan: A multicenter surveillance study, 2012-2013

BACKGROUND/PURPOSE

Intermediate-resistance and heteroresistance to vancomycin in methicillin-resistant Staphylococcus aureus (MRSA) is reported worldwide. A surveillance study in 2003 showed that the prevalence rates of vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA) in Taiwan were 0.2% and 0.7%, respectively. This study aimed to investigate the updated prevalence of VISA and hVISA in Taiwan.

METHODS

MRSA isolates from sterile sites with minimal inhibitory concentrations (MICs) of 1 μg/mL or more to vancomycin were collected from 15 participating hospitals in Taiwan. Enrolled MRSA isolates were submitted to antimicrobial susceptibility testing, staphylococcal cassette chromosome mec (SCCmec) element typing, and multilocus sequence typing. Isolates with vancomycin MIC of 1 μg/mL or 2 μg/mL were screened for vancomycin heterogeneous resistance by Etest glycopeptide-resistance detection (GRD). Those with positive GRD screening results were then analyzed by modified population analysis profiling-area under the curve method for confirmation of vancomycin heteroresistance.

RESULTS

Between 2012 and 2013, a total of 622 MRSA isolates from sterile sites with vancomycin MIC of 1 μg/mL or more were studied. The prevalence rates of hVISA and VISA among these isolates were 10.0% and 2.7%, respectively. The hVISA prevalence increased significantly compared to that in 2003. Compared with vancomycin-susceptible S. aureus, hVISA and VISA isolates were less susceptible to ciprofloxacin, clindamycin, daptomycin, gentamicin, rifampin, and trimethoprim/sulfamethoxazole, and are thus, more likely to have SCCmec II or III element. A twofold increase in either vancomycin or teicoplanin MIC doubled the probability of being hVISA.

CONCLUSION

Growing hVISA prevalence was highly suspected. Longitudinal surveillance of this phenomenon and monitoring of its clinical impact are necessary.

Daptomycin for the Treatment of Infective Endocarditis: Results from European Cubicin(®) Outcomes Registry and Experience (EU-CORE)

INTRODUCTION

The European Cubicin(®) Outcomes Registry and Experience (EU-CORE(SM)) was a retrospective, non-interventional, multicenter study which evaluated the safety and effectiveness of daptomycin therapy in patients with Gram-positive infections including infective endocarditis (IE).

METHODS

Data from the EU-CORE registry were collected for patients with IE who had received at least one dose of daptomycin between January 2006 and April 2012, across 18 countries in Europe (12), Latin America (5) and Asia (1). Clinical outcomes were assessed as success (cured or improved), failure or non-evaluable. Adverse events (AEs) were recorded during treatment and for up to 30 days post-treatment; follow-up data were collected for 2 years.

RESULTS

Of 6075 patients included in the EU-CORE registry, 610 were diagnosed with IE as primary infection; 149 (24.4%) right-sided IE (RIE), 414 (67.9%) left-sided IE (LIE), and 47 (7.7%) with both right- and left-sided IE (BRLIE). Overall clinical success was achieved in 80.0% of patients (RIE 88.6%, LIE 76.6% and BRLIE 82.9%). Success rates for methicillin-resistant Staphylococcus aureus (MRSA) infections were 90.9%, 71.7% and 66.6% in patients with RIE, LIE and BRLIE, respectively. The overall sustained clinical success rate in patients followed for up to 2 years was 86.7% (RIE 93.5%, LIE 88.3% and BRLIE 77.8%). AEs deemed possibly related to daptomycin in the investigator's opinion were reported in 2 (1.3%) RIE, 18 (4.3%) LIE and 1 (2.1%) BRLIE patients. There were 11 (1.8%) patients (2 with RIE, 8 with LIE and 1 with BRLIE) with AEs of creatine phosphokinase elevation reported as possibly related to daptomycin.

CONCLUSION

Data from this real-world clinical setting showed that daptomycin was well tolerated and effective for the treatment of LIE and BRLIE in addition to RIE caused by Gram-positive bacteria, including MRSA. Two-year follow-up data showed that a high proportion of patients had a sustained response.

Antimicrobial heteroresistance: an emerging field in need of clarity

"Heteroresistance" describes a phenomenon where subpopulations of seemingly isogenic bacteria exhibit a range of susceptibilities to a particular antibiotic. Unfortunately, a lack of standard methods to determine heteroresistance has led to inappropriate use of this term. Heteroresistance has been recognized since at least 1947 and occurs in Gram-positive and Gram-negative bacteria. Its clinical relevance may be considerable, since more resistant subpopulations may be selected during antimicrobial therapy. However, the use of nonstandard methods to define heteroresistance, which are costly and involve considerable labor and resources, precludes evaluating the clinical magnitude and severity of this phenomenon. We review the available literature on antibiotic heteroresistance and propose recommendations for definitions and determination criteria for heteroresistant bacteria. This will help in assessing the global clinical impact of heteroresistance and developing uniform guidelines for improved therapeutic outcomes.

Treatment of methicillin-resistant Staphylococcus aureus infections: Importance of high vancomycin minumum inhibitory concentrations

Staphylococcus aureus (SA) infections remain a major cause of morbidity and mortality despite the availability of numerous effective anti-staphylococcal antibiotics. This organism is responsible for both nosocomial and community-acquired infections ranging from relatively minor skin and soft tissue infections to life-threatening systemic infections. The increasing incidence of methicillin-resistant strains has granted an increasing use of vancomycin causing a covert progressive increase of its minimum inhibitory concentration (MIC) (dubbed the MIC “creep”). In this way, the emergence of vancomycin-intermediate SA (VISA) strains and heteroresistant-VISA has raised concern for the scarcity of alternative treatment options. Equally alarming, though fortunately less frequent, is the emergence of vancomycin-resistant SA. These strains show different mechanisms of resistance but have similar problems in terms of therapeutic approach. Ultimately, various debate issues have arisen regarding the emergence of SA strains with a minimum inhibitory concentration sitting on the superior limit of the sensitivity range (i.e., MIC = 2 μg/mL). These strains have shown certain resilience to vancomycin and a different clinical behaviour regardless of vancomycin use, both in methicillin-resistant SA and in methicillin-sensitive SA. The aim of this text is to revise the clinical impact and consequences of the emergence of reduced vancomycin susceptibility SA strains, and the different optimal treatment options known.

The Prevalence of Vancomycin-Intermediate Staphylococcus aureus and Heterogeneous VISA Among Methicillin-Resistant Strains Isolated from Pediatric Population in a Turkish University Hospital

There are limited data regarding the prevalence of vancomycin-intermediate Staphylococcus aureus (VISA)/heterogeneous VISA (hVISA) among pediatric population. Our objective was to determine the distribution of vancomycin and daptomycin minimum inhibitory concentrations (MICs) and explore the phenomenon of vancomycin MIC creep and the VISA/hVISA prevalence among the methicillin-resistant Staphylococcus aureus (MRSA) strains belonging to pediatric population by population analysis profile-area under the curve (PAP-AUC) and Etest macromethod. Vancomycin and daptomycin susceptibilities of 94 pediatric isolates of MRSA were tested by broth microdilution (BMD) and Etest methods. To determine the prevalence of VISA/hVISA, Etest macromethod and PAP-AUC was performed on all isolates. All isolates were susceptible to vancomycin and daptomycin by both BMD and Etest methods. Twenty-eight (29.8%) isolates had vancomycin MICs of 2 μg/ml by BMD. No increase in vancomycin MICs was observed over time. There were no VISA among 94 MRSA tested but 20 (21.3%) hVISA isolates were identified by PAP-AUC. Results of Etest macromethod were compared to PAP-AUC. Etest macromethod was 60.0% sensitive and 90.5% specific. The hVISA isolates represented 53.6% of isolates with vancomycin MICs of 2 μg/ml. Also, 75% of hVISA isolates had vancomycin MICs of 2 μg/ml. To our knowledge, this is the first study investigating the prevalence of VISA/hVISA among MRSA isolated from pediatric patients by PAP-AUC method. Based on our findings, MRSA isolates, which have vancomycin MIC of 2 μg/ml can be investigated for the presence of hVISA. In this study, daptomycin showed potent activity against all isolates and may represent a therapeutic option for MRSA infections.

Vancomycin tolerant, methicillin-resistant Staphylococcus aureus reveals the effects of vancomycin on cell wall thickening

Methicillin-resistant Staphylococcus aureus (MRSA) is an important opportunistic pathogen that causes both healthcare- and community-acquired infections. An increase in the incidence of these infections may lead to a substantial change in the rate of vancomycin usage. Incidence of reduced susceptibility to vancomycin has been increasing worldwide for the last few years, conferring different levels of resistance to vancomycin as well as producing changes in the cell wall structure. The aim of the present study was to determine the effect of vancomycin on cell wall thickening in clinical isolates of vancomycin-tolerant (VT) MRSA obtained from pediatric patients. From a collection of 100 MRSA clinical isolates from pediatric patients, 12% (12/100) were characterized as VT-MRSA, and from them, 41.66% (5/12) exhibited the heterogeneous vancomycin-intermediate S. aureus (hVISA) phenotype. Multiplex-PCR assays revealed 66.66% (8/12), 25% (3/12), and 8.33% (1/12) of the VT-MRSA isolates were associated with agr group II, I, and III polymorphisms, respectively; the II-mec gene was amplified from 83.3% (10/12) of the isolates, and the mecIVa gene was amplified from 16.66% (2/12) of the isolates. Pulsed field electrophoresis (PFGE) fingerprint analysis showed 62% similarity among the VT-MRSA isolates. Thin transverse sections analyzed by transmission electron microscopy (TEM) revealed an average increase of 24 nm (105.55%) in the cell wall thickness of VT-MRSA compared with untreated VT-MRSA isolates. In summary, these data revealed that the thickened cell walls of VT-MRSA clinical isolates with agr type II and SCCmec group II polymorphisms are associated with an adaptive resistance to vancomycin.

Mechanisms of NDV-3 vaccine efficacy in MRSA skin versus invasive infection

Increasing rates of life-threatening infections and decreasing susceptibility to antibiotics urge development of an effective vaccine targeting Staphylococcus aureus. This study evaluated the efficacy and immunologic mechanisms of a vaccine containing a recombinant glycoprotein antigen (NDV-3) in mouse skin and skin structure infection (SSSI) due to methicillin-resistant S. aureus (MRSA). Compared with adjuvant alone, NDV-3 reduced abscess progression, severity, and MRSA density in skin, as well as hematogenous dissemination to kidney. NDV-3 induced increases in CD3+ T-cell and neutrophil infiltration and IL-17A, IL-22, and host defense peptide expression in local settings of SSSI abscesses. Vaccine induction of IL-22 was necessary for protective mitigation of cutaneous infection. By comparison, protection against hematogenous dissemination required the induction of IL-17A and IL-22 by NDV-3. These findings demonstrate that NDV-3 protective efficacy against MRSA in SSSI involves a robust and complementary response integrating innate and adaptive immune mechanisms. These results support further evaluation of the NDV-3 vaccine to address disease due to S. aureus in humans.