Daptomycin MICs Research Articles

Is prediffusion test an alternative to improve accuracy in screening hVISA strains and to detect susceptibility to glycopeptides/lipopeptides?

The characterization of heteroresistant vancomycin-intermediate Staphylococcus aureus strains (hVISA) is even more challenging, as no routine standardized laboratory methods are available. A total of 124 S. aureus isolates recovered from inpatients attended in hospitals of Santa Catarina State, Southern Brazil, were evaluated. The MIC of vancomycin, teicoplanin, and daptomycin was determined by Etest and prediffusion tests using NeoSensitabs® tablets. All isolates were susceptible to vancomycin (MICs: 0.5–3 μg/mL) by Etest. However, according to prediffusion test, 17 isolates presented reduced susceptibility to vancomycin, and of these, 12 were confirmed as hVISA using populational analysis. Considering daptomycin, prediffusion results were in agreement with susceptibility data (MICs), as all isolates were susceptible. Considering that characterizing hVISA is challenging and that MIC determination is not adequate to characterize this phenotype, prediffusion test was a viable alternative to screening hVISA and reduced susceptibility to vancomycin. It was simple and low cost, with accuracy comparable to other well-established methods.

Penetration of daptomycin into bone and synovial fluid in joint replacement.

Daptomycin exhibits clinical activity in the treatment of infections with Gram-positive organisms, including infections due to methicillin-resistant Staphylococcus aureus. However, little is known about its penetration into bone and synovial fluid. The aim of our study was to assess the penetration of daptomycin into bone and synovial fluid after a single intravenous administration. This study was conducted in 16 patients who underwent knee or hip replacement and received a single intravenous dose of 8 mg of daptomycin per kg of body weight prior to surgery. Plasma daptomycin concentrations were measured 1 h after the end of daptomycin infusion and when bone fragments were removed. Daptomycin concentrations were also measured on bone fragments and synovial fluid collected at the same time during surgery. All samples were analyzed with a diode array-high-performance liquid chromatography (HPLC) method. After a single-dose intravenous infusion, bone daptomycin concentrations were above the MIC of daptomycin for Staphylococcus aureus in all subjects, and the median bone penetration percentage was 9.0% (interquartile range [IQR], 4.4 to 11.4). These results support the use of daptomycin in the treatment of Staphylococcus aureus bone and joint infections.

Evaluation of the novel combination of daptomycin plus ceftriaxone against vancomycin-resistant enterococci in an in vitro pharmacokinetic/pharmacodynamic simulated endocardial vegetation model.

Daptomycin has demonstrated synergy with β-lactams against Enterococcus faecium and this combination has been used successfully to treat infections refractory to daptomycin. We investigated daptomycin alone and combined with ceftriaxone against vancomycin-resistant enterococci (VRE) in an in vitro pharmacokinetic/pharmacodynamic simulated endocardial vegetation (SEV) model. Daptomycin (6 and 12 mg/kg/day) with and without 2 g of ceftriaxone every 24 h were evaluated against two clinical E. faecium strains (8019 and 5938) and one Enterococcus faecalis (6981) in a 96 h in vitro pharmacokinetic/pharmacodynamic SEV model. FITC-labelled poly-l-lysine was used to assess β-lactam-induced changes in cell surface charge. For 8019 and 6981, daptomycin 6 mg/kg with ceftriaxone and daptomycin 12 mg/kg alone and in combination with ceftriaxone displayed significantly more activity than daptomycin 6 mg/kg alone from 48 to 96 h (P ≤ 0.005). The addition of ceftriaxone significantly enhanced activity of daptomycin 6 mg/kg against both strains at 96 h (8019, reductions -0.55 versus 3.64 log10 cfu/g; 6981, reductions 1.11 versus 5.67 log10 cfu/g; P < 0.001) and improved daptomycin 12 mg/kg against 8019 at 96 h. Daptomycin 12 mg/kg plus ceftriaxone displayed no appreciable activity against 5938 (daptomycin MIC 32 mg/L). Daptomycin non-susceptibility developed in 8019 and 6981 versus daptomycin 6 mg/kg by 96 h. Ampicillin or ceftriaxone exposure reduced daptomycin surface charge in 8019, resulting in significantly increased FITC-poly-l-lysine binding. The combination of daptomycin and ceftriaxone may be promising for eradicating high-inoculum, deep-seated enterococcal infections. Further research is warranted to examine the enhancement of daptomycin and innate immunity killing of VRE by ceftriaxone and other β-lactams.

Characterization of antibiotic resistance in Listeria spp. isolated from slaughterhouse environments, pork and human infections.

Listeria species are susceptible to most antibiotics. However, over the last decade, increasing reports of multidrug-resistant Listeria spp. from various sources have prompted public health concerns. The objective of this study was to characterize the antibiotic susceptibility of Listeria spp. and the genetic mechanisms that confer resistance. Forty-six Listeria spp. isolates were studied, and their minimal inhibitory concentrations of antibiotics were determined by microdilution using Sensititre standard susceptibility MIC plates. The isolates were screened for the presence of gyrA, parC, lde, lsa(A), lnu(A), and mprF by PCR, and the amplified genes were sequenced. All isolates were susceptible to penicillin, ampicillin, tetracycline, erythromycin, and carbapenems. Resistance to clindamycin, daptomycin, and oxacillin was found among L. monocytogenes and L. innocua, and all species possessed at least intermediate resistance to fluoroquinolones. GyrA, parC, and mprF were detected in all isolates; however, mutations were found only in gyrA sequences. A high daptomycin MIC, as reported previously, was observed, suggesting an intrinsic resistance of Listeria spp. to daptomycin. These results are consistent with reports of emerging resistance in Listeria spp. and emphasize the need for further genotypic characterization of antibiotic resistance in this genus.

Comparison of Commercial Antimicrobial Susceptibility Test Methods for Testing of Staphylococcus aureus and Enterococci against Vancomycin, Daptomycin, and Linezolid

Three commercial antimicrobial susceptibility testing (AST) methods were compared to broth microdilution for testing of Staphylococcus aureus and enterococci against vancomycin, daptomycin, and linezolid. Despite high levels of categorical agreement and essential agreement, vancomycin MICs determined by MicroScan were often 1 log2 concentration higher and MICs determined by Phoenix 1 log2 concentration lower. Daptomycin MICs were 1 to 2 log2 concentrations higher by all AST methods, except Etest, potentially impacting definitive antimicrobial therapy for bloodstream infections due to these organisms.

Human Cathelicidin LL-37 Resistance and Increased Daptomycin MIC in Methicillin-Resistant Staphylococcus aureus Strain USA600 (ST45) Are Associated with Increased Mortality in a Hospital Setting

Bacteremia caused by methicillin-resistant Staphylococcus aureus (MRSA) USA600 has been associated with increased patient mortality. We found that USA600 MRSA exhibited significantly increased resistance to human cathelicidin LL-37 killing and daptomycin MIC creep compared to non-USA600 MRSA. Virulent health care-associated MRSA strains may coevolve innate host defense peptide and antibiotic resistances.

Daptomycin activity tested against 164 457 bacterial isolates from hospitalised patients: Summary of 8 years of a Worldwide Surveillance Programme (2005–2012)

We report the results of 8 years (2005–2012) of the Daptomycin Surveillance Programme Worldwide. Consecutive non-duplicate bacterial isolates (prevalence design) were collected from patients with documented infections in 410 medical centres and were susceptibility tested by reference broth microdilution methods. A total of 164457 Gram-positive isolates were evaluated, including 97542 Staphylococcus aureus, 21413 coagulase-negative staphylococci (CoNS), 29619 enterococci and 15883 β-haemolytic streptococci. The prevalence of daptomycin-non-susceptible isolates was extremely low for all species in all geographic regions. Overall, the highest occurrence of non-susceptible isolates was observed among CoNS (0.19%), followed by Enterococcus faecium (0.18%), S. aureus (0.05%), Enterococcus faecalis (0.02%) and β-haemolytic streptococci (0.00%). Moreover, no trend towards increased daptomycin resistance (non-susceptibility) was observed for any species in any geographic region during the study interval. Against S. aureus, the daptomycin MIC50/90 was 0.25/0.5mg/L in all geographic regions (99.95% susceptible overall). Only 53 daptomycin-non-susceptible S. aureus isolates were observed and the vast majority (49; 92.5%) had a daptomycin MIC value only 1log2 dilution above the published susceptible breakpoint. Daptomycin was also active against CoNS (MIC50/90, 0.25/0.5mg/L; 99.81% susceptible), E. faecalis (MIC50/90, 1/2mg/L; 99.98% susceptible), E. faecium (MIC50/90, 2/4mg/L; 99.82% susceptible) including vancomycin-non-susceptible isolates (4521 isolates; MIC50/90, 2/2mg/L; 99.76% susceptible), and β-haemolytic streptococci (MIC50/90, ≤0.06/0.25mg/L; 100.0% susceptible). In conclusion, daptomycin has remained very active against indicated species worldwide, and no significant year-to-year or regional variation in daptomycin activity has been detected.

Efficacy of daptomycin in the treatment of enterococcal endocarditis: a 5 year comparison with conventional therapy

Enterococcus spp. account for 10% of infective endocarditis (IE). Although daptomycin is a bactericidal drug with in vitro activity against Enterococcus, there is little experience of its use in IE. We analysed the effectiveness of daptomycin in the treatment of enterococcal IE (EIE). This was a retrospective descriptive study comparing the efficacy of daptomycin versus ampicillin/ceftriaxone versus conventional antibiotic regimens (ampicillin or vancomycin ± gentamicin) in EIE. From January 2007 to December 2011, 6 patients with EIE treated with daptomycin monotherapy were compared with 21 patients treated with ampicillin/ceftriaxone and 5 patients treated with ampicillin or vancomycin ± gentamicin. The three groups had similar epidemiological and clinical characteristics. Daptomycin indications were allergy to β-lactams (n = 3), therapy simplification (n = 2), renal failure (n = 2) and Enterococcus faecium resistant to ampicillin/gentamicin (n = 1). Daptomycin MICs ranged from 1 to 2 mg/L and the doses were 6-10 mg/kg/day intravenously. Daptomycin patients had longer duration of bacteraemia (6 versus 1 day, P < 0.01) and greater need of therapy switch due to complications (66.7% versus 0%, P < 0.01). There were no differences regarding duration of hospital stay or mortality. Daptomycin-treated patients more frequently required a therapeutic change due to worse microbiological and clinical response, although mortality was not increased. Our findings do not support the use of daptomycin as single therapy in the treatment of EIE. Its role in combined strategies should be further investigated.

Colistin/daptomycin: an unconventional antimicrobial combination synergistic in vitro against multidrug-resistant Acinetobacter baumannii

The in vitro activity of the combination colistin/daptomycin was evaluated against multidrug-resistant Acinetobacter baumannii clinical isolates. Clonal relationships were assessed by pulsed-field gel electrophoresis. The following synergy studies were undertaken: (i) daptomycin MICs were determined by E-test on Mueller–Hinton agar plates supplemented with a subinhibitory concentration of colistin; and (ii) time–kill methodology using tubes containing an inoculum of 5×105CFU/mL and subinhibitory concentrations of each antibiotic alone or in combination subcultured at 0, 5 and 24h for colony counting. Synergy was defined as ≥2log10CFU/mL decrease of viable colonies compared with colistin alone. Ten colistin-susceptible and four colistin-resistant A. baumannii isolates were tested. Isolates were assigned to nine different clonal types. Enhanced in vitro activity of the combination was detected only against colistin-susceptible isolates; using plates supplemented with colistin, the daptomycin MIC was reduced by 4- to 128-fold. From a total of 30 isolate–concentration combinations in time–kill studies, a synergistic interaction was detected in 16 (53.3%). The combination exhibited synergy against 8 and 12 of these combinations at 5h and 24h, respectively. No antagonism was detected. Colistin alone was bactericidal against two colistin-susceptible isolates at 24h, whereas the combination was bactericidal against 9 colistin-susceptible isolates at 24h. Against all colistin-resistant isolates, the combination exhibited a static effect and indifference in time–kill studies. Potent in vitro synergistic interactions between colistin and daptomycin provide evidence that this unorthodox combination may be beneficial in the treatment of colistin-susceptible multidrug-resistant A. baumannii.

In Vitro Activity of Daptomycin Against VRE and MRSA Strains

Infections with multidrug-resistant (MDR) gram-positive bacteria are gradually increasing in the world. Methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) are among the leading pathogens isolated from nosocomial infections. The increased use of vancomycin in the treatment has led to the development of isolates with reduced susceptibility and resistance. Daptomycin is a novel lipopeptide antibiotic which has a rapid bactericidal activity against infections caused by gram-positive bacteria including MRSA and VRE. The aim of this study was to evaluate the in vitro activity of daptomycin against VRE and MRSA strains isolated from clinical samples of hospitalized patients. A total of 229 isolates of which 118 were VRE and 111 were MRSA have been included in the study. All of the VRE strains were isolated from rectal swab samples and two of them were also resistant to linezolid with the MIC values of 8 µg/ml and 12 µg/ml. Bacterial identification was performed by conventional methods and susceptibility testing of daptomycin was performed by using the broth microdilution method as recommended by CLSI. The MIC values were interpreted according to CLSI susceptibility criteria for daptomycin. Daptomycin MIC50 and MIC90 values for VRE were found as 1 µg/ml and 2 µg/ml, respectively, with a MIC range of 0.125-2 µg/ml. Daptomycin MIC50 and MIC90 values for MRSA were determined as 0.12 µg/ml and 0.5 µg/ml, respectively, and the overall distribution of MIC values ranged between ≤ 0.032-1 µg/ml. All VRE including two linezolid-resistant strains and all MRSA strains (100%) were found susceptible to daptomycin. It was concluded that daptomycin is one of the few alternative agents for the treatment of infections caused by VRE and MRSA, nevertheless it was suggested that daptomycin MICs should better be monitored to prevent treatment failure due to the presence of non-susceptible strains and possible emergence of strains with decreased susceptibility during long-term therapy.

Importance and problems of quality control during daptomycin MIC testing using Etest methodology

Importance and problems of quality control during daptomycin MIC testing using Etest methodology

Vancomycin and daptomycin minimum inhibitory concentration distribution and occurrence of heteroresistance among methicillin-resistant Staphylococcus aureus blood isolates in Turkey

BackgroundThe aim of this study was to determine the distribution of vancomycin and daptomycin MICs among methicillin-resistant Staphylococcus aureus (MRSA) blood isolates, the prevalence of heterogeneous vancomycin-intermediate S. aureus (hVISA) and the relationship between hVISA and vancomycin MIC values.MethodsA total of 175 MRSA blood isolates were collected from seven university hospitals in Turkey. All isolates were tested for susceptibility to vancomycin and daptomycin by reference broth microdilution (BMD) and by standard Etest method. BMD test was performed according to CLSI guidelines and Etest was performed according to the instructions of the manufacturer. All isolates were screened for the presence of the hVISA by using macro Etest (MET) and population analysis profile-area under the curve (PAP-AUC) methods.ResultsThe vancomycin MIC50, MIC90 and MIC ranges were 1, 2, and 0.5-2 μg/ml, respectively, by both of BMD and Etest. The daptomycin MIC50, MIC90 and MIC ranges were 0.5, 1 and 0.125 -1 μg/ml by BMD and 0.25, 0.5 and 0.06-1 μg/ml by Etest, respectively. The vancomycin MIC for 40.6% (71/175) of the MRSA isolates tested was >1 μg/ml by BMD. No vancomycin and daptomycin resistance was found among MRSA isolates. Percent agreement of Etest MICs with BMD MICs within ±1 doubling dilution was 100% and 73.1% for vancomycin and daptomycin, respectively. The prevalence of hVISA among MRSA blood isolates was 13.7% (24/175) by PAP-AUC method. MET identified only 14 of the hVISA strains (sensitivity, 58.3%), and there were 12 strains identified as hVISA that were not subsequently confirmed by PAP-AUC (specificity, 92.1%).ConclusionsAgreement between BMD and Etest MICs is high both for vancomycin and daptomycin. Daptomycin was found to be highly active against MRSA isolates including hVISA. A considerable number of isolates are determined as hVISA among blood isolates. As it is impractical to use the reference method (PAP-AUC) for large numbers of isolates, laboratory methods for rapid and accurate identification of hVISA need to be developed.

A Current Perspective on Daptomycin for the Clinical Microbiologist

Daptomycin is a lipopeptide antimicrobial with in vitro bactericidal activity against Gram-positive bacteria that was first approved for clinical use in 2004 in the United States. Since this time, significant data have emerged regarding the use of daptomycin for the treatment of serious infections, such as bacteremia and endocarditis, caused by Gram-positive pathogens. However, there are also increasing reports of daptomycin nonsusceptibility, in Staphylococcus aureus and, in particular, Enterococcus faecium and Enterococcus faecalis. Such nonsusceptibility is largely in the context of prolonged treatment courses and infections with high bacterial burdens, but it may occur in the absence of prior daptomycin exposure. Nonsusceptibility in both S. aureus and Enterococcus is mediated by adaptations to cell wall homeostasis and membrane phospholipid metabolism. This review summarizes the data on daptomycin, including daptomycin's unique mode of action and spectrum of activity and mechanisms for nonsusceptibility in key pathogens, including S. aureus, E. faecium, and E. faecalis. The challenges faced by the clinical laboratory in obtaining accurate susceptibility results and reporting daptomycin MICs are also discussed.

Daptomycin Activity against Uncommonly Isolated Streptococcal and Other Gram-Positive Species Groups

A total of 1,356 clinical isolates were tested against daptomycin by broth microdilution methods. Daptomycin was active against seven groups of viridans group streptococci (MIC50 and MIC90 values ranging from ≤0.06 and ≤0.06 μg/ml [Streptococcus bovis and Streptococcus dysgalactiae] to 0.5 and 1 μg/ml [Streptococcus mitis, Streptococcus oralis, and Streptococcus parasanguinis], respectively), beta-hemolytic streptococci serogroups C, F, and G (MIC50 and MIC90, ≤0.06 to 0.25 and 0.12 to 0.25 μg/ml, respectively), Corynebacterium spp. (MIC50 and MIC90, ≤0.06 and 0.12 μg/ml, respectively), and Micrococcus spp. (MIC50 and MIC90, ≤0.06 and 0.25 μg/ml, respectively). Listeria monocytogenes exhibited higher daptomycin MICs (MIC50 and MIC90, 2 and 4 μg/ml, respectively) than other tested organisms.

Evaluation of the combination of daptomycin and nafcillin against vancomycin-intermediate Staphylococcus aureus

Continued selective pressure from glycopeptide use has led to non-susceptible strains of Staphylococcus aureus, including vancomycin-intermediate S. aureus (VISA). Though relatively uncommon, VISA presents a particularly difficult clinical challenge when it arises. Pertinent to this investigation is the correlation between vancomycin intermediacy and daptomycin non-susceptibility. The aim of this study was to evaluate the potential for synergy between daptomycin and nafcillin against VISA. Twenty VISA strains were evaluated for daptomycin and nafcillin MICs by broth microdilution in duplicate. Potential for synergy was assessed by time-kill at 0.5× MIC in triplicate. Four strains displaying synergy in time-kill analysis were analysed in an in vitro pharmacokinetic (PK)/pharmacodynamic (PD) model in duplicate over 72 h. In time-kill experiments, 55% of strains (11/20) displayed synergy with the combination. In the PK/PD model, no differences between daptomycin-alone and combination regimens were observed for the strain with the lowest daptomycin MIC (0.5 mg/L). For the strain with a daptomycin MIC of 1 mg/L, 6 mg/kg daptomycin+nafcillin was superior to 6 mg/kg daptomycin alone (P=0.002) and 10 mg/kg daptomycin+nafcillin was superior to all other regimens (P ≤ 0.004). When the daptomycin MIC increased to 2 mg/L, 10 mg/kg daptomycin+nafcillin was superior to 6 mg/kg daptomycin+nafcillin, which was superior to both 6 and 10 mg/kg daptomycin alone (P ≤ 0.019). Daptomycin and nafcillin in combination significantly improved antibacterial activity against VISA. This effect was more pronounced as the daptomycin susceptibility of the strain declined.

Daptomycin Concentrations in Valve Tissue and Vegetation in Patients with Bacterial Endocarditis

In a patient with mitral-aortic native-valve Streptococcus oralis endocarditis, daptomycin concentrations in aortic and mitral valves were 8.6 and 30.8 μg/g, respectively, and 26 μg/g in the mitral vegetation. In the case of porcine-aortic-valve Staphylococcus epidermidis endocarditis, the daptomycin concentrations were 53.1 μg/g in the valve and 18.1 μg/g in perivalvular tissues. Daptomycin achieved apparently adequate tissue concentrations. S. epidermidis was eradicated, whereas Streptococcus oralis persisted, and its daptomycin MIC displayed a 4-fold increase.

Efficacy and safety of intravenous daptomycin in Japanese patients with skin and soft tissue infections

Daptomycin is a lipopeptide antibiotic active against gram-positive organisms and recently approved for marketing in Japan. This study investigates the efficacy and safety of daptomycin in Japanese patients with skin and soft tissue infections (SSTIs) caused by methicillin-resistant Staphylococcus aureus (MRSA) for regulatory filing in Japan. Overall, 111 Japanese patients with SSTI were randomized in this open-label, randomized, active-comparator controlled, parallel-group, multicenter, phase III study. Patients received intravenous daptomycin 4 mg/kg once daily or vancomycin 1 g twice daily for 7–14 days. Efficacy was determined by a blinded Efficacy Adjudication Committee. Among patients with SSTIs caused by MRSA, 81.8 % (95 % CI, 69.1–90.9) of daptomycin recipients and 84.2 % (95 % CI, 60.4–96.6) of vancomycin recipients achieved a successful clinical response at the test-of-cure (TOC) visit. The microbiological success rate against MRSA at the TOC visit was 56.4 % (95 % CI, 42.3–69.7) with daptomycin and 47.4 % (95 % CI, 24.4–71.1) with vancomycin. Daptomycin was generally well tolerated; most adverse events were of mild to moderate severity. The measurement of daptomycin concentration in plasma revealed that patients with mild or moderate impaired renal function showed similar pharmacokinetics profiles to patients with normal renal function. Clinical and microbiological responses, stratified by baseline MRSA susceptibility, suggested that patients infected with MRSA of higher daptomycin MIC showed a trend of lower clinical success with a P value of 0.052 by Cochran–Armitage test. Daptomycin was clinically and microbiologically effective for the treatment of MRSA-associated SSTIs in Japanese patients.

False Daptomycin-Nonsusceptible MIC Results by Microscan Panel PC 29 Relative to Etest Results for Staphylococcus aureus and Enterococci

This study correlated the daptomycin MIC results obtained by Microscan and by Etest in Staphylococcus aureus and enterococci and found that the Microscan panel GP 29 had a high rate of false nonsusceptible results. Of the isolates interpreted as nonsusceptible by Microscan, 87% of Staphylococcus aureus, 90% of Enterococcus faecalis, and 88% of Enterococcus faecium isolates were interpreted as susceptible by Etest. In turn, Etest also has false nonsusceptible results compared to reference methods.

Genotypic and Phenotypic Evaluation of the Evolution of High-Level Daptomycin Nonsusceptibility in Vancomycin-Resistant Enterococcus faecium

Whole-genome sequencing and cell membrane studies of three clonal Enterococcus faecium strains with daptomycin MICs of 4, 32, and 192 μg/ml were performed, revealing nonsynonymous single nucleotide variants in eight open reading frames, including those predicted to encode a phosphoenolpyruvate-dependent, mannose-specific phosphotransferase system, cardiolipin synthetase, and EzrA. Membrane studies revealed a higher net surface charge among the daptomycin-nonsusceptible isolates and increased septum formation in the isolate with a daptomycin MIC of 192 μg/ml.

Evaluation of the Novel Combination of High-Dose Daptomycin plus Trimethoprim-Sulfamethoxazole against Daptomycin-Nonsusceptible Methicillin-Resistant Staphylococcus aureus Using an In Vitro Pharmacokinetic/Pharmacodynamic Model of Simulated Endocardial Vegetations

Daptomycin-nonsusceptible (DNS) Staphylococcus aureus is found in difficult-to-treat infections, and the optimal therapy is unknown. We investigated the activity of high-dose (HD) daptomycin plus trimethoprim-sulfamethoxazole de-escalated to HD daptomycin or trimethoprim-sulfamethoxazole against 4 clinical DNS methicillin-resistant S. aureus (MRSA) isolates in an in vitro pharmacokinetic/pharmacodynamic model of simulated endocardial vegetations (10(9) CFU/g). Simulated regimens included HD daptomycin at 10 mg/kg/day for 14 days, trimethoprim-sulfamethoxazole at 160/800 mg every 12 h for 14 days, HD daptomycin plus trimethoprim-sulfamethoxazole for 14 days, and the combination for 7 days de-escalated to HD daptomycin for 7 days and de-escalated to trimethoprim-sulfamethoxazole for 7 days. Differences in CFU/g (at 168 and 336 h) were evaluated by analysis of variance (ANOVA) with a Tukey's post hoc test. Daptomycin MICs were 4 μg/ml (SA H9749-1, vancomycin-intermediate Staphylococcus aureus; R6212, heteroresistant vancomycin-intermediate Staphylococcus aureus) and 2 μg/ml (R5599 and R5563). Trimethoprim-sulfamethoxazole MICs were ≤0.06/1.19 μg/ml. HD daptomycin plus trimethoprim-sulfamethoxazole displayed rapid bactericidal activity against SA H9749-1 (at 7 h) and R6212 (at 6 h) and bactericidal activity against R5599 (at 72 h) and R5563 (at 36 h). A ≥8 log(10) CFU/g decrease was observed with HD daptomycin plus trimethoprim-sulfamethoxazole against all strains (at 48 to 144 h), which was maintained with de-escalation to HD daptomycin or trimethoprim-sulfamethoxazole at 336 h. The combination for 14 days and the combination for 7 days de-escalated to HD daptomycin or trimethoprim-sulfamethoxazole was significantly better than daptomycin monotherapy (P < 0.05) and trimethoprim-sulfamethoxazole monotherapy (P < 0.05) at 168 and 336 h. Combination therapy followed by de-escalation offers a novel bactericidal therapeutic alternative for high-inoculum, serious DNS MRSA infections.