Abstract
Heteroresistance refers to the presence, within a large population of antimicrobial-susceptible microorganisms, of subpopulations with lesser susceptibilities. Ceftaroline is a novel cephalosporin with activity against methicillin-resistant Staphylococcus aureus (MRSA). The aim of this study was to detect the prevalence of ceftaroline heteroresistance in vitro in a select group of S. aureus strains. There were 57 isolates selected for evaluation, 20 MRSA, 20 vancomycin-intermediate S. aureus (VISA), 7 daptomycin-nonsusceptible S. aureus (DNSSA), 6 linezolid-nonsusceptible S. aureus (LNSSA), and 4 heteroresistant VISA (hVISA) isolates. MICs and minimal bactericidal concentrations were determined using the broth microdilution method according to CLSI guidelines. All of the isolates were analyzed by pulsed-field gel electrophoresis. The staphylococcal cassette chromosome mec element (SCCmec) types were determined by a multiplex PCR. Population analysis profiles (PAPs) were performed to determine heteroresistance for all of the isolates using plates made by adding various amounts of ceftaroline to brain heart infusion agar. The frequencies of resistant subpopulations were 1 in 104 to 105 organisms. We determined that 12 of the 57 (21%) isolates tested were ceftaroline-heteroresistant S. aureus (CHSA). CHSA occurred among strains with reduced susceptibilities to vancomycin, daptomycin, and linezolid but occurred in none of the USA-300 isolates tested. Evaluation of the heteroresistant strains demonstrated that the phenotype was unstable. Further studies are needed to determine whether CHSA has a role in clinical failures and to determine the implications of our study findings.
INTRODUCTION
Ceftaroline is a new parenteral cephalosporin with antimicrobial activity against Staphylococcus aureus strains with reduced susceptibilities to methicillin and vancomycin and has been approved for the treatment of acute bacterial skin and skin structure infections and community-acquired (CA) pneumonia. It is a novel agent because of its avidity for penicillin-binding proteins (PBPs), including PBP2a, which is associated with methicillin resistance (1). There are a few strains of S. aureus that are intermediate to ceftaroline according to the breakpoints established by the Clinical and Laboratory Standards Institute (CLSI) (2, 3).
Heteroresistance refers to the presence, within a larger population of fully antimicrobial-susceptible microorganisms, of subpopulations with lesser susceptibilities (4). Although the clinical significance is still unclear, heteroresistance has been reported among various antimicrobial agents used against S. aureus, including beta-lactams and vancomycin. Typically, the subpopulations with lesser susceptibilities are present at frequencies of 1 subclone in every 105 to 106 colonies. This is why it is difficult to detect these clones in normal broth microdilution MIC testing using an inoculum of 5 × 104 CFU/well. Population analysis profiles (PAPs), which use a larger inoculum size, are considered the most reliable method for detecting heteroresistant subpopulations. The aim of this study was to detect the prevalence of ceftaroline heteroresistance in vitro in a select group of Staphylococcus aureus strains.
MATERIALS AND METHODS
Isolates.
A collection of 57 isolates was selected for evaluation. Methicillin-resistant S. aureus (MRSA) (n = 20), heteroresistant vancomycin-intermediate S. aureus (hVISA) (n = 4), and daptomycin-nonsusceptible S. aureus (DNSSA) (n = 7) isolates were obtained from patients admitted to St. John Hospital and Medical Center (Detroit, MI). VISA (n = 20) and four of the linezolid-nonsusceptible S. aureus (LNSSA) isolates were obtained through the Network on Antimicrobial Resistance in Staphylococcus aureus (NARSA) program (supported under NIAID/NIH contract HHSN272200700055C). Two LNSSA isolates were obtained from Robinson Memorial Hospital in Ohio. Either ceftaroline was not given to these patients, or their exposure was unknown.
Susceptibility testing.
MICs were determined using microdilution tests with cation-adjusted Mueller-Hinton broth. MICs were determined in accordance with CLSI guidelines. MICs were read visually as the lowest drug concentration well with no visible bacterial growth. Minimal bactericidal concentrations (MBC) were determined to be the antibiotic concentration that reduced the number of viable cells by ≥99% as determined by colony counts. We also determined MICs using an Etest strip containing ceftaroline.
Molecular testing.
Staphylococcal cassette chromosome mec element (SCCmec) types were determined by using a multiplex PCR method on all isolates (5). The isolates were analyzed by pulsed-field gel electrophoresis (PFGE) using the restriction enzyme SmaI. The PFGE gel patterns were compared with the development of a dendrogram using GelCompar II software (Applied Maths). Percent similarities were derived from the unweighted-pair group method using arithmetic averages (UPGMA) and based on Dice coefficients. The band position tolerance was set at 1.25, and optimization was set at 0.5%. An isolate was determined to belong to a PFGE strain group (USA-100 to USA-1100) if its similarity coefficient was ≥80% (6). The USA-100 to USA-1100 strains used for comparison were obtained from the NARSA.
Heteroresistance testing procedure.
PAP assays to detect heteroresistance were performed as previously described (7) with the following modifications. Testing plates were prepared by adding ceftaroline to brain heart infusion (BHI) agar (Difco). The BHI agar was prepared according to the manufacturer's instructions. Ceftaroline (CPT) powder was reconstituted and added to the BHI agar plates at concentrations of 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 2.0, 2.5, 3.0, and 4.0 μg/ml. The isolates to be tested were grown overnight on blood agar plates (BAP). The overnight BAP culture was suspended in saline and used to prepare samples at 107 CFU/ml and 104 CFU/ml. Aliquots of the samples containing 107 CFU/ml were than spiral plated (Whitley automatic spiral plater; Microbiology International) onto a drug-free BHI agar plate and onto BHI agar plates containing ceftaroline at all concentrations to determine the presence of heteroresistance. Aliquots of the samples containing 104 CFU/ml were spiral plated onto the drug-free BHI agar plate and onto plates containing 0.25, 0.5, and 0.75 μg/ml of ceftaroline. This was done to obtain an accurate determination of the numbers of CFU/ml. The plates were incubated at 35°C for 48 h in ambient air. The colonies were counted after 48 h using a ProtoCOL automated colony counter (Synbiosis, Frederick, MD, USA). PAPs were generated by plotting the log10 CFU/ml against the antibiotic concentrations. The frequency of resistant subpopulations at the highest drug concentration was calculated by dividing the number of colonies grown on an antibiotic-containing plate by the colony count from the same bacterial inoculum plated onto the antibiotic-free plate (8). We considered any sample with a susceptible ceftaroline MIC and growth in the intermediate or resistant range as heteroresistant. All samples were run three separate times, and the results were averaged for a final result. Colonies that grew on the plates containing the highest concentration of ceftaroline were removed and subcultured daily for 7 days onto antibiotic-free medium. The isolates were subcultured, and MICs were determined daily in order to determine if the resistance was stable (9).
RESULTS
Table 1 provides the data for PAPs for the isolates that did not demonstrate heteroresistance, as there was no growth above 1 μg/ml. Among these 41 isolates, there were 3 hVISA and 14 VISA strains. There were four VISA isolates (Table 2) that showed intermediate susceptibility to ceftaroline and did not demonstrate heteroresistance to ceftaroline. The PAPs disclosed heteroresistance among 12 isolates tested, with growth at ceftaroline concentrations of 1.25 to 3 μg/ml (Table 3). Ceftaroline heteroresistance was seen among 2 of the 20 VISA strains tested, which was not significantly different than 5 of the 20 MRSA strains tested (P = 0.41, Fisher's exact test). The occurrence of ceftaroline heteroresistance also occurred in 2 of the 6 LNSSA, 2 of the 7 DNSSA, and 1 of the 4 hVISA isolates. Note that although there were 9 USA-300 SCCmec type IVa isolates, none of these isolates demonstrated heteroresistance in this study (Table 1). The frequencies of resistance populations ranged from 1.1 × 10−5 to 2.2 × 10−4 (Table 3).
TABLE 1.
Results of samples that did not show growth above 1 μg/ml of ceftaroline on PAP plates
| Sample | Results (μg/ml) for CPTa |
PAP results (log10 CFU/ml) at concentrationb (μg/ml) of: |
SCCmec type | Pulse | Source | Results (μg/ml) for VANc |
Sample type | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| MIC | MBC | Etest | 0 | 0.25 | 0.5 | 0.75 | 1 | MIC | MBC | |||||
| SA-11 | 0.5 | 0.5 | 0.5 | 7.2 | 7.1 | 4.9 | 3.2 | 0 | IVa | USA-300 | Left knee | 1 | 1 | MRSA |
| SA-21 | 0.5 | 0.5 | 0.5 | 7.2 | 7.1 | 4.3 | 3.2 | 0 | IVa | USA-300 | Buttock | 1 | 1 | MRSA |
| SA-23 | 0.5 | 0.5 | 0.5 | 7 | 6.9 | 5.4 | 3.8 | 0 | IVa | USA-300 | Thigh | 1 | 1 | MRSA |
| SA-25 | 0.5 | 1 | 0.5 | 7.1 | 7 | 5 | 3.2 | 0 | IVa | USA-300 | Unknown | 1 | 1 | MRSA |
| TX-O | 0.5 | 0.5 | 0.5 | 7.2 | 7.2 | 6.1 | 3.7 | 0 | IVa | USA-300 | Blood | 1 | 1 | MRSA |
| D-7 | 1 | 1 | 1 | 6.8 | 6.7 | 6.8 | 6 | 0 | II | No match | Sputum | 1 | 1 | MRSA |
| D-10 | 1 | 1 | 1 | 6.9 | 6.9 | 6.8 | 5.4 | 3.1 | II | USA-600 | Unknown | 1 | 1 | MRSA |
| D-17 | 1 | 1 | 1 | 6.9 | 6.9 | 6.9 | 5.8 | 3.7 | II | USA-600 | Wound | 1 | 1 | MRSA |
| D-20 | 0.5 | 1 | 0.75 | 7.4 | 7.4 | 4.8 | 0 | 0 | IVa | USA-300 | Blood | 1 | 1 | MRSA |
| D-21 | 1 | 1 | 1 | 6.8 | 6.7 | 5.5 | 2.9 | 0 | II | USA-600 | Blood | 1 | 1 | MRSA |
| ND-10 | 0.5 | 0.5 | 0.5 | 7.1 | 6.9 | 3.7 | 2.7 | 0 | IV | No match | Respiratory | 0.5 | 1 | MRSA |
| ND-27 | 0.5 | 1 | 1 | 7.1 | 7.1 | 6.4 | 3.3 | 0 | II | USA-100 | Respiratory | 1 | 1 | MRSA |
| ND-29 | 1 | 1 | 1 | 7 | 6.9 | 6.8 | 5.1 | 0 | II | USA-600 | Respiratory | 1 | 1 | MRSA |
| ND-30 | 1 | 1 | 1 | 6.9 | 6.9 | 6.8 | 5.5 | 4.4 | II | USA-600 | Respiratory | 1 | 1 | MRSA |
| END-12 | 0.5 | 1 | 1 | 6.6 | 6.6 | 6.3 | 4.5 | 2.5 | II | USA-100 | Respiratory | 1 | 1 | MRSA |
| JhVISA-1 | 1 | 1 | 1 | 6.7 | 6.7 | 6.6 | 4.9 | 0 | II | No match | Blood | 1 | 1 | hVISA |
| JhVISA-3 | 0.5 | 1 | 1 | 6.8 | 6.9 | 6.7 | 5.6 | 2.7 | II | USA-100 | Blood | 1 | 1 | hVISA |
| JhVISA-4 | 0.5 | 1 | 0.75 | 7.1 | 7 | 5 | 4 | 3.7 | IVa | USA-300 | Blood | 2 | 2 | hVISA |
| DNS-1 | 0.5 | 1 | 1 | 7.1 | 7 | 6.9 | 6.2 | 2.5 | II | USA-100 | Blood | 1 | 2 | DNSSA |
| DNS-4 | 1 | 1 | 1 | 7 | 6.9 | 6.6 | 4.3 | 3.3 | III | No match | Blood | 2 | 2 | DNSSA |
| DNS-5 | 0.5 | 0.5 | 0.75 | 7.1 | 7 | 6.6 | 2.8 | 0 | IVa | USA-300 | Blood | 1 | 2 | DNSSA |
| DNS-6 | 1 | 1 | 0.75 | 7 | 7 | 6.6 | 0 | 0 | II | USA-100 | Blood | 2 | 2 | DNSSA |
| DNS-7 | 0.5 | 1 | 0.75 | 7 | 7 | 6.7 | 4.1 | 2.6 | IVa | USA-300 | Blood | 2 | 2 | DNSSA |
| NRS-1 | 1 | 1 | 0.75 | 6.8 | 6.7 | 6.6 | 6.6 | 4.5 | II | No match | Mu50 | 8 | 8 | VISA |
| NRS-14 | 0.5 | 0.5 | 0.25 | 6.8 | 6.7 | 4.3 | 0 | 0 | MSSA | No match | Eye | 8 | 16 | VISA |
| NRS-17 | 1 | 1 | 0.75 | 6.8 | 6.7 | 5.9 | 0 | 0 | II | USA-100 | Blood | 8 | 8 | VISA |
| NRS-18 | 0.5 | 0.5 | 0.5 | 6.3 | 6.2 | 3.4 | 0 | 0 | II | USA-100 | Wound | 4 | 8 | VISA |
| NRS-19 | 0.5 | 1 | 0.25 | 6.3 | 6.2 | 6.1 | 4.9 | 2.5 | II | No match | Blood | 4 | 4 | VISA |
| NRS-21 | 0.5 | 0.5 | 0.5 | 6.4 | 6.3 | 5.4 | 2.5 | 0 | IVd | USA-500 | Unknown | 4 | 4 | VISA |
| NRS-23 | 1 | 1 | 0.5 | 6.2 | 6 | 5.4 | 3.3 | 0 | II | USA-100 | Bone | 4 | 4 | VISA |
| NRS-49 | 1 | 2 | 0.5 | 6.5 | 6.5 | 6.5 | 5.9 | 3.5 | II | No match | Unknown | 8 | 8 | VISA |
| NRS-51 | 1 | 1 | 1 | 6.9 | 6.9 | 6.5 | 3.1 | 2.5 | II | No match | Bile | 4 | 4 | VISA |
| NRS-52 | 0.25 | 0.25 | 0.25 | 7.2 | 3 | 0 | 0 | 0 | MSSA | No match | Bile | 8 | 16 | VISA |
| NRS-56 | 1 | 1 | 0.75 | 6.3 | 6.2 | 6.2 | 4.2 | 0 | III | No match | Unknown | 4 | 8 | VISA |
| NRS-73 | 0.5 | 0.5 | 0.38 | 7.1 | 7.1 | 4.1 | 0 | 0 | IVd | USA-500 | Wound | 4 | 4 | VISA |
| NRS-126 | 1 | 1 | 1 | 6.9 | 6.9 | 6.4 | 4.3 | 3.5 | II | USA-100 | Blood | 4 | 4 | VISA |
| NRS-272 | 1 | 1 | 0.5 | 6.7 | 6.5 | 5 | 3.7 | 3 | I | No match | Unknown | 4 | 8 | VISA |
| NRS-127 | 0.5 | 1 | 0.75 | 7 | 7.1 | 6.1 | 2.4 | 0 | II | USA-100 | Sputum | 1 | 1 | LNSSA |
| NRS-271 | 1 | 1 | 1 | 7.1 | 7.1 | 6.6 | 4.8 | 4.2 | IV | No match | Unknown | 1 | 1 | LNSSA |
| LNSSA-10 | 0.5 | 1 | 0.75 | 6.9 | 6.9 | 3.6 | 0 | 0 | II | No match | Blood | 1 | 1 | LNSSA |
| LNSSA-11 | 1 | 1 | 1 | 6.7 | 6.6 | 5.9 | 4.4 | 0 | II | USA-100 | Blood | 1 | 1 | LNSSA |
CPT, ceftaroline.
Concentration (μg/ml) of CPT in PAP plates.
VAN, vancomycin.
TABLE 2.
Results of ceftaroline-intermediate organisms that did not show heteroresistance
| Sample | Results (μg/ml) for CPTa |
PAP results (log10 CFU/ml) at concentrationb (μg/ml) of: |
SCCmec type | Pulse | Source | Results (μg/ml) for VANd |
Sample type | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| MIC | MBC | Etest | 0 | 0.25 | 0.5 | 0.75 | 1 | 1.25 | 1.5 | 2 | 2.5c | MIC | MBC | |||||
| NRS-39 | 2 | 2 | 1.5 | 6.4 | 6.3 | 6.2 | 5.4 | 4.2 | 3 | 2.4 | 0 | 0 | I | No match | Urine | 8 | 8 | VISA |
| NRS-54 | 2 | 2 | 1 | 6.8 | 6.8 | 6.6 | 6.2 | 4.6 | 4.3 | 4 | 3.7 | 0 | III | No match | Unknown | 4 | 4 | VISA |
| NRS-65 | 2 | 2 | 2 | 6.3 | 6.3 | 6.3 | 6.2 | 6.1 | 5.2 | 4.6 | 4 | 0 | III | No match | Unknown | 4 | 8 | VISA |
| NRS-283 | 2 | 2 | 1.5 | 6.9 | 6.9 | 6.8 | 6.8 | 6.5 | 5.2 | 4.1 | 2.8 | 2.5 | II | USA-200 | Unknown | 4 | 4 | VISA |
CPT, ceftaroline.
Concentration (μg/ml) of CPT in PAP plates.
For all organisms, there was no growth at a CPT concentration of 3 μg/ml or 4 μg/ml.
VAN, vancomycin.
TABLE 3.
Results of samples that were determined to be heteroresistant
| Sample | Results (μg/ml) for CPTa |
PAP results (log10 CFU/ml) at concentrationb of: |
Frequency | SCCmec type | Pulse | Source | Results (μg/ml) for VANc |
Sample type | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| MIC | MBC | Etest | 0 | 0.25 | 0.5 | 0.75 | 1 | 1.25 | 1.5 | 2 | 2.5 | 3 | 4 | MIC | MBC | ||||||
| D-9 | 1 | 1 | 1 | 7 | 6.9 | 6.9 | 4.7 | 3.8 | 3.3 | 2.7 | 2.3 | 2.1 | 0 | 0 | 1.1 × 10−5 | II | USA-100 | Blood | 1 | 1 | MRSA |
| D-11 | 1 | 1 | 1 | 6.9 | 6.9 | 6.8 | 6.1 | 3.8 | 3.2 | 2.8 | 2.4 | 2 | 0 | 0 | 1.3 × 10−5 | II | USA-600 | Blood | 1 | 1 | MRSA |
| D-27 | 1 | 2 | 1 | 6.9 | 6.9 | 6.8 | 6.1 | 3 | 2.6 | 2.6 | 0 | 0 | 0 | 0 | 5.2 × 10−5 | II | USA-100 | Wound | 1 | 1 | MRSA |
| ND-14 | 1 | 1 | 1 | 7 | 7 | 6.7 | 5.9 | 3.1 | 2.6 | 2.3 | 2.1 | 0 | 0 | 0 | 1.1 × 10−5 | II | USA-100 | Respiratory | 1 | 1 | MRSA |
| ND-33 | 1 | 1 | 1 | 6.8 | 6.8 | 6.7 | 5.9 | 3.1 | 2.5 | 2.2 | 0 | 0 | 0 | 0 | 1.4 × 10−5 | II | No match | Respiratory | 2 | 2 | MRSA |
| DNS-2 | 1 | 1 | 1 | 6.9 | 6.8 | 6.7 | 6.2 | 3.6 | 2.8 | 2.5 | 0 | 0 | 0 | 0 | 4.8 × 10−5 | II | No match | Blood | 2 | 2 | DNSSA |
| DNS-3 | 1 | 2 | 1.5 | 6.9 | 6.8 | 6.7 | 6.5 | 5.9 | 3.1 | 2.7 | 2.4 | 0 | 0 | 0 | 3.8 × 10−5 | II | USA-100 | Blood | 2 | 2 | DNSSA |
| NRS-3 | 1 | 1 | 1.5 | 6.4 | 6.3 | 6.3 | 6.2 | 5.2 | 3 | 2.6 | 2 | 0 | 0 | 0 | 3.8 × 10−5 | II | USA-100 | Peritoneal | 8 | 8 | VISA |
| NRS-118 | 1 | 2 | 1.5 | 6.8 | 6.7 | 6.7 | 6.6 | 6.5 | 5.7 | 4.5 | 3.3 | 2.6 | 2.3 | 0 | 3.2 × 10−5 | I | No match | Respiratory | 8 | 8 | VISA |
| NRS-120 | 1 | 1 | 1 | 6.8 | 6.8 | 6.5 | 5.9 | 4.4 | 2.7 | 2.6 | 0 | 0 | 0 | 0 | 6.9 × 10−5 | IVd | USA-500 | Unknown | 2 | 2 | LNSSA |
| NRS-121 | 1 | 1 | 1 | 6.8 | 6.7 | 6.4 | 5.9 | 4.8 | 3.1 | 0 | 0 | 0 | 0 | 0 | 2.2 × 10−4 | IVd | USA-500 | Unknown | 1 | 2 | LNSSA |
| JhVISA-2 | 1 | 1 | 1 | 6.7 | 6.7 | 6.6 | 4.5 | 3.4 | 3.2 | 3 | 2.5 | 2 | 0 | 0 | 2.1 × 10−5 | II | USA-100 | Blood | 2 | 2 | hVISA |
CPT, ceftaroline.
Concentration (μg/ml) of CPT in PAP plates.
VAN, vancomycin.
The heterogeneous growth in the presence of ceftaroline was determined to be unstable. After daily passages onto a drug-free medium, the MICs of the colonies obtained from the plates with the highest concentrations of ceftaroline returned to the MICs of the native isolates.
DISCUSSION
Among the 57 isolates tested, 12 (21%) of the strains demonstrated heteroresistance by the population analysis profiling method, which is considered an acceptable method for determining heteroresistance. Heteroresistance was found in strains which were nonsusceptible to daptomycin, resistant to linezolid, and intermediate in susceptibility to vancomycin. Finally, we did identify one strain heteroresistant to ceftaroline and vancomycin. The rate of ceftaroline heteroresistance in this study was higher than the rate of vancomycin heteroresistance (2.16% among MRSA isolates) reported by Liu and Chambers (10), who reviewed 14 studies. More recently, it was reported at a higher rate of 8.1%, as noted by Khatib et al. (11).
This rate of ceftaroline heteroresistance may be of concern if heteroresistance is a precursor to resistant isolates. If the selective pressure of prolonged exposure to an antimicrobial agent enhances the likelihood of the emergence of organisms resistant to the therapeutic agent administered, we would expect to see an increasing number of clinical cases resistant to ceftaroline with more widespread use of this agent. The comparison of ceftaroline heteroresistance in this study to vancomycin heteroresistance in other studies may not be valid with the selection bias used in identifying our organisms as opposed to the random selection of MRSA isolates used in the vancomycin heteroresistance studies.
The mechanism for ceftaroline heteroresistance is unknown. We know that these strains are virulent, as they were isolated from clinical infections, including bacteremia, pneumonia, and wound infections. We do not know if these strains are more or less virulent than other ceftaroline-susceptible strains. To date, clinical cases of failure due to ceftaroline heteroresistance have not been reported.
Vancomycin-heteroresistant strains have demonstrated lower growth rates and thicker cell walls than vancomycin-susceptible strains (12). In addition, these strains also produce greater quantities of PBP2 and PBP2′ (13). Several isolates with high ceftaroline MICs (4 μg/ml) obtained from an antibiotic resistance surveillance system demonstrated decreased PBP2a binding affinity due to alterations in the PBP2a (14). Although our study did not evaluate strains for PBP production or cell wall thickness, we did not see a correlation with ceftaroline heteroresistance and vancomycin heteroresistance. Recent in vitro studies of ceftaroline activity against MRSA isolates with reduced vancomycin susceptibility demonstrated increased activity compared with isolates with lower vancomycin MICs (15); however, this observation was not seen among the isolates evaluated in this study.
The clinical significance of ceftaroline heteroresistance is unclear. Heteroresistant strains were mainly SCCmec type II (75%), and no strains were found to be SCCmec type III or IVa. The small sample size of the CA-MRSA isolates in this study limits the generalizability of the results. Studies evaluating the frequency of ceftaroline heteroresistance among a larger set of CA-MRSA isolates should be performed to confirm this finding.
The information in this study should suggest caution by clinicians using ceftaroline in patients with resistance to other anti-MRSA agents, as heteroresistance was seen in isolates demonstrating reduced susceptibilities to daptomycin and linezolid, and increased ceftaroline MICs were noted in four VISA isolates. To date, the occurrence of ceftaroline heteroresistance has not been shown to be a risk factor for the clinical failure of ceftaroline, and the mechanism for the development of these strains is not known. Further work should be done to evaluate the risk factors for and clinical significance of ceftaroline heteroresistance.
ACKNOWLEDGMENT
Funding for this study was provided by Forest Laboratories, Inc.
Footnotes
Published ahead of print 17 March 2014
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