JAC Advance Access published online on February 8, 2008
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkn037
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Original research |
Relationships between vancomycin pharmacodynamics and the emergence of vancomycin-intermediate Staphylococcus aureus (VISA) from heterogeneous VISA in an in vitro pharmacodynamic model
1 Anti-Infective Research Laboratory, Department of Pharmacy Practice, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, Detroit, MI 48201, USA 2 Detroit Receiving Hospital, Detroit, MI 48201, USA 3 University of Wisconsin School of Pharmacy, Madison, WI 53705, USA 4 School of Medicine, Wayne State University, Detroit, MI 48201, USA 5 John D. Dingell Department of Veteran's Affairs Medical Center, Detroit, MI 48201, USA
* Corresponding author. Tel: +1-313-577-4376; Fax: +1-313-577-8915; E-mail: m.rybak{at}wayne.edu
Received 3 August 2007; returned 7 January 2008; revised 8 October 2007; accepted 13 January 2008
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
Objectives: Increasing reports of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) have raised concerns over vancomycin utility in treating infections caused by this pathogen. Previous studies have demonstrated that hVISA precedes the emergence of VISA. This investigation evaluates the pharmacodynamic parameters of vancomycin in vitro against hVISA and the development of reduced susceptibility.
Methods: Two hVISA isolates (Mu3 and MRSA 1629) and one clinical non-hVISA (MRSA 3286) were evaluated at moderate and high inoculum using vancomycin simulations [750–5000 mg every 12 h; free area under the curve (fAUC)/MIC 105–799] in an in vitro pharmacodynamic model over 72 h.
Results: The activity of vancomycin was highly dependent on the bacterial inoculum. At high inoculum, all vancomycin simulations displayed initial killing up to 24 h with no additional activity beyond this time point. Increased vancomycin doses had no impact on overall activity. Although bactericidal activity was achieved in all strains, regardless of the presence of hVISA, 
105cfu/mL of organisms remained after exposure. Doses as high as 5 g every 12 h (fAUC/MIC 799) had little influence on decreasing the hVISA bacterial load. More rapid bactericidal eradication was noted at lower inoculum in the non-hVISA isolate (T99 1.9 h versus 14.1 h and 15.3 h for Mu3 and 1629, respectively; P = 0.001). A 2- to 4-fold increase in vancomycin MIC was noted with both hVISA isolates compared with no increase in non-hVISA at high inoculum.
Conclusions: Vancomycin doses 
2500 mg and 2000 mg every 12 h (fAUC/MIC 374 and 271) suppressed the emergence of MIC increases in Mu3 and 1629, respectively. Our data suggest that at high inoculum, vancomycin has minimal to no activity against hVISA and leads to further reduced susceptibility.
Key Words: resistance , Mu3 , glycopeptides
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
The primary treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections has historically been vancomycin. The use of this glycopeptide antibiotic has been steadily increasing as infection rates due to MRSA continue to increase both in the hospital and community setting.1,2 The increase in vancomycin use has coincided with the emergence of strains with reduced susceptibility to this antibiotic.3,4 In particular, the development of heterogeneous vancomycin-intermediate S. aureus (hVISA) strains with vancomycin MICs below the resistance breakpoint are now a growing concern in the clinical setting.5,6 These organisms appear within the susceptible range, but they possess subpopulations with MICs >2 mg/L. The exact prevalence of these isolates in the clinical setting has not been identified due to the lack of a standardized and practical method of detection. Although reports have indicated prevalence rates ranging from 1.67% to 27% depending on the patient population studied,5,7,8 hVISA infections have been identified in association with clinical failure of vancomycin therapy.5,8 Importantly, we have previously reported that hVISA is increasing in our metropolitan area with 2.2% of MRSA expressing the hVISA phenotype between years 1986 and 1993 compared with 8.3% between years 2003 and 2006.9
The treatment of hVISA infections with vancomycin presents many therapeutic challenges. We have previously reported reduced in vitro activity with vancomycin concentrations at twice the MIC against hVISA compared with standard MRSA.10 Reports of hVISA in patient populations have now identified associations with clinical markers of these infections.5,11 A recent study evaluating the clinical features of hVISA bacteraemia revealed significantly higher rates of morbidity in these subjects compared with standard MRSA. Also, they were more likely to have high bacterial load infections such as endocarditis or prosthetic involvement, low initial vancomycin concentrations (<10 mg/L), and vancomycin failure defined as persistent fever and bacteraemia for >7 days after the start of therapy.5 These therapeutic challenges have raised questions on the utility of vancomycin to treat these infections.12
Although low vancomycin concentrations have been associated with hVISA, the optimal treatment regimen with vancomycin remains unknown. Previous studies have indicated that a vancomycin area under the curve (AUC)/MIC 400 is associated with improved clinical and microbiological response in standard MRSA,13 while an in vivo animal model of MRSA including hVISA suggests that a free (f) AUC/MIC around 500 may enhance efficacy.14 Other studies display the importance of the organism MIC in vancomycin response.15–17 The purpose of this study was to identify bacterial kill at high and moderate inoculum hVISA and non-hVISA isolates utilizing a range of vancomycin regimens corresponding to fAUC/MIC and evaluate the emergence of reduced susceptibility in an in vitro pharmacodynamic model.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
Bacterial strains
Three S. aureus strains were employed: (i) Mu3, the first described isolate with hVISA characteristics from Japan;18 (ii) MRSA 1629, a clinical hVISA isolate from a patient treated with vancomycin from Detroit Receiving Hospital, Detroit, MI, USA; and (iii) MRSA 3286, a clinical non-hVISA control strain from another patient treated at Detroit Receiving Hospital.
Antibiotics, reagents and media
Vancomycin was purchased from a commercial source (Sigma Chemical Co., St Louis, MO, USA).
Mueller–Hinton broth (Difco, Detroit, MI, USA) supplemented with 25 mg/L calcium and 12.5 mg/L magnesium was used for all in vitro pharmacodynamic models for evaluation of various dosing regimens and susceptibility testing of vancomycin. Bacterial colony counts were determined using brain heart infusion (BHI) agar (Difco).
MICs and MBCs of vancomycin were determined by broth microdilution techniques or Etest according to the CLSI.19 MIC/MBC assays were determined at the traditional organism density of 5 x 105 cfu/mL. Samples were incubated at 35°C for 24 h.
The detection of hVISA was determined by population analysis-AUC profile (PAP) and macrodilution Etest (MET) methodology using a 2 McFarland inoculum on BHI agar followed by an incubation period of 48 h.20 Vancomycin population analysis profiles were determined in duplicate on all three test isolates at an inoculum of 109 cfu/mL. Fifty microlitres of this suspension was plated onto vancomycin-containing BHI agar at increasing concentrations (0, 0.5, 1, 1.5, 2, 3, 4 and 8 mg/L) using a Whitley Automatic Spiral Plater (Don Whitley Scientific, West Yorkshire, UK). After incubation at 35°C for 48 h, colony counts (log10 cfu/mL) were determined. Population AUC was determined for each test isolate and compared with the AUC for Mu3, which was performed in parallel for each isolate. The test isolate was considered an hVISA, if the ratio of the AUC of the test isolate to Mu3 was between 0.9 and 1.3.
In vitro pharmacodynamic infection model
The in vitro infection model consisting of a 250 mL one-compartment glass apparatus with a port for samples was utilized for all simulations. Administration of antibiotics and sample removal was performed over the 72 h interval via an injection port. The model apparatus was placed in a 37°C water bath throughout the procedure with a magnetic stir bar for thorough mixing of the drug in the model. Fresh medium was continuously supplied and removed from the compartment along with the drug via a peristaltic pump (Masterflex, Cole-Parmer Instrument Company, Chicago, IL, USA) set to simulate the 6–8 h half-life of vancomycin.21 A starting inoculum of 106 and 109 cfu/mL was utilized for each of the three isolates. Vancomycin was infused over 1 min every 12 h with a fAUC/MIC0–24 range of 105–799 (750–5000 mg every 12 h). Experiments were performed on each strain starting with an fAUC/MIC of 105 to determine kill and change in the MIC. Regimens (fAUC/MIC) were increased until no resistance occurred. Free vancomycin concentrations were utilized to account for estimated 55% protein binding of vancomycin in vivo.22
Bacterial quantification was determined from samples drawn in duplicate from each model over 0–72 h utilizing aseptic techniques. The samples were spread onto BHI agar using an automated spiral dispenser. Colonies were counted after 24 h of incubation at 35°C using a laser colony counter (ProtoCOL, Synbiosis, Cambridge, UK) and cfu/mL were measured. This method results in a lower limit of detection of 2.0 log10 cfu/mL. Antimicrobial carryover was minimized by serial dilution of samples prior to plating when suspected concentrations were close to the MIC.
The total reduction in log10 cfu/mL was determined by plotting time–kill curves based on the number of remaining organisms over the 72 h time period. Bactericidal activity (99.9% kill) was defined as a 3 log10 cfu/mL reduction in colony count from the initial inoculum. Bacteriostatic activity was defined as a <3 log10 cfu/mL reduction in colony count from the initial inoculum, whereas inactive was defined as no observed reductions in initial inocula. The time to achieve a 99.9% bacterial load reduction was determined by linear regression (r2 0.95) or by visual inspection. The fAUC/MIC for vancomycin and its relationship to reduction in cfu/mL was evaluated.
Antimicrobial agent concentrations
Pharmacokinetic samples were obtained through the injection port over 72 h for verification of target antibiotic concentrations. The half-life, fAUC/MIC0–24 and free peak and trough concentrations of vancomycin were determined by the trapezoidal method utilizing PK Analyst software (Version 1.10, MicroMath Scientific Software, Salt Lake City, UT, USA).
Vancomycin concentrations were determined by fluorescence polarization immunoassay (Abbott Diagnostics TDx). This assay has a limit of detection for vancomycin of 2.0 mg/L with a between day percentage coefficient of variation for high (75 mg/L), medium (35 mg/L) and low (7 mg/L) standards of <5% (r2 = 0.99). All samples were stored at –70°C until ready for analysis.
Development of resistance was evaluated at 0, 8, 24, 48 and 72 h. Fifty microlitre samples from each time point in all models were plated on BHI agar screening plates containing vancomycin concentrations of three and six times the respective MIC. Plates were then examined for growth after 48 h of incubation at 35°C. Bacterial growth on the resistance screening plates was verified for changes in MIC values by Etest.
Changes in cfu/mL at 24, 48 and 72 h were compared by two-way analysis of variance (ANOVA) with Tukey's post hoc test. A P value of 
0.05 was considered significant. All statistical analyses were performed using SPSS Statistical Software (Release 14, SPSS, Inc., Chicago, IL, USA).
|
Results |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
All isolates were initially susceptible to vancomycin with an MIC/MBC for Mu3 and hVISA 1629 of 2/4 mg/L. Both of these isolates demonstrated the hVISA phenotype by both MET and PAP. The clinical non-hVISA control strain had an MIC/MBC of 0.5/0.5 mg/L. The pharmacokinetics of the simulated dosing regimens is listed in Table 1 and is comparable to predicted values.
|
The activity of vancomycin was highly dependent on the bacterial inoculum. Figure 1 displays the activity of vancomycin with a range of dosing regimens and inocula against hVISA strains over 72 h. Using an inoculum of 108–109 cfu/mL against both hVISA isolates, vancomycin displayed bactericidal activity at 24 h in the majority of dosing regimens regardless of organism tested. However, additional killing effect beyond 24 h of exposure was minimal. Minimal kill was observed even with supratherapeutic vancomycin doses as high as 5 g every 12 h (fAUC/MIC 799). Also, there was no dose-dependent killing response with any organism at this inoculum. Vancomycin demonstrated killing activity similar to that observed with the hVISA strains against the non-hVISA strain at this high inoculum (Figure 2). Bactericidal activity was also achieved by 24 h of vancomycin therapy with attenuation of activity after this time with doses ranging from 1000 to 4000 mg every 12 h (fAUC/MIC 552–2576). Although bactericidal activity was achieved against all strains, regardless of the presence of hVISA, a high bacterial load of
105 organisms remained by the end of the 72 h.
|
|
Vancomycin simulated doses were also analysed against test strains at a lower inoculum (
106 cfu/mL). The impact of hVISA expression on vancomycin activity was more evident at this inoculum. Rapid bactericidal activity was achieved in non-hVISA strain 3286 by 2 h with vancomycin doses ranging from 1000 to 2000 mg every 12 h (fAUC/MIC 552–1084). Time to achieve bactericidal activity with this strain was much more rapid at the lower inoculum compared with higher inoculum (1.9 versus 27.8 h, P < 0.001). Although vancomycin-killing effects were noted up to 8 h of therapy, minor regrowth (1 log) was demonstrated from 24 to 72 h in all vancomycin doses with this isolate. The activity of vancomycin in hVISA strains at this inoculum was reduced compared with the non-hVISA strain with doses of 1000 to 2000 mg every 12 h (fAUC/MIC 138–271). In both hVISA isolates, bactericidal activity was not achieved and regrowth of 1 log was noted from 24–48 h. A significant difference was noted in T99 between the two hVISAs, Mu3 (14.1 h) and 1629 (15.3 h), and the non-hVISA MRSA 3286 (1.9 h) (P = 0.001). The various vancomycin doses in all three isolates were also analysed for the impacts on MIC post exposure. No MIC changes were noted for any test strain at the lower inoculum. Although increasing dose exposure did not impact the killing activity of vancomycin against hVISA at high inoculum, increased doses prevented the development of MIC increases. The most dramatic increases in MICs occurred in the two hVISA isolates evaluated, displayed in Table 1. In these isolates, MIC increases up to 8 mg/L (4-fold increase) and 4 mg/L (2-fold increase) were noted with 750 mg every 12 h (fAUC/MIC 105) in isolates Mu3 and 1629, respectively. Increasing the dose up to 1750 mg every 12 h (fAUC/MIC 258) did not prevent susceptibility changes in MRSA 1629 (2-fold increase). However, at doses of 2000 mg every 12 h (fAUC/MIC 271) and higher, there was no change in susceptibility noted over the 72 h period. The effect on MIC changes was even more pronounced in isolate Mu3. A standard clinical dose of 1000 mg every 12 h (fAUC/MIC 168) resulted in MIC changes up to 8 mg/L by the end of therapy. Even doses as high as 2000 mg every 12 h (fAUC/MIC 269) could not prevent the emergence of the VISA phenotype, with MICs rising as high as 6 mg/L. Susceptibility changes were minimal with simulated doses of 2250 mg every 12 h (fAUC/MIC 317) resulting in a 1.5-fold MIC increase (3 mg/L). The MIC was stabilized in Mu3 at 2 mg/L with doses of 2500 mg every 12 h (fAUC/MIC 374) or greater. Although no resistance was noted with doses as high as 5 g every 12 h (fAUC/MIC 799) in hVISA, this regimen had minimal activity on overall inoculum reduction and was similar in activity to 1 g every 12 h against these strains.
Vancomycin exposure in the non-hVISA 3286 did not result in susceptibility changes as noted with the two hVISA isolates evaluated. Even though the activity of vancomycin was minimal against this isolate at high inoculum, its MIC remained at 0.5 mg/L throughout the 72 h dosing interval for all tested regimens, as noted in Table 1. Vancomycin simulations against strain 3286 at lower inoculum resulted in rapid bactericidal activity followed by regrowth after 24 h. However, this growth did not result in the recovery of subsequent isolates with MICs beyond 0.5 mg/L. Overall, vancomycin displayed increased activity at lower inoculum and suppressed the emergence of resistance in the non-hVISA strain.
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
Although the prevalence of hVISA in staphylococci remains low, this number may be underrepresented due to a previous lack of clinical reporting and a feasible testing method in the clinical microbiology laboratory. Automated susceptibility devices used in clinical laboratories are unable to detect hVISA due to only a small subpopulation displaying increased MICs. The gold-standard for detection of hVISA has traditionally been by population susceptibility analysis screening, which is time-consuming and labour-intensive. However, a recent study by Wootton et al.20 demonstrated that the MET method is both sensitive and specific for detecting hVISA, which is a more reasonable approach to test clinically. The isolates evaluated in our study were appropriately screened for hVISA by MET and population analyses.
A previous study using pulse-field electrophoresis with Mu3 revealed that this strain is genetically related to Mu50, the first VISA from Japan, and therefore represents an intermediate step from vancomycin susceptible S. aureus to VISA.18 In our in vitro model study with Mu3, we were able to reproduce VISA-like characteristics in dosing regimens ranging from 750–2250 mg every 12 h (fAUC/MIC 105–317) against a simulated high-inoculum infection resulting in up to a 4-fold MIC change. This observation was also confirmed with another clinical hVISA strain (1629), resulting in lower, but still relevant, changes in MIC with therapeutic-dosing simulations. Interestingly, MIC changes were not displayed in the non-VISA clinical isolate evaluated with similar vancomycin doses. Further, MIC increases in hVISA only occurred with the high-inoculum simulations, whereas the MICs at the lower inoculum tested remained stable. We attribute this to the 1000 times greater organism burden in our high-inoculum simulation, resulting in an increased probability of expression of a heterogeneous population.
The vancomycin killing activity in our in vitro model appeared to be significantly affected by inoculum. This effect has been previously demonstrated for vancomycin in vitro and in vivo against non-hVISA MRSA.14,21 Although bactericidal activity was achieved in both the moderate and high inoculum simulations, the former resulted in more rapid bactericidal activity close to detection limits (102 cfu/mL). The high-inoculum simulations in hVISA often resulted in 105 cfu/mL remaining at the end of a wide range of 72 h vancomycin regimens (750–5000 mg every 12 h; fAUC/MIC 105–799). The inoculum effect, combined with our findings of an increased probability of reduced susceptibility in hVISA isolates at this inoculum under vancomycin pressure, presents potential therapeutic hurdles to successful vancomycin treatment of hVISA. The clinical reports of hVISA would support this theory, since these clinical failures have corresponded to high bacterial load infections and complicated and persistent bacteraemia. Also, many of these reports involve infections such as pneumonia, prosthetic-device infection and endocarditis, where penetration of vancomycin may be suboptimal and increase the chance of failure.5,6,11 In non-hVISA isolates, clinical vancomycin MICs have been noted to play an important role in vancomycin response. Recently, Hidayat et al., reported a decreased response rate in S. aureus-infected patients with an MIC of 2 mg/L (62%) compared with an MIC of 1 mg/L (85%). Another study noted similar findings with a longer time to clearance of MRSA bacteraemia in isolates with a vancomycin MIC of 2 mg/L.17 However, it is unknown whether this relationship with time to clearance of MRSA bacteraemia is true in hVISA strains.15
Overall, we were able to demonstrate that vancomycin at doses ranging from 750–2250 mg every 12 h (fAUC/MIC 105–317) had minimal activity against clinical hVISA in an in vitro model and resulted in reduced vancomycin susceptibility in a high-inoculum simulation. More rapid bactericidal activity at a 100-fold lower inoculum was displayed with vancomycin against a clinical non-hVISA isolate with no changes in susceptibility. Our data suggest that vancomycin has no activity against hVISA at high inoculum and leads to the emergence of reduced susceptibilities. The emergence of hVISA and reports of clinical vancomycin failure support the notion of further investigation into novel vancomycin dosing strategies as well as alternative antimicrobial agents to treat this invasive pathogen.
|
Funding |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
There was no external funding of any kind to support this work.
|
Transparency declarations |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
None to declare.
|
Acknowledgements |
|---|
A portion of this work was presented at the Forty-seventh Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, IL, USA, 2007.
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionFundingTransparency declarationsReferences |
|---|
1 . Moran GJ, Krishnadasan A, Gorwitz RJ, et al. Methicillin-resistant S. aureus infections among patients in the emergency department. N Engl J Med (2006) 355:666–74.
2
.
Klevens RM, Morrison MA, Nadle J, et al. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA (2007) 298:1763–71.
3 . Tenover FC, Biddle JW, Lancaster MV. Increasing resistance to vancomycin and other glycopeptides in Staphylococcus aureus. Emerg Infect Dis (2001) 7:327–32.[Web of Science][Medline]
4 . Fridkin SK, Hageman J, McDougal LK, et al. Epidemiological and microbiological characterization of infections caused by Staphylococcus aureus with reduced susceptibility to vancomycin, United States, 1997–2001. Clin Infect Dis (2003) 36:429–39.[CrossRef][Web of Science][Medline]
5 . Charles PG, Ward PB, Johnson PD, et al. Clinical features associated with bacteremia due to heterogeneous vancomycin-intermediate Staphylococcus aureus. Clin Infect Dis (2004) 38:448–51.[CrossRef][Web of Science][Medline]
6
.
Maor Y, Rahav G, Belausov N, et al. Prevalence and characteristics of heteroresistant vancomycin-intermediate Staphylococcus aureus bacteremia in a tertiary care center. J Clin Microbiol (2007) 45:1511–4.
7 . Rybak MJ, Cha R, Cheung CM, et al. Clinical isolates of Staphylococcus aureus from 1987 and 1989 demonstrating heterogeneous resistance to vancomycin and teicoplanin. Diagn Microbiol Infect Dis (2005) 51:119–25.[CrossRef][Web of Science][Medline]
8
.
Bert F, Clarissou J, Durand F, et al. Prevalence, molecular epidemiology, and clinical significance of heterogeneous glycopeptide-intermediate Staphylococcus aureus in liver transplant recipients. J Clin Microbiol (2003) 41:5147–52.
9 . Rybak M, Chin J, Lau K, et al. Increasing prevalence of glycopeptide hetero-resistant S. aureus from the Detroit metropolitan area over a 20-year period (1986–2006). In: Abstracts of the Seventeenth European Congress of Clinical Microbiology and Infectious Diseases, Munich, Germany, 2007. Basel, Switzerland: European Society of Clinical Microbiology and Infectious Diseases. 37. Abstract O32.
10 . LaPlante KL, Rybak MJ. Clinical glycopeptide-intermediate staphylococci tested against arbekacin, daptomycin, and tigecycline. Diagn Microbiol Infect Dis (2004) 50:125–30.[CrossRef][Web of Science][Medline]
11 . Howden BP, Ward PB, Charles PG, et al. Treatment outcomes for serious infections caused by methicillin-resistant Staphylococcus aureus with reduced vancomycin susceptibility. Clin Infect Dis (2004) 38:521–8.[CrossRef][Web of Science][Medline]
12 . Deresinski S. Counterpoint: vancomycin and Staphylococcus aureus—an antibiotic enters obsolescence. Clin Infect Dis (2007) 44:1543–8.[CrossRef][Web of Science][Medline]
13 . Moise-Broder PA, Forrest A, Birmingham MC, et al. Pharmacodynamics of vancomycin and other antimicrobials in patients with Staphylococcus aureus lower respiratory tract infections. Clin Pharmacokinet (2004) 43:925–42.[CrossRef][Web of Science][Medline]
14 . Craig WA, Andes DR. In vivo pharmacodynamics of vancomycin against VISA, heteroresistant VISA, and VSSA in the neutropenic murine thigh-infection model. In: Abstracts of the Forty-sixth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, USA, 2006. Washington, DC, USA: American Society for Microbiology. 16. Abstract A-644.
15
.
Hidayat LK, Hsu DI, Quist R, et al. High-dose vancomycin therapy for methicillin-resistant Staphylococcus aureus infections: efficacy and toxicity. Arch Intern Med (2006) 166:2138–44.
16
.
Sakoulas G, Moise-Broder PA, Schentag J, et al. Relationship of MIC and bactericidal activity to efficacy of vancomycin for treatment of methicillin-resistant Staphylococcus aureus bacteremia. J Clin Microbiol (2004) 42:2398–402.
17
.
Moise PA, Sakoulas G, Forrest A, et al. Vancomycin in vitro bactericidal activity and its relationship to efficacy in clearance of methicillin-resistant Staphylococcus aureus bacteremia. Antimicrob Agents Chemother (2007) 51:2582–6.
18 . Hiramatsu K, Aritaka N, Hanaki H, et al. Dissemination in Japanese hospitals of strains of Staphylococcus aureus heterogeneously resistant to vancomycin. Lancet (1997) 350:1670–3.[CrossRef][Web of Science][Medline]
19 . Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing-Sixteenth Edition: Approved Standard M100-S16 (2006) Wayne, PA, USA: CLSI.
20
.
Wootton M, MacGowan AP, Walsh TR, et al. A multicenter study evaluating the current strategies for isolating Staphylococcus aureus strains with reduced susceptibility to glycopeptides. J Clin Microbiol (2007) 45:329–32.
21
.
LaPlante KL, Rybak MJ. Impact of high-inoculum Staphylococcus aureus on the activities of nafcillin, vancomycin, linezolid, and daptomycin, alone and in combination with gentamicin, in an in vitro pharmacodynamic model. Antimicrob Agents Chemother (2004) 48:4665–72.
22 . Rybak MJ. The pharmacokinetic and pharmacodynamic properties of vancomycin. Clin Infect Dis (2006) 42(Suppl 1):S35–9.[CrossRef][Web of Science][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||