Daptomycin Susceptibility Research Articles

Daptomycin resistance mechanisms in clinically derived Staphylococcus aureus strains assessed by a combined transcriptomics and proteomics approach

The development of daptomycin resistance in Staphylococcus aureus is associated with clinical treatment failures. The mechanism(s) of such resistance have not been clearly defined. We studied an isogenic daptomycin-susceptible (DAP(S)) and daptomycin-resistant (DAP(R)) S. aureus strain pair (616; 701) from a patient with relapsing endocarditis during daptomycin treatment, using comparative transcriptomic and proteomic techniques. Minor differences in the genome content were found between strains by DNA hybridization. Transcriptomic analyses identified a number of genes differentially expressed in important functional categories: cell division; metabolism of bacterial envelopes; and global regulation. Of note, the DAP(R) isolate exhibited reduced expression of the major cell wall autolysis gene coincident with the up-regulation of genes involved in cell wall teichoic acid production. Using quantitative (q)RT-PCR on the gene cadre putatively involved in cationic peptide resistance, we formulated a putative regulatory network compatible with microarray data sets, mainly implicating bacterial envelopes. Of interest, qRT-PCR of this same gene cadre from two distinct isogenic DAP(S)/DAP(R) clinical strain pairs revealed evidence of other strain-dependent networks operative in the DAP(R) phenotype. Comparative proteomics of 616 versus 701 revealed a differential abundance of proteins in various functional categories, including cell wall-associated targets and biofilm formation proteins. Phenotypically, strains 616 and 701 showed major differences in their ability to develop bacterial biofilms in the presence of the antibacterial lipid, oleic acid. Compatible with previous in vitro observations, in vivo-acquired DAP(R) in S. aureus is a complex, multistep phenomenon involving: (i) strain-dependent phenotypes; (ii) transcriptome adaptation; and (iii) modification of the lipid and protein contents of cellular envelopes.

Antimicrobial susceptibility of daptomycin and comparator agents tested against methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci: trend analysis of a 6-year period in US medical centers (2005–2010)

We evaluated daptomycin activity trends against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE; MIC, ≥8 μg/mL) in a 6-year period (2005–2010) following US regulatory release for clinical use. Consecutive, unique patient strains of clinical significance were collected in 32 US medical centers and susceptibility tested in a central laboratory against daptomycin and various comparator agents by reference broth microdilution methods. A total of 22 858 S. aureus (12 181 [53.3%] MRSA), 4312 Enterococcus faecalis (195 [4.5%] VRE), and 2462 Enterococcus faecium (1867 [75.8%] VRE) were evaluated. Daptomycin susceptibility rates were 99.94%, 99.98%, and 99.68% for S. aureus, E. faecalis, and E. faecium, respectively. Among MRSA (daptomycin MIC50/90, 0.25/0.5 μg/mL), only 13 (0.11%) daptomycin-non-susceptible (MIC, ≥2 μg/mL) isolates were observed with no MIC creep over the study interval. Daptomycin was very active against vancomycin-resistant E. faecalis (MIC50/90, 1/2 μg/mL) and E. faecium (MIC50/90, 2/2 μg/mL). Among VRE, only 4 daptomycin-non-susceptible isolates (all E. faecium) were detected. In conclusion, daptomycin demonstrated sustained activity against an extensive collection of clinical isolates of MRSA and VRE from numerous US medical centers over the last 6 monitored years.

Daptomycin non-susceptibility in vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous-VISA (hVISA): implications for therapy after vancomycin treatment failure

The aim of this study was to establish the relationship between reduced vancomycin and daptomycin susceptibility among Australasian vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous-VISA (hVISA) isolates from patients never exposed to daptomycin. Forty-seven stored clinical isolates of hVISA/VISA collected before November 2008 from around Australia and New Zealand were selected. Daptomycin and vancomycin MIC testing was performed using broth microdilution (BMD) and Etest methods. Daptomycin population analysis was performed on a subset of isolates. The percentage of daptomycin non-susceptible isolates was 0% for vancomycin-susceptible S. aureus (VSSA) (Etest and BMD), for hVISA it was 26% by Etest and 15% by BMD, and for VISA 62% by Etest and 38% by BMD. Population analysis profile testing demonstrated daptomycin heteroresistance among the hVISA and VISA strains tested. This is the highest rate of daptomycin non-susceptibility reported among hVISA isolates to date. Clinicians should exhibit caution when using daptomycin in situations where serious hVISA or VISA infection is a possibility.

Lipopeptide Laur-CKK-NH2 dimer preserves daptomycin susceptibility and enhances its activity against Enterococcus faecalis

An experimental study was performed to evaluate both in vitro and in vivo the kind of interaction between the Laur-CKK-NH2 dimer and daptomycin using two Enterococcus faecalis strains with different patterns of susceptibilities. We evaluated whether selection for daptomycin-resistant E. faecalis could be prevented in vitro by combining daptomycin with the Laur-CKK-NH2 dimer. The strains were serially exposed in broth to 2-fold stepwise increasing concentrations of daptomycin alone or in combination with a fixed concentration (0.25×MIC) of the Laur-CKK-NH2 dimer. We also performed an in vitro synergy study. For in vivo studies, a mouse model of enterococcal sepsis was used. In vitro experiments: exposure to daptomycin alone gradually selected for enterococci with increased MICs; and the Laur-CKK-NH2 dimer showed a positive interaction with daptomycin and was able to prevent the resistance. In vivo experiments: the main outcome measures were lethality and quantitative blood cultures; and the Laur-CKK-NH2 dimer combined with daptomycin exhibited the highest efficacy for all main outcome measurements. These results highlight the potential usefulness of combining daptomycin with the Laur-CKK-NH2 dimer. The combination provides a future therapeutic alternative for the treatment of enterococcal severe infections.

Emergence of daptomycin resistance following vancomycin-unresponsive Staphylococcus aureus bacteraemia in a daptomycin-naïve patient—a review of the literature

A patient developed a daptomycin-resistant methicillin-resistant Staphylococcus aureus (MRSA) infection, despite being daptomycin-naïve, in the setting of persistent bacteraemia secondary to vertebral osteomyelitis. Modified population analysis profiling of sequential MRSA blood culture isolates revealed transition from a vancomycin-susceptible phenotype to a vancomycin-intermediate S. aureus (VISA) phenotype through a vancomycin-heteroresistant S. aureus (hVISA) intermediary. Increased cell wall thickening, determined by transmission electron microscopy, correlated with the emergence of daptomycin resistance. This case supports the current hypothesis that MRSA with reduced glycopeptide susceptibility are less susceptible to daptomycin because of a thickened cell wall. This may have significance for the use of daptomycin in salvage therapy. Other predictors of daptomycin resistance include bacteraemic persistence and the presence of high inoculum infections. As resistance may appear de novo and be unstable in vivo, all isolates should have daptomycin susceptibility testing performed. The optimal antibiotic option for salvage therapy of these daptomycin-resistant infections is unknown. However, these findings emphasise the importance of optimising management, including the consideration of early surgical intervention to avoid the emergence of daptomycin resistance, especially in high inoculum infections.

Weissella confusa: an unexpected cause of vancomycin‐resistant gram‐positive bacteremia in immunocompromised hosts

We report the first case of Weissella confusa bacteremia in an allogeneic hematopoietic stem cell transplant patient. After engraftment and discharge, the patient returned with fever and graft failure and was started on an empiric regimen of aztreonam and vancomycin. A blood culture grew an alpha-hemolytic, gram-positive coccus forming pairs and chains, originally thought to be a viridans Streptococcus and a skin contaminant. The isolation of the organism from multiple blood cultures, and the presence of vancomycin resistance prompted identification and additional susceptibility testing. The RapID(™) Str panel, which has W. confusa in its database, provided multiple incorrect identifications. The MicroScan WalkAway 96 SI, using PC-20 or -29 panels, also did not identify this bacterium, because it is not in their database. The organism was identified as W. confusa by 16S rDNA sequencing. Antibiotic susceptibility determination by Etest revealed vancomycin resistance and daptomycin susceptibility. Therapy was changed to daptomycin, and the infection resolved. Additionally, W. confusa sepsis, with multiple positive blood cultures, developed in a patient in the burn unit at our medical center. The patient's blood cultures remained positive until vancomycin was discontinued and daptomycin therapy initiated. Infections with vancomycin-resistant, gram-positive cocci are emerging among immuno compromised hosts. Under appropriate circumstances, clinicians need to request that the laboratory perform susceptibility testing and accurate identification, by nucleic acid sequencing if necessary. Sequencing of 16S rDNA is an important tool in the accurate identification of unusual pathogens.

Daptomycin In Vitro Susceptibility Methodology: a Review of Methods, Including Determination of Calcium in Testing Media

Daptomycin has been available as a gram-positive antimicrobial agent in the United States for over 6 years, and during this time there have been developments in daptomycin susceptibility testing. Daptomycin has a unique requirement for calcium; consequently, a standard calcium concentration is needed when performing in vitro testing. The calcium levels in broth and agar are presented, and with the exception of calcium-adjusted Mueller-Hinton broth, demonstrate variation between types and manufacturers. The reference methodology for daptomycin MIC testing is broth microdilution utilizing Mueller-Hinton broth with 50 μg/ml of calcium. Daptomycin is available in several commercial testing systems, which have been shown to be comparable to the reference method. Specific MICs from several Etest studies have been compared to broth microdilution MICs, and essential agreements were 79.1 to 100% ( Staphylococcus aureus) and 66.7 to 100% (enterococci). The performance of routine quality control is especially important for in vitro testing of daptomycin, and validation of nonsusceptible daptomycin MICs is recommended.

Six cases of daptomycin-non-susceptible Staphylococcus aureus bacteraemia in Singapore

We report what we believe to be the first six cases of daptomycin-non-susceptible Staphylococcus aureus infections from Singapore. These strains were rapidly isolated after bacteraemic patients were switched to daptomycin following initial prolonged unsuccessful therapy with vancomycin, despite confirmation of daptomycin susceptibility just prior to initiating daptomycin therapy. The majority of post-vancomycin therapy strains exhibited marked thickening of their cell walls on electron microscopic examination. In patients with persistent S. aureus bacteraemia, therapeutic failure with daptomycin may occur if used as salvage therapy following vancomycin failure, notwithstanding initial susceptibility testing results.

Daptomycin-oxacillin combinations in treatment of experimental endocarditis caused by daptomycin-nonsusceptible strains of methicillin-resistant Staphylococcus aureus with evolving oxacillin susceptibility (the "seesaw effect").

In vivo development of daptomycin resistance (DAPr) among Staphylococcus aureus strains, especially methicillin-resistant S. aureus (MRSA) strains, in conjunction with clinical treatment failures, has emerged as a major problem. This has raised the question of DAP-based combination regimens to enhance efficacy against such strains. We studied five recent DAP-susceptible (DAPs)/DAPr clinical MRSA strain pairs obtained from patients who failed DAP monotherapy regimens, as well as one DAPs/DAPr MRSA strain pair in which the resistant strain was generated by in vitro passage in DAP. Of note, we identified a DAP-oxacillin (OX) "seesaw" phenomenon in vitro in which development of DAPr was accompanied by a concomitant fall in OX resistance, as demonstrated by 3- to 4-fold decreases in the OX MIC, a susceptibility shift by population analyses, and enhanced early killing by OX in time-kill assays. In addition, the combination of DAP and OX exerted modest improvement in in vitro bactericidal effects. Using an experimental model of infective endocarditis and two DAPs/DAPr strain pairs, we demonstrated that (i) OX monotherapy was ineffective at clearing DAPr strains from any target tissue in this model (heart valve, kidneys, or spleen) and (ii) DAP-OX combination therapy was highly effective in DAPr strain clearances from these organs. The mechanism(s) of the seesaw effect remains to be defined but does not appear to involve excision of the staphylococcal cassette chromosome mec (SCCmec) that carries mecA.

In vitro evaluation of ceftaroline alone and in combination with tobramycin against hospital-acquired meticillin-resistant Staphylococcus aureus (HA-MRSA) isolates

The aim of this study was to evaluate the in vitro activity of ceftaroline and its potential for synergy with tobramycin in comparison with vancomycin against a collection of hospital-acquired meticillin-resistant Staphylococcus aureus (HA-MRSA), including isolates with reduced susceptibility to glycopeptides. Ceftaroline, vancomycin, daptomycin and linezolid susceptibilities were determined for 200 HA-MRSA isolates. Four randomly selected strains [including one vancomycin-intermediate S. aureus (VISA) and one heteroresistant VISA (hVISA)] were evaluated in time–kill experiments with ceftaroline and vancomycin alone or combined with tobramycin at 0.25 and 0.5 times the minimum inhibitory concentration (MIC). MICs for 50% and 90% of the organisms (MIC50 and MIC90, respectively) were both 1mg/L for ceftaroline and were 1mg/L and 2mg/L, respectively, for vancomycin. The same ceftaroline MIC ranges (0.25–2mg/L) were observed for isolates recovered from respiratory tract samples, blood or skin. In time–kill experiments, no synergy was observed at 0.25× MIC against any tested isolates with either ceftaroline or vancomycin. In contrast, the combination of ceftaroline plus tobramycin at 0.5× MIC was synergistic against the two MRSA strains and the hVISA but was indifferent against the VISA isolate. In conclusion, ceftaroline demonstrated antimicrobial activity independently of the specimen source and exhibited lower MICs than vancomycin. Finally, at sub-MIC levels, ceftaroline plus tobramycin displayed significantly greater activity than vancomycin plus tobramycin against MRSA (P<0.01).

Efficacy of vancomycin and daptomycin against Staphylococcus aureus isolates collected over 29 years

We investigated vancomycin and daptomycin efficacy for Staphylococcus aureus isolates at our institution by testing 221 methicillin-sensitive and 299 methicillin-resistant isolates recovered from serious infections between 1979 and 2007 using a microdilution method. The MIC 90's for vancomycin remained constant at 2 μg/mL for methicillin-resistant S. aureus (MRSA) and methicillin-sensitive S. aureus (MSSA). For MSSA, the geometric mean vancomycin MICs remained at 1.1 μg/mL but varied from 1.1 to 1.7 for MRSA. Tolerance to vancomycin was seen in 6.5% of MRSA and 10.5% of MSSA. Daptomycin MIC 90's remained at ≤1 μg/mL, and the daptomycin concentration at which 90% of the strains were killed (MBC 90) remained at 2 μg/ml. Over 29 years, no trend was detected in vancomycin or daptomycin susceptibilities. Daptomycin had excellent inhibitory and bactericidal activities against all strains throughout the years. Although vancomycin's inhibitory activity was consistent over time, vancomycin-tolerant strains whose presence was not predicted by their MIC were found.

Activity of daptomycin against staphylococci collected from bloodstream infections in Spanish medical centers

We used the broth microdilution method to determine the MICs of daptomycin and 13 comparator agents against 319 methicillin-susceptible Staphylococcus aureus isolates, 201 methicillin-resistant S. aureus (MRSA) isolates, and 183 coagulase-negative staphylococci (CoNS). Isolates were consecutively collected from bloodstream infections in 39 Spanish medical centers during a 3-month period (March through May 2008). Among MRSA, 1 isolate with intermediate susceptibility to vancomycin and 6 isolates resistant to linezolid were found. Nonsusceptibility to teicoplanin was detected in 3.9% of CoNS. Daptomycin was highly active against the staphylococcal blood isolates tested—all were inhibited at the daptomycin susceptibility breakpoint of ≤1 μg/mL. Daptomycin retained its activity against the isolates that were resistant to teicoplanin or linezolid, or that had reduced susceptibility to vancomycin. These data suggest that daptomycin could be useful for the treatment of bloodstream infections caused by staphylococci.

Regulation of mprF in Daptomycin-Nonsusceptible Staphylococcus aureus Strains

We used a well-characterized isogenic set of clinical bloodstream Staphylococcus aureus strains to study (i) regulation of mprF-mediated phosphatidylglycerol lysinylation in the contexts of in vitro daptomycin (DAP) nonsuceptibility and (ii) the role of mprF mutation in endovascular virulence. We observed a correlation between increased expression of a mutant mprF gene and reduced in vitro DAP susceptibility. There were no detectable fitness differences between strains in experimental infective endocarditis.

Relationship between Susceptibility to Daptomycin In Vitro and Activity In Vivo in a Rabbit Model of Aortic Valve Endocarditis

Daptomycin is approved for treatment of Staphylococcus aureus bacteremia and right-sided endocarditis. Increases in daptomycin MICs have been associated with failure. A rabbit model of aortic valve endocarditis was used to determine whether MIC correlates with activity in vivo and whether a higher daptomycin dose can improve efficacy. Two related clinical S. aureus strains, one with a daptomycin MIC of 0.5 microg/ml and the other with a MIC of 2 microg/ml, were used to establish aortic valve endocarditis in rabbits. Daptomycin was administered once a day for 4 days at 12 mg/kg of body weight or 18 mg/kg to simulate doses in humans of 6 mg/kg and 10 mg/kg, respectively. Endocardial vegetations, spleens, and kidneys were harvested and quantitatively cultured. The strain with a MIC of 2 microg/ml had a survival advantage over the strain with a MIC of 0.5 microg/ml with >100 times more organisms of the former in endocardial vegetations at the 12-mg/kg dose in a dual-infection model. Both the 12-mg/kg dose and the 18-mg/kg dose completely eradicated the strain with a MIC of 0.5 from vegetations, spleens, and kidneys. The 12-mg/kg dose was ineffective against the strain with a MIC of 2 in vegetations; the 18-mg/kg dose produced a reduction of 3 log(10) units in CFU in vegetations compared to the controls, although in no rabbit were organisms completely eliminated. Increasing the dose of daptomycin may improve its efficacy for infections caused by strains with reduced daptomycin susceptibility.

Genetic Analysis of Factors Affecting Susceptibility ofBacillus subtilisto Daptomycin

Daptomycin is the first of a new class of cyclic lipopeptide antibiotics used against multidrug-resistant, gram-positive pathogens. The proposed mechanism of action involves disruption of the functional integrity of the bacterial membrane in a Ca(2+)-dependent manner. We have used transcriptional profiling to demonstrate that treatment of Bacillus subtilis with daptomycin strongly induces the lia operon including the autoregulatory LiaRS two-component system (homologous to Staphylococcus aureus VraSR). The lia operon protects against daptomycin, and deletion of liaH, encoding a phage-shock protein A (PspA)-like protein, leads to threefold increased susceptibility. Since daptomycin interacts with the membrane, we tested mutants with altered membrane composition for effects on susceptibility. Deletion mutations of mprF (lacking lysyl-phosphatidylglycerol) or des (lipid desaturase) increased daptomycin susceptibility, whereas overexpression of MprF decreased susceptibility. Conversely, depletion of the cell for the anionic lipid phosphatidylglycerol led to increased resistance. Fluorescently labeled daptomycin localized to the septa and in a helical pattern around the cell envelope and was delocalized upon the depletion of phosphatidylglycerol. Together, these results indicate that the daptomycin-Ca(2+) complex interacts preferentially with regions enriched in anionic phospholipids and leads to membrane stresses that can be ameliorated by PspA family proteins.

Frequency of Occurrence and Daptomycin Susceptibility Rates of Gram-positive Organisms Causing Bloodstream

We evaluated the pathogen frequency and the antimicrobial susceptibility patterns of Gram-positive bacteria causing infections in cancer patients. We also compared the antimicrobial susceptibility of isolates from cancer patients to those from non-cancer patients and evaluated the activity of the novel lipopeptide daptomycin against these isolates. A total of 1,222 Gram-positive isolates were collected (one per patient) from bloodstream infections (BSI) in cancer patients at 33 United States (USA) medical centers in 2002-2006 and susceptibility tested by the broth microdilution method. Frequency of occurrence and susceptibility patterns of strains from cancer patients were compared to those from non-cancer patients collected in the same hospitals (10,684 strains). Staphylococcus aureus (39.2%) ranked first among Gram-positive pathogens from cancer patients, followed by Enterococcus spp. (32.2%) and coagulase-negative staphylococci (CoNS; 20.4%). Daptomycin was active against 99.8% of Gram-positive strains collected from patients with cancer at USA hospitals, but vancomycin was only active against 87.6% of strains and showed limited activity against Enterococcus spp. (61.4% susceptible). Only three daptomycin non-susceptible isolates were detected, all with MIC values at one doubling dilution above the susceptible breakpoint. These results indicate that daptomycin has an appropriate spectrum and potency to be used for empirical coverage of Gram-positive infections (especially BSI) in cancer patients in the hospitals surveyed in the USA.

Evaluation of Daptomycin Pharmacodynamics and Resistance at Various Dosage Regimens against Staphylococcus aureus Isolates with Reduced Susceptibilities to Daptomycin in an In Vitro Pharmacodynamic Model with Simulated Endocardial Vegetations

The need to investigate novel dosing regimens and combinations is essential in combating poor treatment outcomes for Staphylococcus aureus bacteremia and endocarditis. We evaluated the impact of simulated standard- and high-dose daptomycin in combination with gentamicin or rifampin against daptomycin-susceptible and nonsusceptible matched strains of S. aureus. These strains were collected from the daptomycin bacteremia and endocarditis clinical trial and consisted of three susceptible strains (MIC, 0.25 mg/liter) and four nonsusceptible isolates (MICs, 2 to 4 mg/liter). Daptomycin regimens of 6 and 10 mg/kg of body weight daily alone and in combination with gentamicin at 5 mg/kg daily or rifampin at 300 mg every 8 h were evaluated using an in vitro model with simulated endocardial vegetations over 96 h. Rapid bactericidal activity, identified by time to 99.9% kill, was displayed in all regimens with the daptomycin-susceptible strains. Concentration-dependent activity was noted by more-rapid killing with the 10-mg/kg/day dose. The addition of gentamicin improved activity in the majority of susceptible isolates. Daptomycin 6-mg/kg/day monotherapy displayed bactericidal activity for only one of the nonsusceptible isolates and for only two isolates with increased doses of 10 mg/kg/day. Combination regimens demonstrated improvement with some but not all nonsusceptible isolates. Three isolates developed a reduction in daptomycin susceptibility with 6-mg/kg/day monotherapy, but this was suppressed with both combination therapy and high-dose daptomycin. These results suggest that high-dose daptomycin therapy and combination therapy may be reasonable treatment options for susceptible isolates; however, more investigations are needed to confirm the variability of these regimens with nonsusceptible isolates.

Detection of daptomycin-nonsusceptible strains using the Neo-Sensitab™ prediffusion method

Two prediffusion methods with daptomycin (DAP) Neo-Sensitabs were evaluated against a challenge set of 30 Staphylococcus aureus isolates and 30 enterococci. DAP Neo-Sensitabs were prediffused for either 8 or 20 h on Mueller–Hinton agar. Inhibition zones were plotted versus Etest MIC values determined on the same prediffused agar. A generalization to the genus level of the manufacturer's suggested Neo-Sensitabs breakpoints for staphylococci and enterococci was used to interpret the results. DAP-susceptible and DAP-nonsusceptible enterococci, Enterococcus faecium in particular, were not reliably discriminated using a 20-h prediffusion method and the manufacturer's suggested breakpoints. Further development of this testing methodology, such as changing the format to a susceptibility screen followed by a confirmatory MIC, is needed to accurately categorize the DAP susceptibility of enterococcal isolates. Prediffusion for either 8 or 20 h with DAP Neo-Sensitabs discriminated between susceptible and nonsusceptible S. aureus with minimal errors. Both prediffusion methods also detected changes in MIC values between isogenic pairs of susceptible and nonsusceptible S. aureus. These results suggest that a multisite evaluation of either prediffusion method with DAP Neo-Sensitabs against a larger collection of S. aureus is warranted.

High rate of decreasing daptomycin susceptibility during the treatment of persistent Staphylococcus aureus bacteremia

Daptomycin is bactericidal against Staphylococcus aureus, with susceptibility defined as a minimal inhibitory concentration (MIC) < or =1 microg/ml. Higher MIC developed in a few cases during therapy. The frequency of MIC rise in persistent bacteremia is unknown. We evaluated all patients with S. aureus bacteremia (SAB) treated with daptomycin (> or =2 days) from 1 April 2004 to 30 October 2006. All patients with post-daptomycin-exposure saved isolates were studied. Daptomycin susceptibility was determined (in duplicate) on all pre- and post-daptomycin-exposure isolates by the broth (Mueller-Hinton) microdilution method. Among 74 treatment courses in 67 patients, 18 were for SAB. Ten had persistent bacteremia (median = 11 days; range = 1-21) and post-daptomycin-exposure saved isolates. The patient age was 29-84 years (median = 57.5 years). Intravascular catheter was the most common source (50%). Most patients (90%) failed therapy prior to starting daptomycin. The initial daptomycin dose was 4 mg/kg in four (40%) cases. The pre-exposure MIC was 0.125-0.5 microg/ml. The post-exposure MIC increased in four cases and was elevated in two cases (60%), to 2 microg/ml in five and 4 microg/ml in one. MIC rise was noted within 5-15 days of exposure and persisted up to 247 days after stopping daptomycin. Pulse-field gel electrophoresis (PFGE) band pattern of isolates with increased MIC revealed 1-3-band differences, implying genetic relatedness. All patients with non-susceptible isolates relapsed or failed therapy. These findings illustrate that daptomycin susceptibility often decreases during the treatment of persistent SAB. Therefore, susceptibility should be closely monitored during therapy.

Diminished vancomycin and daptomycin susceptibility during prolonged bacteremia with methicillin-resistant Staphylococcus aureus

An elderly patient with methicillin-resistant Staphylococcus aureus (MRSA) bacteremia was treated sequentially with vancomycin plus rifampin then daptomycin plus gentamicin. The MRSA strain developed diminished susceptibility to vancomycin (MIC increase and tolerance), daptomycin, and gentamicin, and resistance to rifampin during therapy.