Treatment failure due to methicillin-resistant Staphylococcus aureus (MRSA) with reduced susceptibility to vancomycin

A Vancomycin-Templated DNA-Encoded Library for Combating Drug-Resistant Bacteria

It is an urgent need to tackle the global crisis of multidrug-resistant bacterial infections. We report here an innovative strategy for large-scale screening of new antibacterial agents using a whole bacteria-based DNA-encoded library (DEL) of vancomycin derivatives via peripheral modifications. A bacterial binding affinity assay was established to select the modification fragments in high-affinity compounds. The optimal resynthesized derivatives demonstrated excellently enhanced activity against various resistant bacterial strains and provided useful structures for vancomycin derivatization. This work presents the new concept in a natural product-templated DEL and in antibiotic discovery through bacterial affinity screening, which promotes the fight against drug-resistant bacteria.

Synergistic Antibiofilm Activity between Synthetic Peptides and Ciprofloxacin against Staphylococcus aureus

Staphylococcus aureus is a human pathogen known to be resistant to antibiotics since the mid-20th century and is constantly associated with hospital-acquired infections. S. aureus forms biofilms, which are complex surface-attached communities of bacteria held together by a self-produced polymer matrix consisting of proteins, extracellular DNA, and polysaccharides. Biofilms are resistance structures responsible for increasing bacterial resistance to drugs by 1000 times more than the planktonic lifestyle. Therefore, studies have been conducted to discover novel antibacterial molecules to prevent biofilm formation and/or degrade preformed biofilms. Synthetic antimicrobial peptides (SAMPs) have appeared as promising alternative agents to overcome increasing antibiotic resistance. Here, the antibiofilm activity of eight SAMPs, in combination with the antibiotic ciprofloxacin, was investigated in vitro. Biofilm formation by S. aureus was best inhibited (76%) by the combination of Mo-CBP3-PepIII (6.2 µg mL−1) and ciprofloxacin (0.39 µg mL−1). In contrast, the highest reduction (60%) of the preformed biofilm mass was achieved with RcAlb-PepII (1.56 µg mL−1) and ciprofloxacin (0.78 µg mL−1). Fluorescence microscopy analysis reinforced these results. These active peptides formed pores in the cellular membrane of S. aureus, which may be related to the enhanced ciprofloxacin’s antibacterial activity. Our findings indicated that these peptides may act with ciprofloxacin and are powerful co-adjuvant agents for the treatment of S. aureus infections.

VraCP regulates cell wall metabolism and antibiotic resistance in vancomycin-intermediate Staphylococcus aureus strain Mu50

Objectives

Vancomycin-intermediate Staphylococcus aureus (VISA) is increasingly being reported. Previous studies have shown that vraC and vraP may be involved in vancomycin resistance, although the molecular mechanism remains elusive.

Methods

The vraC (SAV0577), vraP (SAV0578) and vraCP mutants were constructed in Mu50 by allelic replacement. Some common VISA phenotypes were assessed in mutants, such as, susceptibility to the cell wall-associated antibiotics, cell wall thickness, autolysis activity and growth rate. RT-qPCR was performed to reveal the differential genes associated with these phenotypes. The binding abilities of VraC and VraCP to the promoters of target genes were determined by electrophoretic mobility shift assay (EMSA).

Results

VraP forms a stable complex with VraC to preserve their own stability. The vraC, vraP and vraCP mutants exhibited increased susceptibility to the cell wall-associated antibiotics and thinner cell walls compared with the WT strain. Consistent with these phenotypes, RT-qPCR revealed downregulated transcription of glyS, sgtB, ddl and alr2, which are involved in cell wall biosynthesis. Moreover, the transcription of cell wall hydrolysis genes, including sceD, lytM and isaA, was significantly downregulated, supporting the finding that mutants exhibited reduced autolysis rates. EMSA confirmed that both VraC and VraCP can directly bind to the sceD, lytM and isaA promoter regions containing the consensus sequence (5′-TTGTAAN₂AN₃TGTAA-3′), which is crucial for the binding of VraCP with target genes. GFP-reporter assays further revealed VraC and VraCP can enhance promoter activity of sceD to positively regulate its expression.

Conclusions

vraCP plays a significant role in cell wall metabolism and antibiotic resistance in Mu50.

Co-delivery of free vancomycin and transcription factor decoy-nanostructured lipid carriers can enhance inhibition of methicillin resistant Staphylococcus aureus (MRSA)

Bacterial resistance to antibiotics is widely regarded as a major public health concern with last resort MRSA treatments like vancomycin now encountering resistant strains. TFDs (Transcription Factor Decoys) are oligonucleotide copies of the DNA-binding sites for transcription factors. They bind to and sequester the targeted transcription factor, thus inhibiting transcription of many genes. By developing TFDs with sequences aimed at inhibiting transcription factors controlling the expression of highly conserved bacterial cell wall proteins, TFDs present as a potential method for inhibiting microbial growth without encountering typical resistance mechanisms. However, the efficient protection and delivery of the TFDs inside the bacterial cells is a critical step for the success of this technology. Therefore, in our study, specific TFDs against S. aureus were complexed with two different types of nanocarriers: cationic nanostructured lipid carriers (cNLCs) and chitosan-based nanoparticles (CS-NCs). These TFD-carrier nanocomplexes were characterized for size, zeta potential and TFD complexation or loading efficiency in a variety of buffers. In vitro activity of the nanocomplexes was examined alone and in combination with vancomycin, first in methicillin susceptible strains of S. aureus with the lead candidate advancing to tests against MRSA cultures. Results found that both cNLCs and chitosan-based carriers were adept at complexing and protecting TFDs in a range of physiological and microbiological buffers up to 72 hours. From initial testing, chitosan-TFD particles demonstrated no visible improvements in effect when co-administered with vancomycin. However, co-delivery of cNLC-TFD with vancomycin reduced the MIC of vancomycin by over 50% in MSSA and resulted in significant decreases in viability compared with vancomycin alone in MRSA cultures. Furthermore, these TFD-loaded particles demonstrated very low levels of cytotoxicity and haemolysis in vitro. To our knowledge, this is the first attempt at a combined antibiotic/oligonucleotide-TFD approach to combatting MRSA and, as such, highlights a new avenue of MRSA treatment combining traditional small molecules drugs and bacterial gene inhibition.

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.

Mechanism of reduced vancomycin susceptibility conferred by walK mutation in community-acquired methicillin-resistant Staphylococcus aureus strain MW2

Point mutations with unclear molecular mechanisms are often associated with vancomycin resistance in Staphylococcus aureus. Here, we observed that the walK (G223D) mutation caused decreased expression of genes associated with cell wall metabolism, decreased autolytic activity, thickened cell walls, and reduced vancomycin susceptibility. A phosphorylation assay showed that WalK (G223D) exhibited reduced autophosphorylation, which led to reduced phosphorylation of WalR. An electrophoretic mobility shift assay indicated that WalK (G223D)-phosphorylated WalR had a reduced capacity to bind to the atlA promoter.

Inhibitors of bacterial transcription initiation complex formation

Antibiotic resistance is a growing global problem, with very few new compounds in development. Bacterial transcription is an underutilized target for antibiotics, which has been attributed to the similarity of the active site of RNA polymerases (RNAPs) across all domains of life and the ease with which resistance can arise through point mutation at multiple sites within this conserved region. In this study we have taken a rational approach to design a novel set of compounds that specifically target the formation of transcription initiation complexes by preventing the unique bacterial σ initiation factor from binding to RNAP. We have identified the region of RNAP to which these compounds bind and demonstrate that one compound, GKL003, has an inhibition constant in the low nanomolar range. This compound has activity against both Gram-positive and -negative organisms, including a community acquired methicillin-resistant strain of the major pathogen Staphylococcus aureus.

Effects of aggregate and individual antibiotic exposure on vancomycin MICs for Staphylococcus aureus isolates recovered from pediatric patients

We evaluated the evolution of vancomycin MICs for Staphylococcus aureus and their relationship with vancomycin use among hospitalized children. S. aureus isolates recovered from sterile sites were prospectively tested for vancomycin susceptibility using the Etest between 1 April 2000 and 31 March 2008. Vancomycin MICs were grouped into three categories: ≤ 1, 1.5, and 2 μg/ml. The association between vancomycin MICs and aggregate vancomycin use and individual patient vancomycin exposure 6 months prior to the documented infection was assessed. The geometric mean values for vancomycin MICs for S. aureus fluctuated over time without a significant trend (P = 0.146). Of the 436 patients included in the study, 363 (83%) had methicillin-susceptible S. aureus (MSSA) and 73 (17%) had methicillin-resistant S. aureus (MRSA) infections. The rate of isolates with a vancomycin MIC of 2 μg/ml increased from 4% (2 of 46) in 2000 to 2001 to 24% (11 of 46) in 2007 to 2008, despite a decrease in vancomycin use (r = -0.11; P = 0.825). The percentage of isolates with a vancomycin MIC of 2 μg/ml was higher for MRSA (15%; 11 of 73) than for MSSA strains (5.2%; 19 of 363) (χ(2) = 9.2; P = 0.01). Individual patient vancomycin exposure was not associated with a higher vancomycin MIC. In the unadjusted model, in which we compared patients with S. aureus infections with MICs of ≤ 1 μg/ml, the odds ratios of exposure rates for patients with isolates with MICs of 1.5 μg/ml and 2 μg/ml were 1.02 (P = 0.929) and 1.13 (P = 0.767), respectively. In our experience, the geometric means of vancomycin MICs from S. aureus isolates recovered from hospitalized children oscillated over time and were not associated with previous individual patient vancomycin exposure or aggregate vancomycin use.

Evolution of multidrug resistance during Staphylococcus aureus infection involves mutation of the essential two component regulator WalKR

Antimicrobial resistance in Staphylococcus aureus is a major public health threat, compounded by emergence of strains with resistance to vancomycin and daptomycin, both last line antimicrobials. Here we have performed high throughput DNA sequencing and comparative genomics for five clinical pairs of vancomycin-susceptible (VSSA) and vancomycin-intermediate ST239 S. aureus (VISA); each pair isolated before and after vancomycin treatment failure. These comparisons revealed a frequent pattern of mutation among the VISA strains within the essential walKR two-component regulatory locus involved in control of cell wall metabolism. We then conducted bi-directional allelic exchange experiments in our clinical VSSA and VISA strains and showed that single nucleotide substitutions within either walK or walR lead to co-resistance to vancomycin and daptomycin, and caused the typical cell wall thickening observed in resistant clinical isolates. Ion Torrent genome sequencing confirmed no additional regulatory mutations had been introduced into either the walR or walK VISA mutants during the allelic exchange process. However, two potential compensatory mutations were detected within putative transport genes for the walK mutant. The minimal genetic changes in either walK or walR also attenuated virulence, reduced biofilm formation, and led to consistent transcriptional changes that suggest an important role for this regulator in control of central metabolism. This study highlights the dramatic impacts of single mutations that arise during persistent S. aureus infections and demonstrates the role played by walKR to increase drug resistance, control metabolism and alter the virulence potential of this pathogen.

Performance of various testing methodologies for detection of heteroresistant vancomycin-intermediate Staphylococcus aureus in bloodstream isolates

The best screening method for detecting heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA) remains unclear. Using population analysis profiling utilizing the area under the concentration-time curve (PAP-AUC) as the gold standard, we screened 458 consecutive methicillin-resistant S. aureus (MRSA) bloodstream isolates to determine the most accurate and cost-effective testing strategy to detect the presence of heteroresistance. All isolates were also tested using the macromethod Etest (MET) and glycopeptide resistance detection (GRD) Etest. The MIC was determined by several methods, including standard vancomycin Etest, vancomycin broth microdilution (BMD), and Vitek2 testing. Fifty-five (12%) hVISA and 4 (1%) VISA isolates were detected by PAP-AUC. Compared to PAP-AUC, the sensitivities and specificities of MET, GRD Etest, BMD (using a MIC cutoff of ≥ 2 mg/liter), and standard vancomycin Etest (using a MIC cutoff of ≥ 2 mg/liter) were 89 and 55%, 71 and 94%, 82 and 97%, and 71 and 94%, respectively. Combination testing increased the overall testing accuracy by reducing the number of false-positive results. Cost was determined predominately by the number of PAP-AUC runs required following a screening assay. The most cost-effective strategy was BMD (using a MIC cutoff of ≥ 2 μg/ml) as a standalone assay or in combination with PAP-AUC, provided that BMD testing was batched. GRD Etest remained an alternative, with 71% of hVISA isolates detected. Prevalence influenced both cost and test accuracy, with results remaining unchanged for hVISA prevalences of up to 25%. Implementation of any testing strategy would therefore be dependent on balancing cost with accuracy in a given population and clinical context.

Comparison of detection methods for heteroresistant vancomycin-intermediate Staphylococcus aureus, with the population analysis profile method as the reference method

Staphylococcus aureus clinical isolates with vancomycin MICs of 2 μg/ml have been associated with vancomycin therapeutic failure and the heteroresistant vancomycin-intermediate S. aureus (hVISA) phenotype. A population analysis profile (PAP) with an area under the curve (AUC) ratio of ≥ 0.9 for the AUC of the clinical isolate versus the AUC for hVISA strain Mu3 is most often used for determining hVISA, but it is time-consuming and labor-intensive. A collection of 140 MRSA blood isolates with vancomycin MICs of 2 μg/ml by reference broth microdilution and screened for hVISA using PAP-AUC (21/140 [15%] hVISA) were tested by additional methods to detect hVISA. The methods included (i) Etest macromethod using vancomycin and teicoplanin test strips, brain heart infusion (BHI) agar, and a 2.0 McFarland inoculum; (ii) Etest glycopeptide resistance detection (GRD) using vancomycin-teicoplanin double-sided gradient test strips on Mueller-Hinton agar (MHA) with 5% sheep blood and a 0.5 McFarland inoculum; and (iii) BHI screen agar plates containing 4 μg/ml vancomycin and 16 g/liter casein using 0.5 and 2.0 McFarland inocula. Each method was evaluated using PAP-AUC as the reference method. The sensitivity of each method for detecting hVISA was higher when the results were read at 48 h. The Etest macromethod was 57% sensitive and 96% specific, Etest GRD was 57% sensitive and 97% specific, and BHI screen agar was 90% sensitive and 95% specific with a 0.5 McFarland inoculum and 100% sensitive and 68% specific with a 2.0 McFarland inoculum. BHI screen agar with 4 μg/ml vancomycin and casein and a 0.5 McFarland inoculum had the best sensitivity and specificity combination, was easy to perform, and may be useful for clinical detection of hVISA.

Antibiotic susceptibility patterns of Staphylococcus aureus isolates from Australian children

AIM

Staphylococcus aureus is an important cause of serious illness in children. Antibiotic resistance is an international problem and affects initial antibiotic choice. We aimed to describe susceptibility patterns of S. aureus isolates from Australian children to inform optimal empiric treatment of staphylococcal infections in this population.

METHODS

We analysed susceptibility data for all S. aureus isolates from children at Australian tertiary paediatric hospitals in 2006. Susceptibility rates were compared between hospitals and states, and with published studies of S. aureus isolates from Australian adults.

RESULTS

Overall, the proportion of methicillin-resistant S. aureus (MRSA) in Australian children was low (9.8%), and in each state it was less than for the comparable adult population. There were significant differences in susceptibility patterns between different states. Most MRSA isolates were susceptible to clindamycin (73%) and all isolates were reported as susceptible to vancomycin. Susceptibility patterns for isolates from bacteraemic patients were similar to those for isolates from all patients.

CONCLUSIONS

These data support current Australian recommendations for the use of flucloxacillin or a first-generation cephalosporin as initial treatment of non-life-threatening staphylococcal infections. However, broad spectrum antibiotic therapy including agents that are effective against MRSA should be considered for more serious infections. Appropriate specimens should be collected for susceptibility testing to enable directed treatment for MRSA and other resistant organisms. This study highlights the importance of using local, age-specific data in planning antibiotic treatment guidelines, as results vary substantially from city to city and between adults and children.

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.

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.

Daptomycin in bone and joint infections: a review of the literature

INTRODUCTION

To review the pharmacology, pharmacokinetics, efficacy, and safety of daptomycin, a novel antibiotic for the treatment of bone and joint infections, a literature search of relevant articles was conducted.

MATERIALS AND METHODS

A PubMed/MEDLINE search (1990-April 2008) to identify relevant English-language literature was conducted. Search terms included bone and joint infection, osteomyelitis, daptomycin, and methicillin-resistant Staphylococcus aureus (MRSA). Additional articles were identified by reviewing the bibliographies of articles cited. Programs and abstracts from infectious disease meetings were searched, and prescribing information of antibiotics indicated for bone and joint infections consulted. All articles identified from data sources published in English were evaluated.

RESULTS

Caused primarily by Gram-positive pathogens such as S. aureus and, to a lesser extent, Enterococcus faecalis, bone and joint infections are difficult to treat successfully. Surgical intervention and prolonged courses of antibiotics are frequently required, and failure of first-line antibiotic therapy is common. The emergence of S. aureus strains with reduced susceptibility to vancomycin, the longstanding gold standard for bone and joint infections, has complicated the clinical scenario. Few randomized trials comparing the efficacy of different antibiotics for bone and joint infections exist. Daptomycin, a novel intravenous lipopeptide antibiotic, has shown potent in vitro activity against a broad spectrum of Gram-positive bacteria, including many resistant pathogens commonly associated with bone and joint infections such as MRSA and vancomycin-resistant E. faecalis. Early clinical investigation of daptomycin in bone and joint infections unresponsive to antibiotics, such as vancomycin, has found a cure rate of approximately 80%, with a low incidence of adverse events and drug resistance.

CONCLUSION

Further studies are warranted to determine if limited clinical evidence, described in individual case reports and a daptomycin-specific retrospective registry, suggests daptomycin is a promising option for patients with bone and joint infections such as MRSA osteomyelitis.

Vancomycin revisited: a reappraisal of clinical use

Vancomycin has been used for decades to treat serious systemic gram positive infections. Extensive use over time has demonstrated vancomycin is not nephrotoxic even when used in high dosage, i.e., twice the usual dose. Since vancomycin is not nephrotoxic, there is no rationale for dosing vancomycin based on serum vancomycin levels. Since vancomycin is eliminated by GFR, vancomycin dosing should be based on creatinine clearance. Vancomycin obeys "concentration dependent" kinetics and higher than usual doses may be useful in some infections (eg, osteomyelitis). Widespread vancomycin use has resulted in increased VRE prevalence worldwide. Among staphylococci, vancomycin induced cell wall thickening results in "permeability mediated" resistance to vancomycin, as well as other anti-staphylococcal antibiotics. "Permeability mediated" resistance accounts for the common clinical observation that MRSA infections treated with vancomycin often resolve slowly or not at all. Other effective MRSA antibiotics are available (eg, linezolid, daptomycin, minocycline, or tigecycline) and are more reliably effective, do not increase staphylococcal resistance or increase VRE prevalence.

Vancomycin: understanding its past and preserving its future

The increase in vancomycin use in the 1980s to treat antibiotic-associated colitis and methicillin-resistant Staphylococcus aureus (MRSA) is largely responsible for the appearance of vancomycin-resistant enterococcus, which in turn spawned isolated cases of vancomycin-resistant S. aureus. Perhaps most worrisome to clinicians are strains of MRSA that are heteroresistant to vancomycin; these isolates are difficult to detect. Appropriate use of vancomycin coupled with awareness of infection control measures is paramount to abrogating the emergence of new vancomycin-resistant MRSA organisms and preserving its future efficacy. The continued reliance on vancomycin for the treatment of MRSA infections will depend on whether vancomycin resistance can be minimized. Newer antibacterial agents, particularly those with activity toward MRSA and vancomycin-resistant enterococcus, such as linezolid, quinupristin/dalfopristin, daptomycin, and tigecycline, may take a more prominent clinical role when gram-positive bacteria resistance to vancomycin further escalate.

Approaches to serious methicillin-resistant Staphylococcus aureus infections with decreased susceptibility to vancomycin: clinical significance and options for management

PURPOSE OF REVIEW

This review addresses therapeutic approaches to Staphylococcus aureus infections with diminished susceptibility to vancomycin, focusing on recently published data in English language medical literature between June 2006 and July 2007.

RECENT FINDINGS

Knowledge regarding the potential limitations of vancomycin therapy for S. aureus infections continues to emerge. Recent changes include alteration of the Clinical Laboratory and Standards Institute vancomycin breakpoint for S. aureus and questions regarding the utility of the lower breakpoint of 2.0 mg/l. Interest continues in the accessory gene regulator (agr) locus and its impact on the activity of vancomycin. Newer options for drug therapy progress, with strengths and limitations becoming more apparent for each.

SUMMARY

Newer antimicrobial agents active against methicillin-resistant S. aureus such as daptomycin and linezolid continue to show value. Older antimicrobial agents may play an important therapeutic role and warrant further examination. Work is needed to evaluate current agents against methicillin-resistant S. aureus in the setting of elevated vancomycin minimum inhibitory concentrations or clinical failure. Antimicrobial selection for methicillin-resistant S. aureus infections with reduced susceptibility to vancomycin should be governed by disease severity, susceptibility patterns, knowledge of the limitations of current susceptibility testing, and strengths and weaknesses of the agents being considered.

Surveillance of antibacterial resistance in Staphylococcus aureus isolated in Kuwaiti hospitals

OBJECTIVE

To investigate the prevalence of antibiotic resistance among Staphylococcus aureus isolated in Kuwaiti hospitals.

MATERIALS AND METHODS

S. aureus were isolated and identified following standard microbiological methods. Antibacterial susceptibility test was performed by disk diffusion and the measurement of minimum inhibitory concentration with E-test strips.

RESULTS

A total of 1,846 S. aureus isolates were analyzed from 13 hospitals between 1 March and 30 October 2005. They were isolated from 1,765 (95.6%) inpatients and 81 (4.4%) outpatients. Methicillin resistance was detected in 588 (32.0%) of the isolates. The methicillin-resistant S. aureus (MRSA) consisted of 461 (78%) multiresistant and 127 (22%) nonmultiresistant isolates. The nonmultiresistant MRSA consisted of epidemic MRSA-15 and community-associated MRSA. The community-associated MRSA was detected in all hospitals with MRSA, indicating its establishment in Kuwaiti hospitals. The proportion of isolates resistant to gentamicin, kanamycin, erythromycin, tetracycline, ciprofloxacin, fusidic acid and trimethoprim was higher among MRSA than methicillin-susceptible S. aureus (MSSA) isolates. Twenty-four and 22% of MRSA and MSSA isolates, respectively, expressed reduced susceptibility to vancomycin (minimum inhibitory concentration = 3-4 mg/l).

CONCLUSION

The study revealed the presence of methicillin resistance in 32% of S. aureus isolated in Kuwaiti hospitals and revealed an increase in the number of MRSA and MSSA with reduced susceptibility to vancomycin.

Investigation of reduced susceptibility to glycopeptides among methicillin-resistant Staphylococcus aureus isolates from patients in Ireland and evaluation of agar screening methods for detection of heterogeneously glycopeptide-intermediate S. aureus

Methicillin-resistant Staphylococcus aureus (MRSA) isolates (n = 3,189) from 2,990 patients were investigated by agar screening and by the Etest macromethod for reduced susceptibility to glycopeptide. No vancomycin-resistant S. aureus or glycopeptide-intermediate S. aureus (GISA) isolates were detected, but 178 isolates were confirmed as hetero-GISA (hGISA) by vancomycin population analysis profile (vPAP)-area under the curve (AUC) ratio determination and/or teicoplanin PAP (tPAP) methods. Of 139 isolates detected using the recommended Etest macromethod cutoff values of > or =8 mg/liter for both vancomycin and teicoplanin or > or =12 mg/liter for teicoplanin alone, 73 were confirmed as hGISA by vPAP-AUC, 95 were confirmed as hGISA by tPAP, and 108 were confirmed as hGISA by both methods. An Etest macromethod cutoff value of 8 mg/liter for teicoplanin alone detected a further 70 hGISA (17 were confirmed by vPAP-AUC and 70 were confirmed by tPAP). Agar screening utilizing brain heart infusion (BHI) agar containing 6 mg of vancomycin/liter (BHIV6) and Mueller-Hinton (MH) agar containing 8 mg of teicoplanin/liter (MHT8) failed to detect hGISA. MH agar containing 5 mg of teicoplanin/liter (MHT5) and BHI containing 5 mg of teicoplanin/liter (BHIT5) were evaluated using 10-microl volumes of three inoculum concentrations (with densities equivalent to 0.5 and 2.0 McFarland turbidity standards and stationary-phase BHI broth subcultures [MHT5(0.5), MHT5(2.0), MHT5(S), BHIT5(0.5), BHIT5(2.0), and BHIT5(S)]). The sensitivity of all methods except MHT5(0.5) and MHT5(2.0) was 100%. The specificity ranged from 4 to 82%. BHIT5(0.5) yielded the best performance, with a specificity of 84% for detecting isolates with teicoplanin Etest macromethod values of > or =8 mg/liter. Screening on BHIT5(0.5) is useful where screen-positive isolates are investigated with the Etest macromethod and confirmed by vPAP-AUC and tPAP. The prevalence of hGISA among patients with blood culture isolates recovered in Irish hospitals between 1999 and 2003 was 2.6%, whereas the prevalence among patients with isolates from all specimen sites collected during a 2-week survey in 1999 was 12%. The prevalence in one hospital decreased from 5.3% in 2003 to 1.5% in 2004.

Antibiotic resistance in Staphylococcus aureus and its relevance in therapy

Staphylococcus aureus is a major cause of infections. Only approximately 20% of the strains remain sensitive to penicillin. Beta-lactamase stable penicillins such as flucloxacillin form the mainstay of treatment of staphylococcal infection. Meticillin-resistant S. aureus (MRSA) are resistant to all beta-lactam antibiotics. Glycopeptide antibiotics are effective against most MRSA strains but, in the last few years, isolates of MRSA that have reduced susceptibility to glycopeptides (glycopeptide-intermediate S. aureus) have been isolated. Some strains exhibit frank resistance to glycopeptides (vancomycin-resistant S. aureus). Infections due to these strains are difficult to treat. This review summarises the therapeutic options for MRSA, glycopeptide-intermediate S. aureus and vancomycin-resistant S. aureus. Novel therapeutic strategies such as immunotherapy and vaccines are also discussed.

Vancomycin for treatment of invasive, multi-drug resistant Staphylococcus aureus infections

Staphylococcus aureus is a bacterial pathogen responsible for a variety of serious infections and is a frequent cause of nosocomial disease. During the last 60 years, S. aureus has developed increasing in vitro resistance to virtually all antimicrobials. In contrast, vancomycin has maintained a high degree of activity in vitro against this pathogen, although slight changes with in vitro activity could vastly change clinical activity. As a result, vancomycin has become the mainstay of therapy for invasive infections due to methicillin-resistant strains. However, clinical strains of S. aureus with intermediate resistance to vancomycin were reported in 1996, followed in 2002 with reports of isolates that were fully resistant. Although many authorities believe vancomycin remains the drug of choice for most staphylococcal-resistant infections, important issues surrounding its clinical application remain. These include the need for multiple daily dosing, intravenous administration, requirements for serum concentration monitoring, increasing resistance in vitro, modest efficacy rates and (less frequently) treatment-limiting adverse effects. This review addresses these important topics.

Prevalence and characteristics of heteroresistant vancomycin-intermediate Staphylococcus aureus bacteremia in a tertiary care center

Infections with S. aureus with heterogeneous intermediate resistance to vancomycin (hVISA) are occurring more frequently. The detection of these infections, their prevalence, clinical characteristics, and significance are controversial. During 2003 and 2004, all blood culture isolates of methicillin-resistant Staphylococcus aureus (264 patients) at the Sheba Medical Center, Tel Hashomer, Israel, were assessed for hVISA by using the Etest macromethod. A total of 16 patients (6%) were positive for hVISA. Resistance to teicoplanin alone and to vancomycin alone using the Etest macromethod was found in 14 and 10 patients, respectively. Standard MICs to vancomycin were between 1 to 4 mg/ml. Most of these isolates (12 of 16 [75%]) would have been missed without specific testing. The median number of bacteremic days was 4. Seven patients had positive blood cultures for more than 5 days. Twelve patients died, and for eight of these the deaths were directly related to hVISA sepsis. We found that hVISA bacteremia was prevalent in our institution, and we suggest seeking hVISA in patients with persistent S. aureus bacteremia.

Emerging Options for Treatment of Invasive, Multidrug-ResistantStaphylococcus aureusInfections

Limited established treatment options exist for the treatment of serious, invasive infections caused by multidrug-resistant Staphylococcus aureus, most notably nosocomially acquired methicillin-resistant S. aureus (MRSA). Although vancomycin represents the gold standard for therapy of such invasive infections, reports of increasing in vitro resistance to vancomycin, combined with reports of clinical failures (with this and other antistaphylococcal agents), underscore the need for alternative therapies. Older agents with favorable in vitro activity available in both oral and intravenous dose forms include trimethoprim-sulfamethoxazole and clindamycin. Limited clinical data exist to support their routine use as initial therapy in the treatment of invasive disease. However, these and other options (e.g., tetracyclines) are being reexplored in the setting of increasing concern over MRSA acquired in the community setting. Newer treatment options for MRSA include linezolid, quinupristin-dalfopristin, daptomycin, and tigecycline. With the exception of linezolid, these newer agents require intravenous administration. Combination therapy may be considered in select invasive diseases refractory to standard monotherapies. These diseases include infections such as endocarditis, meningitis, and prosthetic device infections. Additional alternatives to vancomycin are under clinical investigation. Those in later stages of development include oritavancin, dalbavancin, telavancin, and ceftobiprole.

Guidelines for the control and prevention of meticillin-resistant Staphylococcus aureus (MRSA) in healthcare facilities

Meticillin-resistant Staphylococcus aureus (MRSA) remains endemic in many UK hospitals. Specific guidelines for control and prevention are justified because MRSA causes serious illness and results in significant additional healthcare costs. Guidelines were drafted by a multi-disciplinary group and these have been finalised following extensive consultation. The recommendations have been graded according to the strength of evidence. Surveillance of MRSA should be undertaken in a systematic way and should be fed back routinely to healthcare staff. The inappropriate or unnecessary use of antibiotics should be avoided, and this will also reduce the likelihood of the emergence and spread of strains with reduced susceptibility to glycopeptides, i.e. vancomycin-intermediate S. aureus/glycopeptide-intermediate S. aureus (VISA/GISA) and vancomycin-resistant S. aureus (VRSA). Screening for MRSA carriage in selected patients and clinical areas should be performed according to locally agreed criteria based upon assessment of the risks and consequences of transmission and infection. Nasal and skin decolonization should be considered in certain categories of patients. The general principles of infection control should be adopted for patients with MRSA, including patient isolation and the appropriate cleaning and decontamination of clinical areas. Inadequate staffing, especially amongst nurses, contributes to the increased prevalence of MRSA. Laboratories should notify the relevant national authorities if VISA/GISA or VRSA isolates are identified.

Glycopeptides: Update on an old successful antibiotic class

The natural product glycopeptides vancomycin and teicoplanin have come to play a significant role in the therapy for Gram-positive bacterial infections. In particular vancomycin is the choice for empiric therapy of these infections primarily due to its activity against and the significance of methicillin-resistant Staphylococcus aureus. While high-level problematic glycopeptide resistance among enterococci was observed initially and continues to increase, the slow creep of vancomycin intermediate susceptibility and the fear of frank resistance among the staphylococci have precipitated increasing work leading to creation of new semisynthetic analogs. These new agents, including dalbavancin and telavancin, are within 1-2 years availability in the clinic. Interestingly, chemical modifications resulting in these second-generation analogs and additional characterization have revealed new mechanisms of antibacterial action, and plasticity regarding additional properties including pharmacokinetics for the drug candidates. The unique beneficial properties of the near term vancomycin replacements, semisynthesis of additional important analogs, and advances in metabolic engineering resulting in novel scaffolds signal a new era for the glycopeptide antibiotics.

Guidelines for the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA)

These evidence-based guidelines have been produced after a literature review of the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA). We have considered the detection of MRSA in screening samples and the detection of reduced susceptibility to glycopeptides in S. aureus. Recommendations are given for the identification of S. aureus and for suitable methods of susceptibility testing and screening for MRSA and for S. aureus with reduced susceptibility to glycopeptides. These guidelines indicate what tests should be used but not when the tests are applicable, as aspects of this are dealt with in guidelines on control of MRSA. There are currently several developments in screening media and molecular methods. It is likely that some of our recommendations will require modification as the new methods become available.

Low-level vancomycin resistance in Staphylococcus aureus--an Australian perspective

Low-level vancomycin resistance in Staphylococcus aureus has emerged as a clinical problem over the past 8 years. The clinical relevance of this resistance has been questioned, and laboratory detection remains difficult and time consuming. There is, however, increasing evidence linking low-level vancomycin resistance with glycopeptide treatment failure in serious Staphylococcus aureus infections. Diagnostic laboratories and clinicians need to be aware of this resistance phenotype, to have procedures in place to detect the resistance, and to have strategies for managing patients with infections caused by resistant strains.

Methicillin-resistant Staphylococcus aureus and beyond: what's new in the world of the golden staph?

Methicillin-resistant Staphylococcus aureus (MRSA) continues to plague our hospitals. With the appearance of isolates that are resistant to vancomycin, now, more than ever, we must direct our efforts to controlling its development and spread. New antimicrobials have become available for treatment, but may only be a short-term answer. Our efforts towards control must be directed towards infection control measures such as improved hand hygiene with user-friendly products, such as alcohol-based hand disinfectants. Intranasal mupirocin may have a place in prevention of surgical site infection, although this role has not yet been clearly defined. Other areas where MRSA control may be effected include prudent controlled use of antibiotics, including surgical prophylaxis.

Efficacy of current agents used in the treatment of Gram-positive infections and the consequences of resistance

The proportion of pathogens causing hospital-onset infections that are resistant to antimicrobial agents continues to increase worldwide. Inadequate antimicrobial therapy is an important factor in the emergence of resistance and is associated with increased mortality. In the USA in 2000, the National Nosocomial Infections Surveillance system reported that >50% of Staphylococcus aureus isolates collected from intensive care units were resistant to methicillin (MRSA). The emergence of community-acquired MRSA is a new concern. MRSA are associated with adverse clinical outcomes and increased hospital costs. The increasing prevalence of MRSA contributes to the use of glycopeptides; however, isolates with intermediate and full resistance to vancomycin and teicoplanin are now being reported. Newer agents, such as the oxazolidinone linezolid, are effective in the treatment of serious Gram-positive infections; however, linezolid-resistant isolates of Enterococcus faecium, Enterococcus faecalis and S. aureus have been reported. Therefore, there is an unmet clinical need for new agents with activity against Gram-positive pathogens. Daptomycin, a lipopeptide with a novel mode of action, was recently approved for the treatment of skin and soft tissue infections in the USA. The two case studies presented herein detail experience with the use of daptomycin in the USA.

Glycopeptide resistance in Staphylococcus aureus: is it a real threat?

Following the discovery in 1992 that resistance to vancomycin could be transferred in vitro from a strain of Staphylococcus haemolyticus to Staphylococcus aureus, the threat that a similar event could happen in vivo has existed. In 1996, the threat became a reality in Japan followed by reports of low-level heteroresistant S. aureus from elsewhere in the world. However, the threat has not gone away; indeed it has become more intimidating within the past 2 years with the isolation of higher-level resistance to vancomycin transferred on a plasmid from Enterococcus faecalis. The mechanisms of resistance are different in each of the types of S. aureus with reduced susceptibility to the glycopeptides: in one hetero-vancomycin intermediate-susceptible S. aureus (hVISA) and vancomycin intermediate-susceptible S. aureus (VISA), cell wall biosynthesis is altered, hindering glycopeptide reaching its target, and in the other vancomycin-resistant S. aureus (VRSA) the target is altered. This review attempts to place into context the threat posed to patients by the appearance of strains of S. aureus more resistant to one of the key therapeutic agents used in our hospitals. It will do so by raising a number of important questions followed by the presentation of experimental evidence that may or may not provide answers that heighten or lower the threat.

Treatment outcomes for serious infections caused by methicillin-resistant Staphylococcus aureus with reduced vancomycin susceptibility

Although infections caused by methicillin-resistant Staphylococcus aureus with reduced vancomycin susceptibility (SA-RVS) have been reported from a number of countries, including Australia, the optimal therapy is unknown. We reviewed the clinical features, therapy, and outcome of 25 patients with serious infections due to SA-RVS in Australia and New Zealand. Eight patients had endocarditis, 9 had bacteremia associated with deep-seated infection, 6 had osteomyelitis or septic arthritis, and 2 had empyema. All patients had received vancomycin before the isolation of SA-RVS, and glycopeptide treatment had failed for 19 patients (76%). Twenty-one patients subsequently received active treatment, which was effective for 16 patients (76%). Eighteen patients received linezolid, which was effective in 14 (78%), including 4 patients with endocarditis. Twelve patients received a combination of rifampicin and fusidic acid. Surgical intervention was required for 15 patients (60%). Antibiotic therapy, especially linezolid with or without rifampicin and fusidic acid, in conjunction with surgical debulking is effective therapy for the majority of patients with serious infections (including endocarditis) caused by SA-RVS.

Antimicrobial resistance in Staphylococcus aureus in Australian teaching hospitals, 1989-1999

An annual survey of antimicrobial resistance in clinical isolates of Staphylococcus aureus was conducted in 21 Australian teaching hospital microbiology laboratories in eight major cities from 1989 to 1999. A total of 19,000 isolates were tested for susceptibility to 18 antimicrobials, with 3795 being methicillin-resistant (MRSA). Resistance to ciprofloxacin in MRSA increased from 4.9% to 75.9%. The proportion of MRSA resistant to erythromycin decreased significantly (99.0%-88.9%), as did that to trimethoprim (98.4%-82.4%) and to tetracycline (96.5%-80.1%). The proportion of MRSA isolated increased in Sydney, Melbourne, Canberra, Adelaide, Perth, and Darwin, but not in Brisbane. The proportion in Hobart peaked in 1994. MRSA in Perth were predominantly non-multiresistant (nmMRSA) throughout the survey (i.e., resistant to less than three of eight indicator antibiotics) due mainly to local strains that originated in the community. The proportion of nmMRSA increased to modest levels in the other cities. In eastern cities, this was due to the appearance of strains closely related to nmMRSA seen in other countries of the southwestern Pacific.

Clinical features associated with bacteremia due to heterogeneous vancomycin-intermediate Staphylococcus aureus

We assessed all episodes of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia at our hospital during a 12-month period (n=53) and compared those due to heterogeneous vancomycin-intermediate S. aureus (hVISA; n = 5, 9.4%) with those due to vancomycin-susceptible MRSA (n=48). Patients with hVISA bacteremia were more likely to have high bacterial load infections (P=.001), vancomycin treatment failure (persistent fever and bacteremia for >7 days after the start of therapy; P<.001), and initially low serum vancomycin levels (P=.006). These clinical markers of hVISA bacteremia may help focus diagnostic efforts and treatment.

Methicillin-resistant Staphylococcus aureus: impact on dermatology practice

Methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) emerged in the 1960s and is now commonly seen in hospitals, clinics and, since the mid-1990s, the community. Risk factors for the acquisition of MRSA include chronic dermatoses, underlying medical illnesses, attending healthcare facilities, use of prescription antibacterials, surgery, intravenous lines, hospitalization in an intensive care unit, and proximity to patients colonized with MRSA. Recent community-associated strains often occur in patients without these risk factors. Staphylococci are readily spread from person to person and readily contaminate the environment. Infection control measures thus involve identifying the infected patients, separating them from other non-infected patients, cleaning of the environment and, most important of all, scrupulous attention to hand hygiene. Alcoholic antiseptic hand rubs offer an alternative to antiseptic hand washes and increase compliance. Treatment of MRSA skin infections is challenging. Topical agents such as mupirocin or fusidic acid can be used, but the organisms often become resistant. Systemic therapy involves non-beta-lactams. Parenteral treatment is generally with glycopeptides such as vancomycin; oral therapy is more complex. Monotherapy with quinolones, rifampin (rifampicin), and fusidic acid often results in the development of resistance and so, if any of these agents are chosen it should be in combination. There are no data on combination therapy, although rifampin-containing combinations are often chosen. Fourth-generation quinolones and linezolid are expensive but promising alternatives.

Emergence of community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infection in Queensland, Australia

OBJECTIVES

To investigate the incidence and epidemiology of non-multiresistant methicillin-resistant Staphylococcus aureus (nmMRSA) infection in south-east Queensland, Australia.

STUDY DESIGN

A retrospective survey was done of hospital records of all patients who had non-multiresistant MRSA isolated at Ipswich Hospital (a 250-bed general hospital, 40 km south-west of Brisbane, Queensland, Australia) between March 2000 and June 2001. Laboratory typing of these isolates was done with antibiogram, pulsed-field gel electrophoresis, bacteriophage typing and coagulase gene typing.

RESULTS

There were 44 infections caused by nmMRSA. Seventeen infections (39%) occurred in patients from the south-west Pacific Islands (predominantly Samoa, Tonga and New Zealand). Laboratory typing showed that the isolates in Pacific Islanders were Pacific Island strains, and 16/17 of these infections were community acquired. Twenty-three infections (52%) occurred in Caucasians. Eleven of the isolates from Caucasians (48%) were a new predominantly community-acquired strain that we have termed the 'R' pulsotype, nine (39%) were Pacific Island strains, and three (13%) were health care institution-associated strains. Four infections occurred in patients who were not Caucasians or Pacific Islanders. Overall, 34 of all 44 infections (77%) were community acquired.

CONCLUSIONS

Non-multiresistant MRSA infection, relatively frequently observed in Pacific Islanders in south-east Queensland, is now a risk for Caucasians as well, and is usually community acquired. Clinicians should consider taking microbiological specimens for culture and antimicrobial susceptibility testing in patients with suspected staphylococcal infections who are not responding to empirical therapy with beta-lactam antibiotics.

Hospital infection control in Australia

Australia is a large country divided into six states and two territories, each of which has infection control programmes. This paper looks at the organization of infection control in Australia, as well as describing the national bodies involved and recent state initiatives in infection control.

The prevalence and mechanisms of vancomycin resistance in Staphylococcus aureus

The emergence of Staphylococcus aureus resistant to vancomycin has caused considerable concern. Such strains are currently rare, although they have been isolated from many areas of the world. Considerable controversy surrounds strains of S. aureus displaying heterogeneous resistance to vancomycin regarding their definition and methods for detection. This has led to considerable variance in estimates of prevalence (0-1.3%-20% in Japan) and has hindered efforts to define the clinical relevance of these strains. The mechanism of resistance involves a complex reorganization of cell wall metabolism, leading to a grossly thickened cell wall with reduced peptidoglycan cross-linking. There may be many different ways in which strains achieve this endpoint. Current knowledge and theories are summarized.

11: Antibiotic resistance

Antibiotic resistance is a consequence of antibiotic use - we need to use antibiotics less and to use them prudently. Plans to combat antibiotic resistance were recently proposed by the World Health Organization, a United States interagency taskforce and the Australian Joint Expert Technical Advisory Committee on Antibiotic Resistance. Prudent antibiotic use includes not using antibiotics when benefit is minimal (eg, in many respiratory tract infections), using narrow-spectrum antibiotics whenever possible and using optimal dosages and regimens. The need for antibiotic therapy can be reduced by preventing infections through vaccination, infection control measures and improved sanitation. Surveillance of antibiotic resistance is needed to target interventions for minimising antibiotic use. More research is needed into new antibiotics and regimens and into improving medical devices and protocols to prevent infection. Some simple changes to practice could reduce development and spread of antibiotic resistance