Vancomycin-intermediate Staphylococcus aureus in Korea

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.

Model-informed precision dosing in vancomycin treatment

Introduction: While vancomycin remains a widely prescribed antibiotic, it can cause ototoxicity and nephrotoxicity, both of which are concentration-associated. Overtreatment can occur when the treatment lasts for an unnecessarily long time. Using a model-informed precision dosing scheme, this study aims to develop a population pharmacokinetic (PK) and pharmacodynamic (PD) model for vancomycin to determine the optimal dosage regimen and treatment duration in order to avoid drug-induced toxicity. Methods: The data were obtained from electronic medical records of 542 patients, including 40 children, and were analyzed using NONMEM software. For PK, vancomycin concentrations were described with a two-compartment model incorporating allometry scaling. Results and discussion: This revealed that systemic clearance decreased with creatinine and blood urea nitrogen levels, history of diabetes and renal diseases, and further decreased in women. On the other hand, the central volume of distribution increased with age. For PD, C-reactive protein (CRP) plasma concentrations were described by transit compartments and were found to decrease with the presence of pneumonia. Simulations demonstrated that, given the model informed optimal doses, peak and trough concentrations as well as the area under the concentration-time curve remained within the therapeutic range, even at doses smaller than routine doses, for most patients. Additionally, CRP levels decreased more rapidly with the higher dose starting from 10 days after treatment initiation. The developed R Shiny application efficiently visualized the time courses of vancomycin and CRP concentrations, indicating its applicability in designing optimal treatment schemes simply based on visual inspection.

Decreased susceptibility to vancomycin and other mechanisms of resistance to antibiotics in Staphylococcus epidermidis as a therapeutic problem in hospital treatment

Multidrug-resistant coagulase-negative staphylococci represent a real therapeutic challenge. The aim of the study was to emphasize the importance of heteroresistance to vancomycin presence in methicillin-resistant strains of S. epidermidis. The research comprised 65 strains of S. epidermidis. Heteroresistance to vancomycin was detected with the use of the agar screening method with Brain Heart Infusion and a population profile analysis (PAP test). In addition, types of cassettes and genes responsible for resistance to antibiotics for 22 multidrug resistant strains were determined. Our investigations showed that 56 of 65 S. epidermidis strains were phenotypically resistant to methicillin. The tested strains were mostly resistant to erythromycin, gentamicin, clindamycin, and ciprofloxacin. Six strains showed decreased susceptibility to vancomycin and their heterogeneous resistance profiles were confirmed with the PAP test. All tested multi-resistant strains exhibited the mecA gene. More than half of them possessed type IV cassettes. ant(4')-Ia and aac(6')/aph(2''), ermC and tetK genes were most commonly found. The described phenomenon of heteroresistance to vancomycin in multidrug resistant bacteria of the Staphylococcus genus effectively inhibits a therapeutic effect of treatment with this antibiotic. That is why it is so important to search for markers that will enable to identify heteroresistance to vancomycin strains under laboratory conditions.

Vancomycin heteroresistance among methicillin-resistant clinical isolates S. haemolyticus, S. hominis, S. simulans, and S. warneri

Besides being an essential part of the skin microbiome, coagulase-negative staphylococci are the etiological factors of serious infections. The aim of the study was to evaluate the heteroresistance to vancomycin and the potential antimicrobial efficacy of teicoplanin and daptomycin against the multiresistant strains of S. haemolyticus, S. hominis, S. warneri, and S. simulans. The study covered 80 clinical coagulase-negative staphylococci. Teicoplanin, vancomycin, and daptomycin MICs for the tested strains were determined according to EUCAST recommendation. The vanA and vanB genes were searched. The brain heart infusion screen agar method detected vancomycin heteroresistance. The population analysis profile method and analysis of autolytic activity were applied for the strains growing on BHI containing 4 mg/L vancomycin. Seven S. haemolyticus, two S. hominis, and two S. warneri strains presented a heterogeneous resistance to vancomycin. Their subpopulations were able to grow on a medium containing 4-12 mg/L of vancomycin. Monitoring heteroresistance to peptide antibiotics, which are often the last resort in staphylococcal infections, is essential due to the severe crisis in antibiotic therapy and the lack of alternatives to treat infections with multiresistant strains. Our work highlights the selection of resistant strains and the need for more careful use of peptide antibiotics.

A Case of Vancomycin-Induced Drug Reaction with Eosinophilia, Systemic Symptoms and Multiorgan Involvement Proven Using Lymphocyte Transformation Test

Drug-induced hypersensitivity syndrome (DiHS), also referred to as drug reaction with eosinophilia and systemic symptoms (DRESS), is a rare but potentially life-threatening condition induced by drug hypersensitivity that leads to significant morbidity and mortality and often occurs in patients undergoing combination antibiotic therapy. Due to a recent increase in the incidence of methicillin-resistant Staphylococcus aureus infections, the occurrence of vancomycin-induced DiHS/DRESS has increased rapidly. However, because of insufficient pharmacogenetic data on vancomycin-induced drug eruptions in Asians coupled with the risk of re-eliciting the symptoms by provocation tests, confirmation of the culprit drug in vancomycin-induced DiHS/DRESS is often challenging. Here, we report a case of vancomycin-induced DiHS/DRESS, where the causal relationship was confirmed using a lymphocyte transformation test (LTT). A 51-year-old woman was treated with combination antibiotics, including vancomycin, for infective pericarditis. The patient subsequently developed fever, facial edema, generalized rash followed by multiple internal organ involvement, including the kidney, lung, liver, and heart. Thus, based on the International Registry of Severe Cutaneous Adverse Reaction (RegiSCAR) criteria, the case was diagnosed as 'definite' DiHS/DRESS, although the culprit drug was obscured by combination antibiotic therapy. The LTT confirmed that vancomycin, but not other glycopeptide antibiotics, specifically induced T-cell proliferation in this case. Collectively, our case suggests that clinicians can utilize LTT to identify the causative medication of DiHS/DRESS when the clinical information is limited to defining the culprit drug.

Molecular detection of vancomycin and methicillin resistance in Staphylococcus aureus isolated from food processing environments

Staphylococcus aureus is a well-known foodborne pathogen. The aim of this study was to investigate the presence of S. aureus isolated from serving utensils in food processing environments in Mymensingh city, Bangladesh and to determine their antibiogram and resistance determinants. A total of 120 environmental samples were collected from different food settings. Isolation and identification were conducted using conventional biochemical tests. Molecular identification of isolates and detection of methicillin and vancomycin resistance were done using primer-specific polymerase chain reaction (PCR) targeting Tuf, nuc, mecA, and mecC genes. Antibiotic sensitivity tests were performed, and resistance genes were also detected by amplifying bla TEM , vanA, vanB, and vanC genes. Among the 120 samples, 81 (67.5%) were positive for Staphylococcus spp. and 41 (50.62%) were positive for the nuc-gene. Among the 41 isolates, 5 (12.20%) were positive for mecA, but none were positive for the mecC gene. A total of 12.2% of the isolates were vanC-positive, of which 4 isolates (9.76%) were also positive for the mecA gene. Antibiotic sensitivity testing revealed that all S. aureus isolates (100%) from hotel samples were sensitive to ciprofloxacin and chloramphenicol, 90.32% were sensitive to doxycycline, and 80.65% were sensitive to streptomycin. Conversely, all isolates (100%) were resistant to ampicillin, and 29.03% were resistant to vancomycin. All S. aureus isolates obtained from non-hotel samples were susceptible to chloramphenicol, ceftriaxone, ciprofloxacin, doxycycline, meropenem, and vancomycin; however, 40% of isolates were resistant to novobiocin. Among the hotel isolates, 29 (93.55%) of the ampicillin-resistant isolates harbored the blaTEM gene while 5 (55.55%) of the vancomycin-resistant isolates harbored the vanC gene. Four of the five vanC positive isolates were also positive for the mecA gene. The presence of methicillin-resistant S. aureus (MRSA) which is also vancomycin-resistant in food processing environments is a threat to public health. This is the first report on the molecular detection of methicillin and vancomycin-resistant S. aureus isolated from food processing environments in Bangladesh.

Reduced Susceptibility and Resistance to Vancomycin of Staphylococcus aureus: A Review of Global Incidence Patterns and Related Genetic Mechanisms

Staphylococcus aureus is a Gram-positive bacterium causing a wide range of infections ranging from cutaneous infections to endocarditis and bacteremia. Beta-lactamases such as penicillin and, subsequently, methicillin have been used in the treatment of S. aureus infections. With the emergence of methicillin-resistant Staphylococcus aureus (MRSA), vancomycin, a bacterial cell wall synthesis inhibitor, has been used as the treatment of choice for MRSA infections.

However, over the past few decades, there have been reports of reduced susceptibility and resistance of S. aureus to vancomycin globally, most recently from Michigan, United States, in July 2021. Based on the minimum inhibitory concentration (MIC) of the antibiotic against S. aureus, there are three strains of resistance, vancomycin-intermediate Staphylococcus aureus (VISA), vancomycin-resistant Staphylococcus aureus (VRSA), and heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA).

The increasing prevalence of VISA and VRSA infections is a cause of global concern. This qualitative review of peer-reviewed research publications aims to describe the cases of VISA and VRSA reported in the literature globally and summarizes the genetic mechanisms implicated in their resistance. The most common mechanism implicated in VRSA infections is the vanA operon, while cell wall thickening is responsible for VISA infections. This review aims to perform a global comparison between the MIC corresponding to the strength of resistance to vancomycin and the presence of the vanA operon. In this review, VISA and VRSA are noted to be most susceptible to quinupristin-dalfopristin and linezolid, respectively. Maintaining active systemic surveillance for such infections, employing strict infection control measures, and continuing to mitigate indiscriminate and irrational use of antibiotics are some of the actions that can be undertaken to reduce the incidence and transmission of VISA, VRSA, and hVISA infections worldwide.

Global prevalence and distribution of vancomycin resistant, vancomycin intermediate and heterogeneously vancomycin intermediate Staphylococcus aureus clinical isolates: a systematic review and meta-analysis

Vancomycin-resistant Staphylococcus aureus (VRSA), Vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA) are subject to vancomycin treatment failure. The aim of the present study was to determine their precise prevalence and investigate prevalence variability depending on different years and locations. Several international databases including Medline (PubMed), Embase and Web of Sciences were searched (data from 1997 to 2019) to identify studies that addressed the prevalence of VRSA, VISA and hVISA among human clinical isolates around the world. Subgroup analyses and meta-regression were conducted to indicate potential source of variation. Publication bias was assessed using Egger's test. Statistical analyses were conducted using STATA software (version 14.0). Data analysis showed that VRSA, VISA and hVISA isolates were reported in 23, 50 and 82 studies, with an overall prevalence of 1.5% among 5855 S. aureus isolates, 1.7% among 22,277 strains and 4.6% among 47,721 strains, respectively. The overall prevalence of VRSA, VISA, and hVISA before 2010 was 1.2%, 1.2%, and 4%, respectively, while their prevalence after this year has reached 2.4%, 4.3%, and 5.3%. The results of this study showed that the frequency of VRSA, VISA and hVISA after 2010 represent a 2.0, 3.6 and 1.3-fold increase over prior years. In a subgroup analysis of different strain origins, the highest frequency of VRSA (3.6%) and hVISA (5.2%) was encountered in the USA while VISA (2.1%) was more prevalent in Asia. Meta-regression analysis showed significant increasing of VISA prevalence in recent years (p value ≤ 0.05). Based on the results of case reports (which were not included in the calculations mentioned above), the numbers of VRSA, VISA and hVISA isolates were 12, 24 and 14, respectively, among different continents. Since the prevalence of VRSA, VISA and hVISA has been increasing in recent years (especially in the Asian and American continents), rigorous monitoring of vancomycin treatment, it's the therapeutic response and the definition of appropriate control guidelines depending on geographical regions is highly recommended and essential to prevent the further spread of vancomycin-resistant S. aureus.

Synergistic effect of linezolid with fosfomycin against Staphylococcus aureus in vitro and in an experimental Galleria mellonella model

BACKGROUND/PURPOSES

Treatment of Staphylococcus aureus infections is challenging owing to widespread multidrug resistance. There is now considerable interest in the potential of combination therapies. Although linezolid/fosfomycin combination appears to be a promising treatment option based on in vitro data, further preclinical work is needed. In this study, the Galleria mellonella system was employed to study the in vivo efficacy of this combination in order to determine whether it should be explored further for the treatment of S. aureus infections.

METHODS

The antimicrobial activity of linezolid and fosfomycin alone and in combination was assessed versus four S. aureus. Synergy studies were performed using the microtitre plate chequerboard assay and time-kill methodology. The in vivo activity of linezolid/fosfomycin combination was assessed using a G. mellonella larvae model.

RESULTS

The combination of linezolid and fosfomycin was synergistic and bacteriostatic against four tested strains. Treatment of G. mellonella larvae infected with lethal doses of S. aureus resulted in significantly enhanced survival rates when low-dose of combination has no significant differences with high-dose combination (P > 0.05), G. mellonella hemolymph burden of S. aureus suggest that combination therapy with rapid and sustained bacteriostatic activity compared monotherapy.

CONCLUSION

This work indicated that linezolid combination with fosfomycin has synergistic effect against S. aureus in vitro and in an experimental G. mellonella model, and it suggests that high-dose of linezolid and fosfomycin may not necessary.

Vancomycin heteroresistance in coagulase negative Staphylococcus blood stream infections from patients of intensive care units in Mansoura University Hospitals, Egypt

BACKGROUND

Vancomycin heteroresistance in coagulase negative Staphylococci (CoNS) is a recent health concern especially in serious infections like bloodstream infections as it may lead to failure of therapy. Little information is available about the prevalence vancomycin heteroresistance in CoNS causing bloodstream infections in intensive care units (ICUs) patients of Mansoura University Hospitals (MUHs).

METHODS

This prospective study enrolled 743 blood samples collected from ICUs patients presented with clinical manifestations of bloodstream infections over the period extending from January 2014 to March 2016. Samples were processed, coagulase negative Staphylococci were identified by routine microbiological methods and the absence of coagulase activity. Species were identified by API Staph 32. Oxacillin resistant CoNS were identified by cefoxitin disc diffusion method. Susceptibility testing of isolated CoNS to vancomycin was carried out using vancomycin agar dilution method. Mec A gene detection by PCR was done for oxacillin resistant isolates. Screening for vancomycin heteroresistance was done on brain heart infusion (BHI) agar containing 4 μg/mL vancomycin. Confirmation of vancomycin heteroresistance was carried out by population analysis profile (PAP).

RESULTS

A total of 58 isolates were identified as CoNS from patients of clinically suspected bloodstream infections. The identified species were 33 (56.9%) Staphylococcus epidermidis, 12 (20.7%) Staphylococcus capitis, 7 (12.1%) Staphylococcus haemolyticus, and 3 isolates (5.2%) Staphylococcus lugdunesis. Three isolates were unidentified by API Staph 32. Forty-four (75.9%) isolates were oxacillin resistant. Mec A gene was detected in all oxacillin resistant isolates. All isolates had susceptible vancomycin MICs by agar dilution. Nine isolates (15.5%) could grow on BHI agar containing 4 μg/mL vancomycin. These isolates showed heterogeneous profile of resistance to vancomycin by population analysis profile.

CONCLUSIONS

Vancomycin heteroresistant CoNS causing bloodstream infections is growing unrecognized health hazard in ICUs patients. These isolates have susceptible vancomycin MICs. Screening methods are recommended and should be considered to improve clinical outcome in these high risk patients.

Molecular characterization of vancomycin-resistant Staphylococcus aureus strains isolated from clinical samples: A three year study in Tehran, Iran

Introduction

Emergence of vancomycin-intermediate Staphylococcus aureus (VISA) and vancomycin-resistant S. aureus (VRSA) strains has led to great concern in global public health in both developing and developed countries. This study investigated distribution and molecular characterization of VRSA strains in Tehran’s hospitals using a combination of molecular typing methods.

Materials and methods

A total of 1789 S. aureus isolates obtained between 2014 and 2017 and were characterized using antibiogram, SCCmec typing, spa typing, and multilocus-sequence typing. Resistance to vancomycin was determined by E-test method. After confirmation of the isolated VRSA strain, genetic analysis was performed by evaluating vanA and vanB genes presence.The presence of resistance (ermA, ermB, ermC, mupA, msrA, msrB, tetM, ant (4΄)-Ia, aac (6΄)-Ie/aph (2˝), aph (3΄)-IIIa) and toxin (etb, eta, pvl, tst) encoding genes was investigated by the polymerase chain reaction (PCR) technique.

Results

Of all S. aureus tested isolates, four isolates were confirmed as VRSA isolates and two isolates confirmed as VISA isolates. ST5- SCCmec II/t002 and ST239-SCCmec III/t037 strains had MIC values of 512μg/ml, ST239-SCCmec III/t037 and ST8-SCCmecIV/t008 strains had MIC values of 64μg/ml and ST22-SCCmec IV/t790 and ST239-SCCmec III/t030 strains had MIC values ≥ 8 μg/ml. pvl-encoding gene was confirmed in ST8-SCCmecIV/t008 and ST22-SCCmec IV/t790 strains. The isolates differed in the carriage of resistance and toxin encoding genes.

Conclusions

The study revealed the existence of VRSA strains in capital of Iran, Tehran. To our knowledge, this is the first report of ST239-SCCmec III/t037 as VRSA strain. These findings support the need for future surveillance studies on VRSA strains to keep the emergence and transmission of these isolates to a minimum.

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.

Systematic Review and Meta-Analysis of the Epidemiology of Vancomycin-Intermediate and Heterogeneous Vancomycin-Intermediate Staphylococcus aureus Isolates

BACKGROUND

Vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA) are associated with vancomycin treatment failure, and are becoming an increasing public health problem. Therefore, we undertook this study of 91 published studies and made subgroup comparisons of hVISA/VISA incidence in different study years, locations, and types of clinical samples. We also analyzed the genetic backgrounds of these strains.

METHODS

A systematic literature review of relevant articles published in PubMed and EMBASE from January 1997 to August 2014 was conducted. We selected and assessed journal articles reporting the prevalence rates of hVISA/VISA.

RESULTS

The pooled prevalence of hVISA was 6.05% in 99,042 methicillin-resistant S. aureus (MRSA) strains and that of VISA was 3.01% in 68,792 MRSA strains. The prevalence of hVISA was 4.68% before 2006, 5.38% in 2006-2009, and 7.01% in 2010-2014. VISA prevalence was 2.05%, 2.63%, and 7.93%, respectively. In a subgroup analysis of different isolation locations, the prevalence of hVISA strains was 6.81% in Asia and 5.60% in Europe/America, and that of VISA was 3.42% and 2.75%, respectively. The frequencies of hVISA isolated from blood culture samples and from all clinical samples were 9.81% and 4.68%, respectively, and those of VISA were 2.00% and 3.07%, respectively. The most prevalent genotype was staphylococcal cassette chromosome mec (SCCmec) II, which accounted for 48.16% and 37.74% of hVISA and VISA, respectively. Sequence Type (ST) 239 was most prevalent.

CONCLUSION

The prevalence of hVISA/VISA has been increasing in recent years, but has been grossly underestimated. Its incidence is higher in Asia than in Europe/America. hVISA is isolated from blood culture samples more often than from other samples. These strains are highly prevalent in epidemic MRSA strains. This study clarifies the epidemiology of hVISA/VISA and indicates that the detection of these strains and the control of nosocomial infections must be strengthened.

Bacteriological profile and antimicrobial susceptibility patterns of blood culture isolates among febrile patients in Mekelle Hospital, Northern Ethiopia

Bacterial bloodstream infections are a major public health problem, which leads to high morbidity and mortality of patients. On time diagnosis and appropriate medication will be the best way to save the lives of affected ones. The aim of the present study was to determine the bacterial profile of bloodstream infections and their antibiotic susceptibility pattern in Mekelle Hospital. Cross sectional study method was carried out in 514 (269 females and 245 males) febrile patients in Mekelle hospital from March to October 2014. Standard bacteriological methods were used for blood collection, bacterial isolation and antimicrobial susceptibility pattern. Out of the total 514 febrile patients, 144 (28%) culture positive were isolated. Staphylococcus aureus 54 (37.5%), Coagulase-negative staphylococci 44 (30.6%), Escherichia coli 16 (3.1%), Citrobacter spp. 9 (1.7%) and Salmonella typhi 8 (1.6%) were the most dominant isolates, collectively accounting for >90% of the isolates. Antimicrobial resistance pattern for gram positive and gram negative bacteria was 0-83.3% and 0-100%, respectively. High resistance was seen to Trimethoprim-sulphamethoxazole 101 (70.1%), Oxacillin 65 (62.5%), Ceftriaxone 79 (58.9%) and Doxycycline 71 (49.3%). Fifty-nine percent of the isolated bacteria in this study were multi drug resistant. Most bacterial isolates were sensitive to Gentamicin, Ciprofloxacin and Amoxicillin clavulanic acid. All gram positive isolates in this current study were sensitive to vancomycin. Prevalence of bacterial isolates in blood was high. It also reveals isolated bacteria species developed multi drug resistance to most of the antibiotics tested, which highlights for periodic surveillance of etiologic agent, antibiotic susceptibility to prevent further emergence and spread of resistant bacterial pathogens.

New epidemiology of Staphylococcus aureus infection in Asia

Not only is Asia the most populous region in the world, but inappropriate therapy, including self-medication with over-the-counter antimicrobial agents, is a common response to infectious diseases. The high antibiotic selective pressure among the overcrowded inhabitants creates an environment that is suitable for the rapid development and efficient spread of numerous multidrug-resistant pathogens. Indeed, Asia is among the regions with the highest prevalence rates of healthcare-associated methicillin-resistant Staphylococcus aureus (HA-MRSA) and community-associated methicillin-resistant S. aureus (CA-MRSA) in the world. Most hospitals in Asia are endemic for multidrug-resistant methicillin-resistant S. aureus (MRSA), with an estimated proportion from 28% (in Hong Kong and Indonesia) to >70% (in Korea) among all clinical S. aureus isolates in the early 2010s. Isolates with reduced susceptibility or a high level of resistance to glycopeptides have also been increasingly identified in the past few years. In contrast, the proportion of MRSA among community-associated S. aureus infections in Asian countries varies markedly, from <5% to >35%. Two pandemic HA-MRSA clones, namely multilocus sequence type (ST) 239 and ST5, are disseminated internationally in Asia, whereas the molecular epidemiology of CA-MRSA in Asia is characterized by clonal heterogeneity, similar to that in Europe. In this review, the epidemiology of S. aureus in both healthcare facilities and communities in Asia is addressed, with an emphasis on the prevalence, clonal structure and antibiotic resistant profiles of the MRSA strains. The novel MRSA strains from livestock animals have been considered to constitute a public health threat in western countries. The emerging livestock-associated MRSA strains in Asia are also included in this review.

The msaABCR operon regulates resistance in vancomycin-intermediate Staphylococcus aureus strains

Vancomycin-intermediate Staphylococcus aureus (VISA) strains present an increasingly difficult problem in terms of public health. However, the molecular mechanism for this resistance is not yet understood. In this study, we define the role of the msaABCR operon in vancomycin resistance in three clinical VISA strains, i.e., Mu50, HIP6297, and LIM2. Deletion of the msaABCR operon resulted in significant decreases in the vancomycin MIC (from 6.25 to 1.56 μg/ml) and significant reductions of cell wall thickness in strains Mu50 and HIP6297. Growth of the mutants in medium containing vancomycin at concentrations greater than 2 μg/ml resulted in decreases in the growth rate, compared with the wild-type strains. Mutation of the msaABCR operon also reduced the binding capacity for vancomycin. We conclude that the msaABCR operon contributes to resistance to vancomycin and cell wall synthesis in S. aureus.

Teicoplanin as an effective alternative to vancomycin for treatment of MRSA infection in Chinese population: a meta-analysis of randomized controlled trials

Objective

To evaluate whether teicoplanin could be an alternative to vancomycin for treatment of MRSA infection in Chinese population using a meta-analysis in randomized controlled trials.

Methods

The following databases were searched: Chinese Biomedical Literature database (CBM), Chinese Journal Full-text database (CNKI), Wanfang database, Medline database, Ovid database and Cochrane Library. Articles published from 2002 to 2013 that studied teicoplanin in comparison to vancomycin in the treatment of MRSA infected patients were collected. Overall effects, publishing bias analysis and sensitivity analysis on clinical cure rate, microbiologic eradication rate and adverse events rate were performed by using Review Manager 5.2 and Stata 11.0 softwares.

Results

Twelve articles met entry criteria. There was no statistically significant difference between the two groups regarding the clinical cure rate (risk ratio [RR], teicoplanin vs vancomycin, 0.94; 95% CI, 0.74∼1.19; P = 0.60), microbiological cure rate (risk ratio [RR], teicoplanin vs vancomycin, 0.99; 95% CI, 0.91∼1.07; P = 0.74) and adverse event rate (risk ratio [RR], teicoplanin vs vancomycin, 0.86; 95% CI, 0.40∼1.84; P = 0.70).

Conclusions

The meta-analysis results indicate that the two therapies are similar in both efficacy and safety, thus teicoplanin can act as an effective alternative to vancomycin for treating patients infected by MRSA.

Comprehensive identification of mutations responsible for heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA)-to-VISA conversion in laboratory-generated VISA strains derived from hVISA clinical strain Mu3

Heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) spontaneously produces VISA cells within its cell population at a frequency of 10(-6) or greater. We established a total of 45 VISA mutant strains independently obtained from hVISA Mu3 and its related strains by one-step vancomycin selection. We then performed high-throughput whole-genome sequencing of the 45 strains and their parent strains to identify the genes involved in the hVISA-to-VISA phenotypic conversion. A comparative genome study showed that all the VISA strains tested carried a unique set of mutations. All of the 45 VISA strains carried 1 to 4 mutations possibly affecting the expression of a total of 48 genes. Among them, 32 VISA strains carried only one gene affected by a single mutation. As many as 20 genes in more than eight functional categories were affected in the 32 VISA strains, which explained the extremely high rates of the hVISA-to-VISA phenotypic conversion. Five genes, rpoB, rpoC, walK, pbp4, and pp2c, were previously reported as being involved in vancomycin resistance. Fifteen remaining genes were newly identified as associated with vancomycin resistance in this study. The gene most frequently affected (6 out of 32 strains) was cmk, which encodes cytidylate kinase, followed closely by rpoB (5 out of 32), encoding the β subunit of RNA polymerase. A mutation prevalence study also revealed a sizable number of cmk mutants among clinical VISA strains (7 out of 38 [18%]). Reduced cytidylate kinase activity in cmk mutant strains is proposed to contribute to the hVISA-to-VISA phenotype conversion by thickening the cell wall and reducing the cell growth rate.

Genetic Characterization of Methicillin Resistant and Sensitive, Vancomycin Intermediate Staphylococcus aureus Strains Isolated from Different Iranian Hospitals

Background. Global concerns have been raised due to upward trend of Vancomycin Intermediate Staphylococcus aureus (VISA) and Vancomycin Resistant Staphylococcus aureus (VRSA) reports which mean casting doubt on the absolute effectiveness of the last line of antibiotic treatment for S. aureus, vancomycin. Hence, epidemiological evaluation can improve global health care policies. Methodology. 171 Isolates of Staphylococcus aureus were collected from different types of clinical samples in selected hospitals in Isfahan, Mashhad, and Tehran, Iran. Then, they were evaluated by agar screening, disk diffusion, and MIC method to determine their resistance to vancomycin and methicillin. The isolated VISA strains were then confirmed with genetic analysis by the evaluation of mecA and vanA genes, SCCmec, agr, and spa type, and also toxin profiles. MLST was also performed. Results and Conclusion. Our data indicated that 67% of isolated S. aureus strains were resistant to methicillin. Furthermore, five isolates (2.9%) had intermediate resistance to vancomycin (VISA). In contrast to usual association of VISA with MRSA strains, we found two isolates of MSSA-VISA. Therefore, our data suggests a probable parallel growing trend of VISA towards MSSA, along with MRSA strains. However, more samples are required to confirm these primarily data. Moreover, genetic analysis of the isolated VISA strains revealed that these strains are endemic Asian clones.

Genetic characterization of a vancomycin-resistant Staphylococcus aureus isolate from the respiratory tract of a patient in a university hospital in northeastern Iran

Emergence of vancomycin-intermediate Staphylococcus aureus (VISA) and vancomycin-resistant S. aureus (VRSA) strains has led to global concerns about treatments for staphylococcal infections. These strains are currently rare even though there is an upward trend in their reported incidence. Therefore, appropriate screening and epidemiological evaluation of VRSA strains can affect future global health care policies. Isolates of Staphylococcus aureus were obtained from various clinical samples and were then evaluated with agar screening, disk diffusion, and MIC methods to determine resistance to vancomycin and methicillin. After confirmation of the isolated VRSA strain, genetic analysis was performed by evaluating mecA and vanA gene presence, SCCmec, agr, and spa types, and toxin profiles. Multilocus sequence typing (MLST) and plasmid analysis were also performed. The VRSA strain was resistant to oxacillin (MIC of 128 μg/ml) and vancomycin (MIC of 512 μg/ml). Disk diffusion antimicrobial susceptibility tests showed resistance to oxacillin, vancomycin, levofloxacin, ciprofloxacin, trimethoprim-sulfamethoxazole, clindamycin, rifampin, and tetracycline. The isolate was susceptible to minocycline and gentamicin. PCRs were positive for the mecA and vanA genes. Other genetic characteristics include SCCmec type III, agr I, spa type t037, and sequence type (ST) 1283. The plasmid profile shows five plasmids with a size of ~1.7 kb to >10 kb. The isolated VRSA strain was obtained from a critically ill hospitalized patient. Genetic analysis of this strain suggested that the strain was a methicillin-resistant S. aureus (MRSA) clone endemic in Asia that underwent some genetic changes, such as mutation in the gmk gene and acquisition of the vanA gene.

First report of vancomycin-intermediate resistance in sequence type 72 community genotype methicillin-resistant Staphylococcus aureus

Vancomycin-intermediate resistance has not been previously reported among sequence type 72 (ST72) methicillin-resistant Staphylococcus aureus (MRSA) isolates of SCCmec type IV (ST72-MRSA-IV), which are distinctive community genotype strains in Korea. We report the first case of vancomycin treatment failure due to development of vancomycin-intermediate resistance in infection caused by an ST72-MRSA-IV isolate.

walK and clpP mutations confer reduced vancomycin susceptibility in Staphylococcus aureus

Vancomycin-intermediate Staphylococcus aureus (VISA) is generated from vancomycin-susceptible Staphylococcus aureus by multiple spontaneous mutations. We previously reported that sequential acquisition of mutations in the two-component regulatory systems vraSR and graRS was responsible for the VISA phenotype of strain Mu50. Here we report on the identification of a novel set of regulator mutations, a deletion mutation in two-component regulatory system walRK (synonyms, vicRK and yycFG), and a truncating mutation in a proteolytic regulatory gene, clpP, responsible for the raised vancomycin resistance in a laboratory-derived VISA strain, LR5P1-V3. The contributory effect of the two mutations to vancomycin resistance was confirmed by introducing the walK and clpP mutations into the vancomycin-susceptible parent strain N315LR5P1 by a gene replacement procedure. The vancomycin MIC of N315LR5P1 was raised from 1 to 2 mg/liter by the introduction of the walK or clpP mutation, but it was raised to 4 mg/liter by the introduction of both the walK and clpP mutations. The vancomycin MIC value of the double mutant was equivalent to that of strain LR5P1-V3. Like VISA clinical strains, LR5P1-V3 and the double mutant strain LR5P1walK*clpP* exhibited a thickened cell wall, slow growth, and decreased autolytic activity. Transcriptional profiles of the mutants with gene replacements demonstrated that introduction of both the walK and clpP mutations could alter expression of dozens or hundreds of genes, including those involved in cell envelope and cellular processes, intermediary metabolism, and information pathway. A mutation prevalence study performed on 39 worldwide clinical VISA strains showed that 61.5, 7.7, 10.3, and 20.5% of VISA strains harbored mutations in walRK, clpP, graRS, and vraSR, respectively. The mutation of walRK was most frequently carried by VISA strains. Together, these results suggested that the mutations of walK and clpP identified in LR5P1-V3 constitute a new combination of genetic events causing vancomycin resistance in Staphylococcus aureus.

Efficacy of fosfomycin and its combination with linezolid, vancomycin and imipenem in an experimental peritonitis model caused by a Staphylococcus aureus strain with reduced susceptibility to vancomycin

The objective of this study was to evaluate the in vitro and in vivo efficacies of therapies including fosfomycin against clinical Staphylococcus aureus isolates with reduced susceptibility to vancomycin (hGISA). Time-kill curves were performed over 24 h. Peritonitis in C57BL/6 mice was induced by intraperitoneal inoculation of 10(8) CFU/ml. Four hours later (0 h), therapy was started and the treatment groups were: control (not treated), fosfomycin (100 mg/kg/5 h), vancomycin (60 mg/kg/5 h), imipenem (30 mg/kg/5 h), fosfomycin plus linezolid, fosfomycin plus vancomycin and fosfomycin plus imipenem, receiving subcutaneous therapy over 25 h. Bacterial counts in peritoneal fluid, bacteraemia and mortality rates were determined. In vitro, fosfomycin showed a synergistic effect when combined with the other antimicrobials tested. In the animal model, fosfomycin combinations were effective and significantly reduced the bacteraemia rates achieved in the control, imipenem and vancomycin groups (p < 0.05). The best combination in vivo was fosfomycin plus imipenem. Also, fosfomycin plus linezolid was significantly better than vancomycin alone, reducing the bacterial concentration in the peritoneal fluid. In conclusion, in vitro and in vivo, fosfomycin in combination with linezolid, vancomycin or imipenem exerted a good activity. Fosfomycin plus imipenem was the most active combination, decreasing 3 log CFU/ml, and appears to be a promising combination for clinical practice.

Methicillin and Vancomycin Resistant S. aureus in Hospitalized Patients

S. aureus is the major bacterial cause of skin, soft tissue and bone infections, and one of the commonest causes of healthcare-associated bacteremia. Hospital-associated methicillin-resistant S. aureus (MRSA) carriage is associated with an increased risk of infection, morbidity and mortality. Screening of high-risk patients at the time of hospital admission and decolonization has proved to be an important factor in an effort to reduce nosocomial transmission. The electronic database Pub Med was searched for all the articles on "Establishment of MRSA and the emergence of vancomycin-resistant S. aureus (VRSA)." The search included case reports, case series and reviews. All the articles were cross-referenced to search for any more available articles. A total of 88 references were obtained. The studies showed a steady increase in the number of vancomycin-intermediate and vancomycin-resistant S. aureus. Extensive use of vancomycin creates a selective pressure that favors the outgrowth of rare, vancomycin-resistant clones leading to heterogenous vancomycin intermediate S. aureus hVISA clones, and eventually, with continued exposure, to a uniform population of vancomycin-intermediate S. aureus (VISA) clones. However, the criteria for identifying hVISA strains have not been standardized, complicating any determination of their clinical significance and role in treatment failures. The spread of MRSA from the hospital to the community, coupled with the emergence of VISA and VRSA, has become major concern among healthcare providers. Infection-control measures, reliable laboratory screening for resistance, appropriate antibiotic prescribing practices and avoidance of blanket treatment can prevent long-term emergence of resistance.

Emergence of vancomycin-intermediate Staphylococcus aureus from predominant methicillin-resistant S. aureus clones in a Korean hospital

The genetic and epidemiological features of four vancomycin-intermediate Staphylococcus aureus (VISA) isolates obtained from a Korean hospital were evaluated in this study. The VISA isolates were genotyped as sequence type (ST) 5-staphylococcal cassette chromosome mec (SCCmec) II variant (n=2) and ST239-SCCmec III (n=2), which were derived from the predominant methicillin-resistant S. aureus (MRSA) clones in Korean hospitals. One VISA isolate was acquired during vancomycin treatment, whereas three VISA isolates were obtained from the patients who had not previously been exposed to glycopeptides. As VISA is likely to arise from the predominant MRSA clones and may then possibly spread between patients, the emergence of VISA should be monitored with great care in hospitals.

Methicillin-resistant Staphylococcus aureus (MRSA) as a cause of nosocomial wound infections

Postoperative wound infections represent about 16% of hospital-acquired infections. Staphylococcus aureus is the most common cause of nosocomial wound infections. Increased frequency of Methicillin-resistant Staphylococcus aureus (MRSA) in hospitalized patients and possibility of vancomycin resistance requires permanent control of MRSA spread in the hospital.The purpose of this study was to analyse the frequency of Methicillin-resistant Staphylococcus aureus (MRSA) in the swabs taken from the surgical wounds, the presence of MRSA infection in surgical departments and to examine antimicrobial susceptibility of MRSA isolates. Wound swabs were examined from January 2006 to December 2008. The isolates were identified by conventional methods. Antimicrobial susceptibility testing was performed by Kirby-Bauer disc-diffusion method as per NCCLS guidelines.A total of 5755 wound swabs were examined: 938 (16,3%) swabs were sterile and 4817 (83,7%) were positive. Staphylococcus aureus was isolated in 1050 (22,0%) swabs and it was the most common cause of wound infections. MRSA was isolated from 12,4% samples in 2006, from 6,7% samples in 2007 and from 3,7% samples during 2008. Wound infections caused by MRSA dominated in the department of plastic surgery (24,4%) and in the department of orthopaedic surgery (24,1%). Antimicrobial susceptibility testing showed that 73% of MRSA isolates were with the same antibiotic sensitivity pattern (antibiotyp)-sensitive only to vancomycin, tetracycline, fucid acid and trimethoprim/sulfamethoxasole. Our results show decreasing of MRSA infection in the surgical wards. These results appear to be maintained with strategies for preventing nosocomial infection: permanent education, strong application of protocols and urging the implementation of strict infection control policy.

First detection of an invasive Staphylococcus aureus strain (D958) with reduced susceptibility to glycopeptides in Saudi Arabia

Strain D958, a methicillin-resistant Staphylococcus aureus strain with reduced susceptibility to vancomycin, was isolated from a 69-year-old Saudi male patient presenting with severe sepsis immediately after admission. Despite high serum levels of vancomycin, the same S. aureus strain was isolated from five blood culture sets during 1 week. Treatment failure under therapeutic levels of vancomycin prompted us to investigate the resistance profile of this strain in further detail. The MIC values for vancomycin as determined by Etest and microdilution were 3.0 and 2.0 mg/liter, respectively, and remained unchanged during the treatment course. The macro-Etest method showed a MIC of 4 mg/liter. The strain showed liquid vancomycin and lysostaphin MBCs of 2.0 and 5.0 mg/liter, respectively. The isolates were confirmed as heterogeneously vancomycin-intermediate S. aureus (hVISA) by vancomycin population analysis profile. The areas under these curves were similar for Mu3 and D958 for vancomycin and teicoplanin (ratio values were 1 and 1.1 for vancomycin and teicoplanin, respectively). Extensive genotyping and molecular characterization demonstrated that the strain harbored a staphylococcal cassette chromosome mec element (SCCmec) type III cassette and was of sequence type ST241, a single-locus variant of the successful multiresistant clone ST239. Microarray results demonstrated that D958 contained numerous resistance determinants (generally plasmid or phage encoded). These results suggest that this strain is constitutively expressing an altered susceptibility to vancomycin. Further studies are warranted to assess the clonal distribution of such strains displaying reduced susceptibility to vancomycin prior to any antimicrobial therapy.

Nanoconjugated vancomycin: new opportunities for the development of anti-VRSA agents

More than 90% of Staphylococcus strains are resistant to penicillin. In 1961 S. aureus developed resistance to methicillin (MRSA), invalidating almost all antibiotics, including the most potent beta-lactams. Vancomycin, a glycopeptide antibiotic, was used for the treatment of MRSA in 1980. Vancomycin inhibits the bio-synthesis of peptidoglycan and the assembly of NAM-NAG-polypeptide into the growing peptidoglycan chain. Vancomycin resistant S. aureus (VRSA) first appeared in the USA in 2002. Folic acid tagged chitosan nanoparticles are used as Trojan horses to deliver vancomycin into bacterial cells. These nanoparticles are biocompatible and biodegradable semisynthetic polymers. These nanosized vehicles enhance the transport of vancomycin across epithelial surfaces and show its efficient drug action, which has been understood from studies of the minimum inhibitory concentration and minimum bactericidal concentration of nanoparticles of a chitosan derivative loaded with vancomycin. Tolerance values distinctly show that vancomycin loaded into nanoconjugate is very effective and has a strong bactericidal effect on VRSA.

Reduced vancomycin susceptibility in Staphylococcus aureus, including vancomycin-intermediate and heterogeneous vancomycin-intermediate strains: resistance mechanisms, laboratory detection, and clinical implications

The emergence of vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) over the past decade has provided a challenge to diagnostic microbiologists to detect these strains, clinicians treating patients with infections due to these strains, and researchers attempting to understand the resistance mechanisms. Recent data show that these strains have been detected globally and in many cases are associated with glycopeptide treatment failure; however, more rigorous clinical studies are required to clearly define the contribution of hVISA to glycopeptide treatment outcomes. It is now becoming clear that sequential point mutations in key global regulatory genes contribute to the hVISA and VISA phenotypes, which are associated predominately with cell wall thickening and restricted vancomycin access to its site of activity in the division septum; however, the phenotypic features of these strains can vary because the mutations leading to resistance can vary. Interestingly, changes in the staphylococcal surface and expression of agr are likely to impact host-pathogen interactions in hVISA and VISA infections. Given the subtleties of vancomycin susceptibility testing against S. aureus, it is imperative that diagnostic laboratories use well-standardized methods and have a framework for detecting reduced vancomycin susceptibility in S. aureus.

The inhibitory effect of vancomycin ointment on the manifestation of MRSA keratitis in rabbits

The purpose of this study was to investigate an experimental model of methicillin-resistant Staphylococcus aureus (MRSA) keratitis, and to evaluate the inhibitory effect of vancomycin ointment on the manifestation of keratitis in this model. For the development of the MRSA keratitis model, 16 rabbits received 0.3-ml intrastromal injections of 1 x 10(7) colony forming units/ml MRSA inoculated to each of their corneas. After the MRSA inoculation, the rabbits were examined for a period of 14 days. Vehicle only was applied to each left eye for 2 days as an experimental control. Prototype vancomycin ointments, at concentrations of 0.03%, 0.1%, 0.3%, and 1.0%, were applied to each right eye for 2 days. The treatment commenced immediately after the MRSA inoculation and was administered five times a day at 2-h intervals. Four rabbits were used for each vancomycin ointment concentration. Anterior segments were examined daily, and the minimum inhibitory concentration of vancomycin (percentage in the ointment) for treating MRSA keratitis was determined on day 2. On day 14, no recurrences of infection were observed in the eyes that had not exhibited keratitis on day 2. Forty-eight hours post-inoculation, all left eyes demonstrated severe conjunctivitis, stromal infiltrates at injection sites, and fibroid reactions in the anterior chamber. MRSA keratitis was completely inhibited by 0.3% and 1.0% vancomycin ointments. Although the 0.1% ointment generally could prevent keratitis, punctate infiltrates were found in some eyes. Numerous infiltrates were found in eyes treated with 0.03% ointment. In conclusion, the present study substantiates the view that vancomycin ointment has a potent inhibitory effect upon the manifestations of MRSA keratitis, and indicates that a 0.3% concentration is necessary for effective treatment.

Linezolid for the treatment of infections caused by Gram-positive pathogens in China

In this randomised, double-blind, comparator-controlled, multicentre study conducted in China, 142 hospitalised patients aged 18-75 years with pneumonia (n=80) or complicated skin and soft-tissue infection (cSSTI) (n=62) due to suspected or known Gram-positive pathogens were randomised (1:1) to receive either linezolid 600mg (n=71) or vancomycin 1g in patients aged < or =60 years or 0.75g in patients aged >60 years (n=71) intravenously every 12h. The duration of treatment was 10-21 days for patients with pneumonia and 7-21 days for patients with cSSTI. Clinical outcomes were assessed at end-of-treatment (EOT) visit and follow-up (FU) visit 7-28 days post therapy. Staphylococcus aureus was the most common pathogen at baseline and most of these isolates were resistant to meticillin. All isolates were susceptible to linezolid and vancomycin. For the evaluable patients, the effective treatment rate for linezolid was higher than that for vancomycin at EOT (86.9% (53/61) vs. 61.7% (37/60)) and at FU (83.1% (49/59) vs. 64.9% (37/57)). Pathogen eradication rates for the microbiologically evaluable patients at FU were 79.2% (42/53) for linezolid and 61.5% (32/52) for vancomycin. The incidence of drug-related adverse events (AEs) was 25.4% (18/71) for linezolid and 16.9% (12/71) for vancomycin. Four (5.6%) linezolid-treated and eight (11.3%) vancomycin-treated patients discontinued the study drug because of an AE. Linezolid was well tolerated and effective for the treatment of infections caused by Gram-positive pathogens, including meticillin-resistant S. aureus.

Rapid detection of Staphylococcus aureus strains having reduced susceptibility to vancomycin using a chemiluminescence-based drug-susceptibility test

Current drug-susceptibility tests used routinely in clinical laboratories sometimes fail to identify strains of Staphylococcus aureus with reduced susceptibility to vancomycin. To solve this problem, we have developed a more sensitive and rapid method that measures bacterial metabolic activity by a chemiluminescence-based technique. This method is able to discriminate such strains from vancomycin-susceptible S. aureus with a sensitivity and specificity of > 95%. This rapid and reliable method appears to be promising for detection of vancomycin-intermediate S. aureus strains in clinical laboratories, and may supersede classical susceptibility testing.

Treatment of hospital-acquired pneumonia caused by methicillin-resistant Staphylococcus aureus

Nosocomial pneumonia and ventilator-assisted pneumonia may be polymicrobial and can be caused by a wide spectrum of pathogens. Potentially multidrug-resistant microorganisms often represent the 'core' pathogens of the most severe infections. Among Gram-positive pathogens, methicillin-resistant Staphylococcus aureus (MRSA) plays a key role, mainly in mechanically ventilated patients or in patients with specific risk factors. The mainstay of treatment for MRSA pneumonia has been glycopeptide antibiotics, i.e. vancomycin and, to a lesser extent, teicoplanin. However, owing to its insufficient penetration into lung compartments, vancomycin may result in therapeutic failure or slow clinical responses. Moreover, vancomycin serum levels must be monitored in order to minimise nephrotoxicity and to maximise the concentration in the lung. Finally, the emergence of staphylococci isolates with reduced susceptibility to vancomycin suggests that glycopeptides should no longer be considered as first-line antibacterial agents for Gram-positive lung infections. Among new therapeutic options, linezolid may be an appropriate choice for MRSA pulmonary infections owing to its good pharmacokinetic profile in the lung and its acceptable tolerability, especially in patients with renal insufficiency or in those receiving other nephrotoxic agents. However, to contain the increasing emergence of drug resistance among hospitalised patients, these novel antimicrobial agents should be used judiciously, restricting their use to patients not responsive to, or intolerant of, glycopeptides. Other new drugs under development appear to be promising and deserve further evaluation.

The rationale for revising the Clinical and Laboratory Standards Institute vancomycin minimal inhibitory concentration interpretive criteria for Staphylococcus aureus

The Clinical and Laboratory Standards Institute (formerly, the NCCLS) established the susceptibility and resistance breakpoints for minimal inhibitory concentration (MIC) and disk diffusion testing of vancomycin against isolates of Staphylococcus aureus > 20 years ago. The disk diffusion breakpoints were modified in 1998 when it was recognized that vancomycin-intermediate S. aureus strains were not detected by this method. In 2006, the vancomycin MIC breakpoints for S. aureus were lowered (from < or = 4 microg/mL to < or = 2 microg/mL for "susceptible," from 8-16 microg/mL to 4-8 microg/mL for "intermediate," and from > or = 32 microg/mL to > or = 16 microg/mL for "resistant") to increase detection of heterogeneously resistant isolates of S. aureus. This decision reflected a growing amount of microbiological and clinical data indicating that isolates of S. aureus are less likely to respond to vancomycin therapy when the vancomycin MICs are > or = 4 microg/mL.

Exploring glycopeptide-resistance in Staphylococcus aureus: a combined proteomics and transcriptomics approach for the identification of resistance-related markers

Background

To unravel molecular targets involved in glycopeptide resistance, three isogenic strains of Staphylococcus aureus with different susceptibility levels to vancomycin or teicoplanin were subjected to whole-genome microarray-based transcription and quantitative proteomic profiling. Quantitative proteomics performed on membrane extracts showed exquisite inter-experimental reproducibility permitting the identification and relative quantification of >30% of the predicted S. aureus proteome.

Results

In the absence of antibiotic selection pressure, comparison of stable resistant and susceptible strains revealed 94 differentially expressed genes and 178 proteins. As expected, only partial correlation was obtained between transcriptomic and proteomic results during stationary-phase. Application of massively parallel methods identified one third of the complete proteome, a majority of which was only predicted based on genome sequencing, but never identified to date. Several over-expressed genes represent previously reported targets, while series of genes and proteins possibly involved in the glycopeptide resistance mechanism were discovered here, including regulators, global regulator attenuator, hyper-mutability factor or hypothetical proteins. Gene expression of these markers was confirmed in a collection of genetically unrelated strains showing altered susceptibility to glycopeptides.

Conclusion

Our proteome and transcriptome analyses have been performed during stationary-phase of growth on isogenic strains showing susceptibility or intermediate level of resistance against glycopeptides. Altered susceptibility had emerged spontaneously after infection with a sensitive parental strain, thus not selected in vitro. This combined analysis allows the identification of hundreds of proteins considered, so far as hypothetical protein. In addition, this study provides not only a global picture of transcription and expression adaptations during a complex antibiotic resistance mechanism but also unravels potential drug targets or markers that are constitutively expressed by resistant strains regardless of their genetic background, amenable to be used as diagnostic targets.

Microbiological features of vancomycin in the 21st century: minimum inhibitory concentration creep, bactericidal/static activity, and applied breakpoints to predict clinical outcomes or detect resistant strains

The results of vancomycin susceptibility tests document that the drug continues to have activity against a wide variety of gram-positive pathogens. The subsequent emergence of vancomycin-resistant enterococci, the persistent failure of vancomycin therapy against strains tested as susceptible, and the more recent discoveries of vancomycin-intermediate or -resistant Staphylococcus aureus strains have compromised the use of vancomycin. Although analyses of surveillance studies fail to demonstrate "minimum inhibitory concentration creep" among populations of wild-type enterococci, streptococci, or staphylococci, enterococci with acquired resistance to vancomycin continue to evolve. The dominantly used automated commercial tests poorly recognize vancomycin-intermediate S. aureus, heteroresistant vancomycin-intermediate S. aureus, and vancomycin-resistant S. aureus isolates, which necessitates the use of expensive supplemental screening tests. Monitoring for appropriate serum levels of vancomycin and determinations of the bactericidal activity of vancomycin appear to best predict clinical outcome, thus creating additional diagnostic burdens for clinical laboratories. Improvements in current test methods with breakpoint criteria and expanded use of the vancomycin bactericidal assays to detect "tolerant" strains will be required to increase the value of vancomycin treatment or to refocus therapy toward the use of newer, alternative agents.

Synergy of arbekacin-based combinations against vancomycin hetero-intermediate Staphylococcus aureus

This study was undertaken to evaluate the in vitro activities of arbekacin-based combination regimens against vancomycin hetero-intermediate Staphylococcus aureus (hetero-VISA). Combinations of arbekacin with vancomycin, rifampin, ampicillin-sulbactam, teicoplanin, or quinupristin-dalfopristin against seven hetero-VISA strains and two methicillin-resistant S. aureus strains were evaluated by the time-kill assay. The combinations of arbekacin with vancomycin, teicoplanin, or ampicillin-sulbactam showed the synergistic interaction against hetero-VISA strains. Data suggest that these arbekacin-based combination regimens may be useful candidates for treatment options of hetero-VISA infections.

Novel mechanism of antibiotic resistance originating in vancomycin-intermediate Staphylococcus aureus

As an aggressive pathogen, Staphylococcus aureus poses a significant public health threat and is becoming increasingly resistant to currently available antibiotics, including vancomycin, the drug of last resort for gram-positive bacterial infections. S. aureus with intermediate levels of resistance to vancomycin (vancomycin-intermediate S. aureus [VISA]) was first identified in 1996. The resistance mechanism of VISA, however, has not yet been clarified. We have previously shown that cell wall thickening is a common feature of VISA, and we have proposed that a thickened cell wall is a phenotypic determinant for vancomycin resistance in VISA (L. Cui, X. Ma, K. Sato, et al., J. Clin. Microbiol. 41:5-14, 2003). Here we show the occurrence of an anomalous diffusion of vancomycin through the VISA cell wall, which is caused by clogging of the cell wall with vancomycin itself. A series of experiments demonstrates that the thickened cell wall of VISA could protect ongoing peptidoglycan biosynthesis in the cytoplasmic membrane from vancomycin inhibition, allowing the cells to continue producing nascent cell wall peptidoglycan and thus making the cells resistant to vancomycin. We conclude that the cooperative effect of the clogging and cell wall thickening enables VISA to prevent vancomycin from reaching its true target in the cytoplasmic membrane, exhibiting a new class of antibiotic resistance in gram-positive pathogens.

Antibiotic management of ventilator-associated pneumonia due to antibiotic-resistant gram-positive bacterial infection

Gram-positive cocci, in particular Staphylococcus aureus, account for as much as one-third of all cases of hospital-acquired pneumonia, and treatment has become increasingly complex as the proportion of resistant isolates has increased. Methicillin-resistant S. aureus is of particular concern because this pathogen is now associated with hospital-acquired, ventilator-associated, community-acquired, and healthcare-associated pneumonia. Antibiotic therapy for ventilator-associated pneumonia is challenging because it can be caused by multiple pathogens, which can be resistant to multiple drugs. This article reviews the epidemiology of ventilator-associated pneumonia and describes options for antibiotic treatment. Particular attention is paid to pneumonia due to methicillin-resistant S. aureus. Studies suggest that vancomycin, the traditional treatment for ventilator-associated pneumonia, may not be the best option for this type of pneumonia and that other antibiotics, such as linezolid and clindamycin, might be better choices. New antibiotics with activity against methicillin-resistant S. aureus are under investigation and may soon become available for clinical use. Studies are needed to define the optimal choice of antibiotic for pneumonias caused by this organism, and these choices will need to be balanced with the need to minimize the emergence of resistance.

DNA microarray-based identification of genes associated with glycopeptide resistance in Staphylococcus aureus

Six pairs of transcription profiles between glycopeptide-intermediate S. aureus (GISA [or vancomycin-intermediate S. aureus; VISA]) and glycopeptide-susceptible S. aureus (vancomycin-susceptible S. aureus [VSSA], including glycopeptide-susceptible isogenic mutants from VISA) strains were compared using a microarray. Ninety-two open reading frames which were or tended to be increased in transcription in VISA in at least five out of six array combination pairs were evaluated for their effects on glycopeptide susceptibility by introducing these genes one by one into VSSA strain N315 to construct an overexpression library. By screening the library, 17 genes including 8 novel genes were identified as associated with glycopeptide resistance since their experimental overexpression reduced vancomycin and/or teicoplanin susceptibility of N315. The raised MICs of vancomycin and teicoplanin were 1.25 to 3.0 and 1.5 to 6.0 mg/liter, respectively, as compared to 1.0 mg/liter of N315. Three of these genes, namely graF, msrA2, and mgrA, also raised the oxacillin MIC from 8.0 mg/liter for N315 to 64 to approximately 128 mg/liter when they were overexpressed in N315. Their contribution to vancomycin and beta-lactam resistance was further supported by gene knockout and trans-complementation assay. By using a plasmid-based promoter-green fluorescent protein gene (gfp) transcriptional fusion system, graF promoter-activated cells were purified, and subsequent susceptibility tests and Northern blot analysis demonstrated that the cells with up-regulated activity of graF promoter showed reduced susceptibility to vancomycin, teicoplanin, and oxacillin. In addition, cell morphology studies showed that graF and msrA2 overexpression increased cell wall thickness of N315 by factors of 23.91 and 22.27%, respectively, accompanied by glycopeptide MIC increments of 3- to 6-fold, when they were overexpressed in N315. Moreover, extended experiments and analyses indicate that many of the genes identified above are related to the cell wall biosynthetic pathway, including active nutrient transport systems. We propose that the genes which raise glycopeptide resistance in S. aureus function toward altering the cell wall metabolic pathway.

Experimental study on the efficacy of combinations of glycopeptides and β-lactams against Staphylococcus aureus with reduced susceptibility to glycopeptides

OBJECTIVES

The combination of glycopeptides and beta-lactams has been proposed as an alternative therapy against infections due to Staphylococcus aureus with reduced susceptibility to glycopeptides, though its role is still controversial. Our aim was to evaluate the efficacy (decrease in bacterial concentration after 24 h therapy) of these combinations both in vitro and in vivo.

METHODS

Four strains of S. aureus with different glycopeptide susceptibility (MICs of vancomycin from 1 to 8 mg/L) were used. In vitro experiments were performed by means of time-kill curves while we used the mouse peritonitis model for in vivo evaluation.

RESULTS

Combinations of glycopeptides and beta-lactams showed synergy in in vitro time-kill curves against the four staphylococcal strains, the highest efficacy being detected against the glycopeptide-intermediate S. aureus (GISA) strain (MIC = 8 mg/L) (Deltalog 24 h = -3.19 cfu/mL for vancomycin at 1/2 x MIC and oxacillinat 1/64 x MIC versus -0.56 cfu/mL for vancomycin alone at 1/2 x MIC). On the other hand, no significant increase in efficacy was observed in vivo in the experimental model. The efficacy of the combinations decreased in correlation to the decreasing susceptibility of the strains to glycopeptides, showing only residual activity against the GISA strain (Deltalog 24 h = -1.42 cfu/mL for vancomycin and cloxacillin versus -1.22 cfu/mL for vancomycin).

CONCLUSIONS

In the in vivo setting we were unable to demonstrate the synergism between glycopeptides and beta-lactams observed in vitro; nor did combinations show antagonism against any of the strains. Though the usefulness of these combinations cannot be totally ruled out in highly specific clinical conditions, it seems unlikely that they will provide a serious therapeutic alternative in most hGISA and GISA infections in the coming years.