High-level Daptomycin Resistance Research Articles

Defective pgsA contributes to increased membrane fluidity and cell wall thickening in Staphylococcus aureus with high-level daptomycin resistance.

The cationic lipopeptide antimicrobial daptomycin has become an essential tool for combating infections with Staphylococcus aureus that display reduced susceptibility to β-lactams or vancomycin. Since daptomycin's activity is based on interaction with the negatively charged membrane of S. aureus, routes to daptomycin-resistance occur through mutations in the lipid biosynthetic pathway surrounding phosphatidylglycerols and the regulatory systems that control cell envelope homeostasis. Therefore, there are many avenues to achieve daptomycin resistance and several different, and sometimes contradictory, phenotypes of daptomycin-resistant S. aureus, including both increased and decreased cell wall thickness and membrane fluidity. This study is significant because it demonstrates the unexpected influence of a lipid biosynthesis gene, pgsA, on membrane fluidity and cell wall thickness in S. aureus with high-level daptomycin resistance.

Phylogeny, Virulence, and Antimicrobial Resistance Gene Profiles of Enterococcus faecium Isolated from Australian Feedlot Cattle and Their Significance to Public and Environmental Health.

The extent of similarity between E. faecium strains found in healthy feedlot beef cattle and those causing extraintestinal infections in humans is not yet fully understood. This study used whole-genome sequencing to analyse the antimicrobial resistance profile of E. faecium isolated from beef cattle (n = 59) at a single feedlot and compared them to previously reported Australian isolates obtained from pig (n = 60) and meat chicken caecal samples (n = 8), as well as human sepsis cases (n = 302). The E. faecium isolated from beef cattle and other food animal sources neither carried vanA/vanB responsible for vancomycin nor possessed gyrA/parC and liaR/liaS gene mutations associated with high-level fluoroquinolone and daptomycin resistance, respectively. A small proportion (7.6%) of human isolates clustered with beef cattle and pig isolates, including a few isolates belonging to the same sequence types ST22 (one beef cattle, one pig, and two human isolates), ST32 (eight beef cattle and one human isolate), and ST327 (two beef cattle and one human isolate), suggesting common origins. This provides further evidence that these clonal lineages may have broader host range but are unrelated to the typical hospital-adapted human strains belonging to clonal complex 17, significant proportions of which contain vanA/vanB and liaR/liaS. Additionally, none of the human isolates belonging to these STs contained resistance genes to WHO critically important antimicrobials. The results confirm that most E. faecium isolated from beef cattle in this study do not pose a significant risk for resistance to critically important antimicrobials and are not associated with current human septic infections.

Membrane Phenotypic, Metabolic and Genotypic Adaptations of Streptococcus oralis Strains Destined to Rapidly Develop Stable, High-Level Daptomycin Resistance during Daptomycin Exposures.

The Streptococcus mitis-oralis subgroup of viridans group streptococci are important human pathogens. We previously showed that a substantial portion of S. mitis-oralis strains (>25%) are 'destined' to develop rapid, high-level, and stable daptomycin (DAP) resistance (DAP-R) during DAP exposures in vitro. Such DAP-R is often accompanied by perturbations in distinct membrane phenotypes and metabolic pathways. The current study evaluated two S. oralis bloodstream isolates, 73 and 205. Strain 73 developed stable, high-level DAP-R (minimum inhibitory concentration [MIC] > 256 µg/mL) within 2 days of in vitro DAP passage ("high level" DAP-R [HLDR]). In contrast, strain 205 evolved low-level and unstable DAP-R (MIC = 8 µg/mL) under the same exposure conditions in vitro ("non-HLDR"). Comparing the parental 73 vs. 73-D2 (HLDR) strain-pair, we observed the 73-D2 had the following major differences: (i) altered cell membrane (CM) phospholipid profiles, featuring the disappearance of phosphatidylglycerol (PG) and cardiolipin (CL), with accumulation of the PG-CL pathway precursor, phosphatidic acid (PA); (ii) enhanced CM fluidity; (iii) increased DAP surface binding; (iv) reduced growth rates; (v) decreased glucose utilization and lactate accumulation; and (vi) increased enzymatic activity within the glycolytic (i.e., lactate dehydrogenase [LDH]) and lipid biosynthetic (glycerol-3-phosphate dehydrogenase [GPDH]) pathways. In contrast, the 205 (non-HLDR) strain-pair did not show these same phenotypic or metabolic changes over the 2-day DAP exposure. WGS analyses confirmed the presence of mutations in genes involved in the above glycolytic and phospholipid biosynthetic pathways in the 73-D2 passage variant. These data suggest that S. oralis strains which are 'destined' to rapidly develop HLDR do so via a conserved cadre of genotypic, membrane phenotypic, and metabolic adaptations.

Is Corynebacterium striatum an emerging prosthetic joint infection pathogen and how should it be treated?

The aim of this study was to assess the incidence of Corynebacterium striatum prosthetic joint infections (PJI) to determine if an increase has occurred recently. Moreover, susceptibility testing was conducted on C. striatum preserved isolates to determine antibiotic options for these infections. Retrospective review of PJI cases was conducted from 1/2017 through 1/2021 compared to 1/2021 through 7/2022 to determine how many cases of C. striatum have occurred for each of these time points. From these cases, demographics, outcomes and risk factors for C. striatum PJI were recorded. The preserved clinical isolates from these cases were tested for susceptibility to different antibiotics. A statistically significant increase in the proportion of C. striatum PJI cases (1.98 to 7.84, p=0.0489) has occurred over the past 16 months at a single institution. Chronic wounds and exposure to daptomycin were associated with the majority of these cases. Susceptibility testing of the clinical isolates showed uniform susceptibility to vancomycin, linezolid and dalbavancin. Uniform resistance was seen with ciprofloxacin, tetracycline and doxycycline as well. Interestingly, 85.7% of the isolates displayed inducible daptomycin resistance after overnight exposure to daptomycin. C. striatum is an emerging PJI pathogen. It is important for clinicians to be cognizant that this pathogen can have inducible high level daptomycin resistance and that daptomycin is likely not a reliable antibiotic for these infections. While vancomycin and linezolid are the traditional antibiotics to use in these infections, other antibiotics such as dalbavancin, may also have utility, but more research is needed to determine the effectiveness of this antibiotic in C. striatum infections.

Combinations of Daptomycin plus Ceftriaxone, but Not Ascending Daptomycin Dose-Regimens, Are Effective in Experimental Endocarditis Caused by Streptococcus mitis-oralis Strains: Target Tissue Clearances and Prevention of Emergence of Daptomycin-Resistance.

The Streptococcus mitis-oralis subgroup of the viridans group streptococci (VGS) are the most common cause of infective endocarditis (IE) in many parts of the world. These organisms are frequently resistant in vitro to standard β-lactams (e.g., penicillin; ceftriaxone [CRO]), and have the notable capacity for rapidly developing high-level and durable daptomycin resistance (DAP-R) during exposures in vitro, ex vivo, and in vivo. In this study, we used 2 prototypic DAP-susceptible (DAP-S) S. mitis-oralis strains (351; and SF100), which both evolved stable, high-level DAP-R in vitro within 1 to 3 days of DAP passage (5 to 20 μg/mL DAP). Of note, the combination of DAP + CRO prevented this rapid emergence of DAP-R in both strains during in vitro passage. The experimental rabbit IE model was then employed to quantify both the clearance of these strains from multiple target tissues, as well as the emergence of DAP-R in vivo under the following treatment conditions: (i) ascending DAP-alone dose-strategies encompassing human standard-dose and high-dose-regimens; and (ii) combinations of DAP + CRO on these same metrics. Ascending DAP-alone dose-regimens (4 to 18 mg/kg/d) were relatively ineffective at either reducing target organ bioburdens or preventing emergence of DAP-R in vivo. In contrast, the combination of DAP (4 or 8 mg/kg/d) + CRO was effective at clearing both strains from multiple target tissues (often with sterilization of bio-burdens in such organs), as well as preventing the emergence of DAP-R. In patients with serious S. mitis-oralis infections such as IE, especially caused by strains exhibiting intrinsic β-lactam resistance, initial therapy with combinations of DAP + CRO may be warranted.

1703. Efficacy of Selected Metabolic Inhibitors for the Prevention and Resensitization of High-Level Daptomycin Resistance in Streptococcus mitis-oralis In Vitro and in an Ex Vivo Simulated Endocarditis Model

Abstract Background Rapid development of stable, high-level daptomycin-resistance (DAP-R) is frequent among Streptococcus mitis-oralis strains during exposure to DAP in vitro and in vivo. Metabolomic analyses of in vitro-derived daptomycin-resistant (DAP-R) S. mitis-oralis strains (351D10) revealed substantial glycolytic pathway differences vs. its DAP-susceptible (DAP-S) parental strain, 351. To define translatability of such metabolic changes, we assessed combinations of DAP + strategic metabolic inhibitors likely to impact such pathways, including: oxamic acid [OXA], trimetazidine [TMZ) and 6-mercaptopurine [6-MP]. We assessed these combinations vs. our DAP-S/DAP-R isogenic strain pair both in vitro and ex vivo for: bactericidal and synergistic activities; prevention of high-level DAP-R in DAP-S cells; and/or resensitization of DAP-R cells Methods MICs. E test and microbroth dilution In vitro antimicrobial combination assays. Time-kill curves (24 hr) and serial passaging in DAP (10 d). Ex vivo IE model: SEVs (simulated endocardial vegetation’s) were quantitatively cultured at serial time-points post-exposures to DAP +/- inhibitors (0, 4, 8, 24, 32, 48 hr). Results In vitro , combinations of DAP + OXA or DAP + TMZ, as well as OXA or TMZ monotherapy, exerted bactericidal effect against both strains. Moreover, DAP + 6-MP yielded both a bactericidal, as well as synergistic activity against DAP-S 351 (but not against DAP-R 351 D10). None of the combinations prevented the development of DAP-R or resensitized the DAP-R strain to a DAP-S phenotype in vitro. In the ex vivo SEV model, using PK/PD simulations of humanized drug doses, DAP + OXA was the most effective regimen for both the DAP-S 351 and DAP-R 351 D10 strains; both OXA alone and DAP + OXA prevented DAP-R evolution, although not resensitizing the DAP-R strain to a DAP-S phenotype. Conclusion Combinations of DAP plus specific metabolic inhibitors represents a promising approach to enhance killing of S. mitis-oralis strains, as well as to potentially forestall DAP-R emergence. Also, they do provide a solid platform upon which to explore other potential metabolic modifiers for their ability to enhance the efficacy of DAP, based on definable metabolic perturbations in DAP-R S. mitis-oralis. Disclosures All Authors: No reported disclosures.

Evaluating the Rapid Emergence of Daptomycin Resistance in Corynebacterium: a Multicenter Study.

Members of the genus Corynebacterium are increasingly recognized as pathobionts and can be very resistant to antimicrobial agents. Previous studies have demonstrated that Corynebacterium striatum can rapidly develop high-level daptomycin resistance (HLDR) (MIC, ≥256 μg/ml). Here, we conducted a multicenter study to assay for this in vitro phenotype in diverse Corynebacterium species. Corynebacterium clinical isolates (n = 157) from four medical centers were evaluated. MIC values to daptomycin, vancomycin, and telavancin were determined before and after overnight exposure to daptomycin to identify isolates able to rapidly develop daptomycin nonsusceptibility. To investigate assay reproducibility, 18 isolates were evaluated at three study sites. In addition, the stability of daptomycin nonsusceptibility was tested using repeated subculture without selective pressure. The impact of different medium brands was also investigated. Daptomycin nonsusceptibility emerged in 12 of 23 species evaluated in this study (C. afermentans, C. amycolatum, C. aurimucosum, C. bovis, C. jeikeium, C. macginleyi, C. pseudodiphtheriticum, C. resistens, C. simulans, C. striatum, C. tuberculostearicum, and C. ulcerans) and was detected in 50 of 157 (31.8%) isolates tested. All isolates displayed low (susceptible) MIC values to vancomycin and telavancin before and after daptomycin exposure. Repeated subculture demonstrated that 2 of 9 isolates (22.2%) exhibiting HLDR reverted to a susceptible phenotype. Of 30 isolates tested on three medium brands, 13 (43.3%) had differences in daptomycin MIC values between brands. Multiple Corynebacterium species can rapidly develop daptomycin nonsusceptibility, including HLDR, after a short daptomycin exposure period.

1546. Efficacy of Daptomycin Combinations Against Daptomycin-Resistant Enterococcus faecium Differs by β-lactam

BackgroundWe have previously demonstrated that daptomycin (DAP) combinations with β-lactams offer enhanced bactericidal activity and prevent the emergence of resistance in Enterococcus faecium infections. Although the mechanisms of DAP resistance in enterococci are not fully comprehended, they are associated with alterations in cell envelope phospholipids assembly which leads to either repulsion of the drug from cell exterior or diversion from the cell septum. In this context, we sought to evaluate combinations of DAP with a panel of β-lactams including ampicillin (AMP), amoxicillin (AMX), ceftaroline (CPT), ceftriaxone (CRO) and ertapenem (ERT).Methods E. faecium R497 harboring liaSFR mutations (DAP MIC of 16 mg/L) was evaluated in a simulated endocardial vegetation (SEV) pharmacokinetic and pharmacodynamic model over 336 h at a starting inoculum of 109 log10 CFU/g. DAP 10 mg/kg/day combinations with AMP, AMX (2 g continuous infusion), CPT 600 mg q 12 h, CRO 2g q 24 h or ERT 1 g q 24 h were evaluated. The emergence of DAP resistance was determined daily over the course of treatment.ResultsDAP alone was not bactericidal and high-level DAP resistance was observed (MIC increase from 16 to 64 µg/mL). Combination of DAP+AMP offered a significant reduction in log10CFU/g amounts (Up to 7 log10 CFU/g and to detection limits) in 24h in with no emergence of DAP resistance. DAP 10+ AMX caused 6–6.5 log10 CFU/g reduction and counts were maintained around the detection limit while demonstrating no increased resistance. Dose de-escalation with AMP indicated that even DAP 4 mg/kg/d with AMP (2g) combination, reached detection limit at 168 h with no further resistance. None of the CPT, CRO or ERT regimens in combination with DAP was effective against R497 and elevated DAP MICs (>64 µg/mL) was observed during the 14-day model.ConclusionCombination of DAP+AMP offered the most encouraging results against E. faecium R497, while DAP+AMX caused enhanced reduction. The reason for this discrepancy in various β-lactam activity may be related to diverse β-lactam targeting or affinity toward PBP proteins. Further dissection of our observations is warranted to understand the optimized DAP-β-lactam combination and consequently improve patient outcomes and prevention of resistance. Disclosures All authors: No reported disclosures.

Rapid Emergence of Daptomycin Resistance in Nonstriatum Corynebacterium Species: A Multicenter Study

Abstract Members of the genus Corynebacterium are increasingly recognized as causes of opportunistic infection; some species can be multidrug resistant, posing a treatment challenge. Daptomycin is frequently used as therapy of last resort in this setting, but previous work from our group demonstrated the ability of C striatum clinical isolates to rapidly develop high-level resistance to daptomycin, both in vivo and in vitro. Here, our objective was to expand this investigation into a multicenter study evaluating multiple Corynebacterium species. Corynebacterium strains from three tertiary-care academic medical centers (total, n = 76; site 1, n = 44; site 2, n = 15; site 3, n = 17) were evaluated, representing 16 species. Isolates were identified during routine clinical testing and reported to species level in accordance with each laboratory’s standard operating procedures. Identification of each species was confirmed using both VITEK MS and Bruker BioTyper MALDI-TOF MS. MICs to daptomycin (Etest), vancomycin (Etest), and telavancin (Liofilchem) at baseline were determined using gradient diffusion methods on Mueller-Hinton agar with blood (Hardy Diagnostics). Each isolate was then inoculated in duplicate to 5 mL Tryptic Soy Broth. A daptomycin Etest was submerged in one tube from each pair, and growth was observed after 24-hour incubation. If turbidity was observed in the tube with daptomycin, MICs for each of the 3 antimicrobials were reassessed. High-level daptomycin resistance emerged in 24 strains: C aurimucosum (1/1 isolate tested), C bovis (1/2), C jeikeium (2/11), C macginleyi (3/3), C resistens (1/1), C simulans (1/1), C striatum (14/14 isolates), and C ulcerans (1/1). The majority of these isolates had MIC values >256 µg/mL following exposure to daptomycin. Forty-eight other isolates remained susceptible to daptomycin: C afermentans (1/1), C amycolatum (19/20), C diphtheriae (1/1), C jeikeium (7/11), C kroppenstedtii (2/2), C propinquum (3/3), C pseudodiphtheriticum (6/6), C tuberculostearicum (0/6), and C urealyticum (0/3). Many of these isolates did not undergo MIC testing postdaptomycin exposure in broth due to complete lack of growth. Among those that did (n = 19), the median daptomycin MIC was 0.38 µg/mL (mean 0.42 µg/mL; range 0.023-1.0 µg/mL). One isolate of C bovis and two isolates of C jeikeium yielded variable susceptibility to daptomycin; a subset of resistant colonies grew adjacent to the gradient diffusion strip. Upon isolation and further MIC testing, these colonies maintained high-level resistance. In addition, one isolate of C amycolatum exhibited high-level daptomycin resistance (MIC >256 µg/mL) prior to in vitro exposure. All isolates in the cohort were susceptible to vancomycin and telavancin, both before and after daptomycin exposure. Our findings suggest that multiple Corynebacterium species can rapidly develop high-level daptomycin resistance after a short period of exposure to this antimicrobial. This finding has important clinical implications, especially in the treatment of invasive infections or infections of indwelling medical devices.

Evaluation of daptomycin combinations with cephalosporins or gentamicin against Streptococcus mitis group strains in an in vitro model of simulated endocardial vegetations (SEVs)

Among viridans group streptococcal infective endocarditis (IE), the Streptococcus mitis group is the most common aetiological organism. Treatment of IE caused by the S. mitis group is challenging due to the high frequency of β-lactam resistance, drug allergy and intolerability of mainstay antimicrobial agents such as vancomycin or gentamicin. Daptomycin has been suggested as an alternative therapeutic option in these scenarios based on its excellent susceptibility profile against S. mitis group strains . However, the propensity of many S. mitis group strains to rapidly evolve stable, high-level daptomycin resistance potentially limits this approach. We evaluated the activity of 6 mg/kg/day daptomycin alone or in combination with gentamicin, ceftriaxone or ceftaroline against two daptomycin-susceptible S. mitis group strains over 96 h in a pharmacokinetic/pharmacodynamic model of simulated endocardial vegetations. Daptomycin alone was not bactericidal and high-level daptomycin resistance evolved at 96 h in both organisms. Combinations of daptomycin + ceftriaxone and daptomycin + ceftaroline demonstrated enhanced killing activity compared with each antibiotic alone and prevented emergence of daptomycin resistance at 96 h. Use of gentamicin as an adjunctive agent neither improved the efficacy of daptomycin nor prevented the development of daptomycin resistance. Addition of ceftriaxone or ceftaroline to daptomycin improves the bactericidal activity against S. mitis group strains and prevents daptomycin resistance emergence. Further investigation with combinations of daptomycin and β-lactams in a large number of strains is warranted to fully elucidate the clinical implications of such combinations for treatment of S. mitis group IE.

Streptococcus mitis and S. oralis Lack a Requirement for CdsA, the Enzyme Required for Synthesis of Major Membrane Phospholipids in Bacteria.

ABSTRACTSynthesis and integrity of the cytoplasmic membrane are fundamental to cellular life. Experimental evolution studies have hinted at unique physiology in the Gram-positive bacteria Streptococcus mitis and S. oralis. These organisms commonly cause bacteremia and infectious endocarditis (IE) but are rarely investigated in mechanistic studies of physiology and evolution. Unlike in other Gram-positive pathogens, high-level (MIC ≥ 256 μg/ml) daptomycin resistance rapidly emerges in S. mitis and S. oralis after a single drug exposure. In this study, we found that inactivating mutations in cdsA are associated with high-level daptomycin resistance in S. mitis and S. oralis IE isolates. This is surprising given that cdsA is an essential gene for life in commonly studied model organisms. CdsA is the enzyme responsible for the synthesis of CDP-diacylglycerol, a key intermediate for the biosynthesis of all major phospholipids in prokaryotes and most anionic phospholipids in eukaryotes. Lipidomic analysis by liquid chromatography-mass spectrometry (LC-MS) showed that daptomycin-resistant strains have an accumulation of phosphatidic acid and completely lack phosphatidylglycerol and cardiolipin, two major anionic phospholipids in wild-type strains, confirming the loss of function of CdsA in the daptomycin-resistant strains. To our knowledge, these daptomycin-resistant streptococci represent the first model organisms whose viability is CdsA independent. The distinct membrane compositions resulting from the inactivation of cdsA not only provide novel insights into the mechanisms of daptomycin resistance but also offer unique opportunities to study the physiological functions of major anionic phospholipids in bacteria.

Impact of High-Level Daptomycin Resistance in the Streptococcus mitis Group on Virulence and Survivability during Daptomycin Treatment in Experimental Infective Endocarditis.

Among the viridans group streptococci, the Streptococcus mitis group is the most common cause of infective endocarditis. These bacteria have a propensity to be β-lactam resistant, as well as to rapidly develop high-level and durable resistance to daptomycin (DAP). We compared a parental, daptomycin-susceptible (DAPs) S. mitis/S. oralis strain and its daptomycin-resistant (DAPr) variant in a model of experimental endocarditis in terms of (i) their relative fitness in multiple target organs in this model (vegetations, kidneys, spleen) when animals were challenged individually and in a coinfection strategy and (ii) their survivability during therapy with daptomycin-gentamicin (an in vitro combination synergistic against the parental strain). The DAPr variant was initially isolated from the cardiac vegetations of animals with experimental endocarditis caused by the parental DAPs strain following treatment with daptomycin. The parental strain and the DAPr variant were comparably virulent when animals were individually challenged. In contrast, in the coinfection model without daptomycin therapy, at both the 106- and 107-CFU/ml challenge inocula, the parental strain outcompeted the DAPr variant in all target organs, especially the kidneys and spleen. When the animals in the coinfection model of endocarditis were treated with DAP-gentamicin, the DAPs strain was completely eliminated, while the DAPr variant persisted in all target tissues. These data underscore that the acquisition of DAPr in S. mitis/S. oralis does come at an intrinsic fitness cost, although this resistance phenotype is completely protective against therapy with a potentially synergistic DAP regimen.

Pronounced heterogeneity observed in high-level daptomycin-resistant viridans group streptococci.

Viridans group streptococci (VGS) have demonstrated high-level daptomycin resistance (HLDR) upon daptomycin exposure. This study evaluated the extent of heterogeneity and whether dose escalation or combination therapy could prevent resistance development. Five VGS strains (daptomycin MICs 0.25-2mg/L) were evaluated. In vitro models utilised simulated daptomycin dosages of 4, 6, 8 and 12mg/kg with estimated fCmax of 4.1, 6.6, 8.6 and 12.9mg/L, respectively. Time-kill studies included fCmax simulations of daptomycin alone or combined with ceftriaxone, gentamicin, linezolid, rifampicin or vancomycin. Population analyses were performed on daptomycin-containing and non-containing agar plates. Extreme heterogeneity was observed in four strains with daptomycin population MICs 4-512-fold higher than broth microdilution. Whilst Streptococcus gordonii 1649 did not consistently develop HLDR, its population MIC was above the established daptomycin breakpoint. In vitro modelling demonstrated initial kill by daptomycin in all strains within 8h, with substantial re-growth by 24h despite increasing daptomycin. Daptomycin kill curves also displayed resistance development by 24h. However, synergy or additivity was noted for most regimens and strains. Synergy was most notable with daptomycin plus linezolid or rifampicin. Overall, daptomycin plus ceftriaxone or gentamicin were the most potent regimens. Gentamicin or rifampicin with daptomycin were least additive. For combination regimens with colonies isolated at 24h, HLDR was reduced 16-64-fold (MICs 4-16mg/L). Daptomycin monotherapy for VGS led to rapid development of HLDR likely due to extreme heterogeneity. Combination therapy suppressed or minimised the degree of resistance although the mechanism remains unknown.

Characterization of high-level daptomycin resistance in Viridans group Streptococci developed upon in vitro exposure to daptomycin.

Viridans group streptococci (VGS) are part of the normal flora that may cause bacteremia, often leading to endocarditis. We evaluated daptomycin against four clinical strains of VGS (MICs = 1 or 2 μg/ml) using an in vitro-simulated endocardial vegetation model, a simulated bacteremia model, and kill curves. Daptomycin exposure was simulated at 6 mg/kg of body weight and 8 mg/kg every 24 h for endocardial and bacteremia models. Total drug concentrations were used for analyses containing protein (albumin and pooled human serum), and free (unbound) drug concentrations (93% protein bound) were used for analyses not containing protein. Daptomycin MICs in the presence of protein were significantly higher than those in the absence of protein. Despite MICs below or at the susceptible breakpoint, all daptomycin regimens demonstrated limited kill in both pharmacodynamic models. A reduction of approximately 1 to 2 log10 CFU was seen for all isolates and dosages except daptomycin at 6 mg/kg, which achieved a reduction of 2.7 log10 CFU/g against one strain (Streptococcus gordonii 1649) in the endocardial model. Activity was similar in both pharmacodynamic models in the presence or absence of protein. Similar activity was noted in the kill curves over all multiples of the MIC. Regrowth by 24 h was seen even at 8× MIC. Postexposure daptomycin MICs for both pharmacodynamic models increased to >256 μg/ml for all isolates by 24 and 72 h. Despite susceptibility to daptomycin by standard MIC methods, these VGS developed high-level daptomycin resistance (HLDR) after a short duration following drug exposure not attributed to modification or inactivation of daptomycin. Further evaluation is warranted to determine the mechanism of resistance and clinical implications.

Evolution of high-level daptomycin resistance in Enterococcus faecium during daptomycin therapy is associated with limited mutations in the bacterial genome

Evolution of high-level daptomycin resistance in Enterococcus faecium during daptomycin therapy is associated with limited mutations in the bacterial genome

Early In Vitro and In Vivo Development of High-Level Daptomycin Resistance Is Common in Mitis Group Streptococci after Exposure to Daptomycin

The development of high-level daptomycin resistance (HLDR; MIC of ≥ 256 mg/liter) after exposure to daptomycin has recently been reported in viridans group streptococcus (VGS) isolates. Our study objectives were as follows: to know whether in vitro development of HLDR after exposure to daptomycin was common among clinical isolates of VGS and Streptococcus bovis; to determine whether HLDR also developed during the administration of daptomycin to treat experimental endocarditis caused by the daptomycin-susceptible, penicillin-resistant Streptococcus mitis strain S. mitis 351; and to establish whether combination with gentamicin prevented the development of HLDR in vitro and in vivo. In vitro studies were performed with 114 VGS strains (mitis group, 92; anginosus group, 10; mutans group, 8; and salivarius group, 4) and 54 Streptococcus bovis strains isolated from 168 consecutive patients with infective endocarditis diagnosed between 1995 and 2010. HLDR was only observed after 24 h of exposure to daptomycin in 27% of the mitis group, including 27% of S. mitis isolates, 47% of S. oralis isolates, and 13% of S. sanguis isolates. In our experimental model, HLDR was detected in 7/11 (63%) and 8/12 (67%) isolates recovered from vegetations after 48 h of daptomycin administered at 6 mg/kg of body weight/24 h and 10 mg/kg/24 h, respectively. In vitro, time-kill experiments showed that daptomycin plus gentamicin was bactericidal against S. mitis 351 at tested concentrations of 0.5 and 1 times the MIC and prevented the development of HLDR. In vivo, the addition of gentamicin at 1 mg/kg/8 h to both daptomycin arms prevented HLDR in 21 out of 23 (91%) rabbits. Daptomycin plus gentamicin was at least as effective as vancomycin plus gentamicin. In conclusion, HLDR develops rapidly and frequently in vitro and in vivo among mitis group streptococci. Combining daptomycin with gentamicin enhanced its activity and prevented the development of HLDR in most cases.

Native Valve Endocarditis Caused by Corynebacterium striatum with Heterogeneous High-Level Daptomycin Resistance: Collateral Damage from Daptomycin Therapy?

We describe a patient who developed Corynebacterium striatum native valve endocarditis after receiving two 6-week courses of daptomycin for the treatment of methicillin-resistant Staphylococcus aureus bacteremia and osteomyelitis. The organism exhibited in vitro heteroresistance to daptomycin, with two subpopulations showing daptomycin susceptibility (MIC of ≤ 0.094 μg/ml) and high-level resistance to daptomycin (MIC of ≥ 256 μg/ml). The selection of daptomycin-resistant Gram-positive skin flora with the potential of causing invasive disease may be a concern during prolonged courses of daptomycin.