Doripenem (formerly S-4661), a new 1-β-methyl carbapenem, was challenged with a worldwide collection of 394 drug-refractory isolates. For endemic extended-spectrum β-lactamase- and stably derepressed AmpC-producing enteric bacilli, the doripenem MICs at which 90% of the isolates were inhibited (MIC90s) were 0.03 to 0.5 μg/ml, generally lower than those of comparator carbapenems. A greater proportion of strains among carbapenem-resistant nonfermentative gram-negative bacilli were inhibited by doripenem at ≤4 μg/ml, and doripenem was the most active carbapenem (MIC90, 1 to 4 μg/ml) against penicillin-resistant streptococci.
The antimicrobial activity of BMS 284756, a novel des-F(6)-quinolone, was comparatively evaluated against 257 Streptococcus pneumoniae, 198 Haemophilus influenzae, and 88 Moraxella catarrhalis strains isolated in Latin America between July and September of 1999 as part of the SENTRY Antimicrobial Surveillance Program. Nearly 28.0% of S. pneumoniae strains were nonsusceptible to penicillin. The rank order of quinolone potency versus S. pneumoniae was BMS 284756 (MIC at which 90% of isolates were inhibited [MIC90], 0.12 μg/ml) > trovafloxacin (MIC90, 0.25 μg/ml) > gatifloxacin (MIC90, 0.5 μg/ml) > levofloxacin and ciprofloxacin (MIC90, 1 to 2 μg/ml). All S. pneumoniae strains that were not susceptible to other quinolones were inhibited by BMS 284756 at ≤2 μg/ml. The overall prevalence of β-lactamase production was 15.2% in H. influenzae and 98.9% in M. catarrhalis. BMS 284756 showed excellent potency and spectrum against this group of pathogens, inhibiting all isolates at ≤0.12 μg/ml. BMS 284756 exhibited activity similar to those displayed by the new fluoroquinolones, such as levofloxacin, trovafloxacin, or gatifloxacin, and could be a therapeutic option for empirical treatment of community-acquired respiratory tract infections.
Salmonella enterica serotype Typhi and nontyphoidal Salmonella remain major causes of morbidity and mortality worldwide. Ampicillin, trimethoprim-sulfamethoxazole, and chloramphenicol no longer provide reliable coverage of Salmonella, and fluoroquinoloes have emerged as first-line treatment options. Due to mounting evidence of decreased in vitro susceptibility and diminished clinical response to fluoroquinolone therapy, it has been suggested that the NCCLS breakpoints for the salmonellae be reevaluated. We utilized an in vitro infection model to determine which pharmacokinetic-pharmacodynamic (PK-PD) measure was most closely linked to fluoroquinolone activity against salmonellae and the magnitude that was predictive of efficacy. Monte Carlo simulation was utilized to determine the probability of attaining potential susceptibility breakpoints for three fluoroquinolones. The free-drug area under the concentration-time curve from 0 to 24 h/MIC ratio was the PK-PD measure most predictive of efficacy, and a ratio of 105 corresponded to 90% of maximal activity. Simulation results suggested susceptible breakpoints of 0.12 μg/ml for ciprofloxacin and gatifloxacin and 0.25 μg/ml for levofloxacin. These proposed breakpoints correspond to the MIC separating the wild-type susceptible organism population from those strains possessing single-step mutations in the quinolone resistance-determining region. These results that integrate PK-PD measures and fluoroquinolone MIC distributions in the genetic context of examined Salmonella isolates clearly demonstrate that the prudent use of a lower susceptibility breakpoint minimizes the probability of clinical failure or delayed response in fluoroquinolone-treated patients.
LB 11058 is a novel parenteral cephalosporin with a C-3 pyrimidinyl-substituted vinyl sulfide group and a C-7 2-amino-5-chloro-1,3-thiazole group. This study evaluated the in vitro activity and spectrum of LB 11058 against 1,245 recent clinical isolates, including a subset of gram-positive strains with specific resistant phenotypes. LB 11058 was very active against Streptococcus pneumoniae. The novel cephalosporin was 8- to 16-fold more potent than ceftriaxone, cefepime, or amoxicillin-clavulanate against both penicillin-intermediate and -resistant S. pneumoniae. LB 11058 was also very active against both β-hemolytic streptococci (MIC at which 90% of isolates were inhibited [MIC90], ≤0.008 μg/ml) and viridans group streptococci (MIC90, 0.03 to 0.5 μg/ml), including penicillin-resistant strains. Among oxacillin-susceptible Staphylococcus aureus, LB 11058 MIC results varied from 0.06 to 0.25 μg/ml (MIC50, 0.12 μg/ml), while among oxacillin-resistant strains LB 11058 MICs varied from 0.25 to 1 μg/ml (MIC50, 1 μg/ml). Coagulase-negative staphylococci showed an LB 11058 susceptibility pattern similar to that of S. aureus, with all isolates being inhibited at ≤1 μg/ml. LB 11058 also showed reasonable in vitro activity against Enterococcus faecalis, including vancomycin-resistant strains (MIC50, 1 μg/ml), and Bacillus spp. (MIC50, 0.25 μg/ml); however, it was less active against Enterococcus faecium (MIC50, >64 μg/ml) and Corynebacterium spp. (MIC50, 32 μg/ml). Against gram-negative pathogens, LB 11058 showed activity against Haemophilus influenzae (MIC90, 0.25 to 0.5 μg/ml) and Moraxella catarrhalis (MIC90, 0.25 μg/ml), with MICs not influenced by β-lactamase production. In conclusion, LB 11058 demonstrated a broad antibacterial spectrum and was highly active against gram-positive bacteria, particularly against multidrug-resistant staphylococci and streptococci.
In over a decade (2002 to 2012) of Staphylococcus aureus surveillance testing on 62,195 isolates, dalbavancin was demonstrated to be active against isolates that were either susceptible or nonsusceptible to daptomycin, linezolid, or tigecycline. Nearly all (99.8%) multidrug-resistant methicillin-resistant S. aureus isolates were inhibited by dalbavancin at ≤0.12 μg/ml (MIC50/90, 0.06/0.06 μg/ml), the current U.S. Food and Drug Administration (U.S. FDA) breakpoint. Overall, only 0.35% of the monitored S. aureus isolates had a dalbavancin MIC of either 0.25 or 0.5 μg/ml (i.e., were nonsusceptible).
Oritavancin is a recently approved lipoglycopeptide antimicrobial agent with activity against Gram-positive pathogens. Its extended serum elimination half-life and concentration-dependent killing enable single-dose treatment of acute bacterial skin and skin structure infections. At the time of regulatory approval, new agents, including oritavancin, are not offered in the most widely used susceptibility testing devices and therefore may require application of surrogate testing using a related antimicrobial to infer susceptibility. To evaluate vancomycin as a predictive susceptibility marker for oritavancin, 26,993 recent Gram-positive organisms from U.S. and European hospitals were tested using reference MIC methods. Organisms included Staphylococcus aureus, coagulase-negative staphylococci (CoNS), beta-hemolytic streptococci (BHS), viridans group streptococci (VGS), and enterococci (ENT). These five major pathogen groups were analyzed by comparing results with FDA-approved susceptible breakpoints for both drugs, as well as those suggested by epidemiological cutoff values and supported by pharmacokinetic/pharmacodynamic analyses. Vancomycin susceptibility was highly accurate (98.1 to 100.0%) as a surrogate for oritavancin susceptibility among the indicated pathogen species. Furthermore, direct MIC comparisons showed high oritavancin potencies, with vancomycin/oritavancin MIC90 results of 1/0.06, 2/0.06, 0.5/0.12,1/0.06, and >16/0.06 μg/ml for S. aureus, CoNS, BHS, VGS, and ENT, respectively. In conclusion, vancomycin demonstrated acceptable accuracy as a surrogate marker for predicting oritavancin susceptibility when tested against the indicated pathogens. In contrast, 93.3% of vancomycin-nonsusceptible enterococci had oritavancin MIC values of ≤0.12 μg/ml, indicating a poor predictive value of vancomycin for oritavancin resistance against these organisms. Until commercial oritavancin susceptibility testing devices are readily available, isolates that when tested show vancomycin susceptibility can be inferred to be susceptible to oritavancin by using FDA-approved breakpoints.
Telavancin had MIC50, MIC90, and MIC100 values of 0.03, 0.06, and 0.12 μg/ml, respectively, against methicillin-susceptible Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and non-multidrug-resistant (non-MDR) and MDR subsets. MRSA with elevated MIC values for vancomycin (2 to 4 μg/ml) or daptomycin (1 to 2 μg/ml) had telavancin MIC50 (0.06 μg/ml) values 2-fold higher than those of isolates with lower MIC results (MIC50, 0.03 μg/ml). However, telavancin had MIC90 and MIC100 results of 0.06 and 0.12 μg/ml (100% susceptible), respectively, regardless of the MRSA subset.
Reports of an increased clinical incidence of pertussis and the development of resistance by Bordetella pertussis to erythromycin prompted the collection and testing of recent clinical isolates from patients in northern California against a range of antimicrobial agents by the Etest (AB BIODISK, Solna, Sweden) method. All isolates were fully susceptible to all eight agents tested (MIC, ≤0.38 μg/ml), including newer fluoroquinolones, such as gatifloxacin (MIC of which 90% of the isolates tested are inhibited, 0.006 μg/ml), which may be used in cases of adolescent or adult pertussis. Continued surveillance of B. pertussis isolates appears to be a prudent practice.
We evaluated doripenem-resistant Acinetobacter baumannii-Acinetobacter calcoaceticus complex (ACB; n = 411) and Enterobacteriaceae (n = 92) isolates collected from patients from 14 European and Mediterranean countries during 2009 to 2011 for the presence of carbapenemase-encoding genes and clonality. Following susceptibility testing, carbapenem-resistant (doripenem MIC, >2 μg/ml) isolates were screened for carbapenemases. New β-lactamase genes were expressed in a common background and susceptibility was tested. Class 1 integrons were sequenced. Clonality was evaluated by pulsed-field gel electrophoresis and multilocus sequence typing (Pasteur scheme). Relative expression of β-lactam intrinsic resistance mechanisms was determined for carbapenemase-negative Enterobacteriaceae. ACB and Enterobacteriaceae displayed 58.9 and 0.9% doripenem resistance, respectively. blaOXA-23, blaOXA-58, and blaOXA-24/OXA-40 were detected among 277, 77, and 29 ACB, respectively (in 8, 6, and 5 countries). Ten Turkish isolates carried blaGES-11 or blaGES-22. GES-22 (G243A and M169L mutations in GES-1) had an extended-spectrum β-lactamase profile. A total of 33 clusters of ≥2 ACB isolates were observed, and 227 isolates belonged to sequence type 2/international clone II. Other international clones were limited to Turkey and Israel. Doripenem-resistant Enterobacteriaceae increased significantly (0.7 to 1.6%), and 15 blaKPC-2- and 22 blaKPC-3-carrying isolates, mostly belonging to clonal complexes 11 and 258, were observed. Enterobacteriaceae isolates producing OXA-48 (n = 16; in Turkey and Italy), VIM-1 (n = 10; in Greece, Poland, and Spain), VIM-26 (n = 1; in Greece), and IMP-19, VIM-4, and the novel VIM-35 (n = 1 each from Poland) were detected. VIM-35 had one substitution compared to VIM-1 (A235T) and a similar susceptibility profile. One or more resistance mechanisms were identified in 4/6 carbapenemase-negative Enterobacteriaceae. This broad evaluation confirms results from country-specific surveys and shows a highly diverse population of carbapenemase-producing ACB and Enterobacteriaceae in Europe and Mediterranean countries.
OprD loss and hyperexpression of AmpC, MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM were evaluated among 120 Pseudomonas aeruginosa isolates collected during 2012 in U.S. hospitals and selected based on ceftazidime MIC values (1 to >32 μg/ml). AmpC derepression (10-fold greater than that with the control) and OprD loss (decreased/no band) were the most prevalent resistance mechanisms: 47.5 and 45.8% of the isolates were considered positive, respectively. Elevated expression of the efflux pumps MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM was observed in 32.5, 8.3, 0.0, and 28.4% of the isolates, respectively. A total of 21 different combinations of resistance mechanisms were noted, and the most prevalent included AmpC derepression with OprD loss with and without efflux hyperexpression (38 and 10 isolates, respectively). A total of 26 isolates had no changes in the resistance mechanisms tested and had lower MIC values for all β-lactams or β-lactam/β-lactamase inhibitor combinations analyzed. OprD loss had a strong correlation with elevated MIC results for imipenem and meropenem (median MIC values of 8 and 4 μg/ml, respectively), with all combinations displaying OprD loss also displaying elevated median MIC values for these carbapenems (4 to >8 μg/ml). AmpC expression levels were greater in isolates displaying elevated cefepime, ceftazidime, or piperacillin-tazobactam MIC values (≥4, ≥4, and ≥16 μg/ml, respectively). Isolates displaying derepressed AmpC had ceftolozane-tazobactam MIC values ranging from 1 to 16 μg/ml. No strong correlation was noticed with MIC values for this β-lactam/β-lactamase inhibitor combination and OprD loss or hyperexpression of efflux systems. Two KPC-producing isolates were detected among 16 isolates displaying ceftolozane-tazobactam MIC values of ≥8 μg/ml.
It is important to understand the relationship between antibiotic exposure and the selection of drug resistance in the context of therapy exposure. We sought to identify the ceftolozane-tazobactam exposure necessary to prevent the amplification of drug-resistant bacterial subpopulations in a hollow-fiber infection model. Two Pseudomonas aeruginosa challenge isolates were selected for study, a wild-type ATCC strain (ceftolozane-tazobactam MIC, 0.5 mg/liter) and a clinical isolate (ceftolozane-tazobactam MIC, 4 mg/liter). The experiment duration was 10 days, and the ceftolozane-tazobactam dose ratio (2:1) and dosing interval (every 8 h) were selected to approximate those expected to be used clinically. The studied ceftolozane-tazobactam dosing regimens ranged from 62.5/31.25 to 2,000/1,000 mg per dose in step fold dilutions. Negative-control arms included no treatment and tazobactam at 500 mg every 8 h. Positive-control arms included ceftolozane at 1 g every 8 h and piperacillin-tazobactam dosed at 4.5 g every 6 h. For the wild-type ATCC strain, resistance was not selected by any ceftolozane-tazobactam regimen evaluated. For the clinical isolate, an inverted-U-shaped function best described the relationship between the amplification of a drug-resistant subpopulation and drug exposure. The least (62.5/31.25 mg) and most (2,000/1,000 mg) intensive ceftolozane-tazobactam dosing regimens did not select for drug resistance. Drug resistance selection was observed with intermediately intensive dosing regimens (125/62.5 through 1,000/500 mg). For the intermediately intensive ceftolozane-tazobactam dosing regimens, the duration until the selection for drug resistance increased with dose regimen intensity. These data support the selection of ceftolozane-tazobactam dosing regimens that minimize the potential for on-therapy drug resistance selection.
Totals of 8.7% (103/1,190) and 21.0% (249/1,190) of the Streptococcus pneumoniae isolates recovered from specimens collected in the United States during the 2011-2012 AWARE (Assessing Worldwide Antimicrobial Resistance Evaluation) Surveillance Program were ceftriaxone nonsusceptible according to the CLSI (≤1 μg/ml for susceptible) and EUCAST (≤0.5 μg/ml for susceptible) criteria, respectively. Decreased susceptibility to ceftriaxone (MIC, 1 μg/ml) was frequently observed among serotypes 19A (51.4%; 128/249) and 35B (29.7%; 74/249), which were most often observed in the East South Central and South Atlantic U.S. Census regions. Ceftaroline (MIC50/90, 0.12/0.25 μg/ml) remained active (≥96.8% susceptible) when tested against these less susceptible isolates.
Ceftobiprole medocaril is a newly approved drug in Europe for the treatment of hospital-acquired pneumonia (HAP) (excluding patients with ventilator-associated pneumonia but including ventilated HAP patients) and community-acquired pneumonia in adults. The aim of this study was to evaluate the in vitro antimicrobial activity of ceftobiprole against prevalent Gram-positive and -negative pathogens isolated in Europe, Turkey, and Israel during 2005 through 2010. A total of 60,084 consecutive, nonduplicate isolates from a wide variety of infections were collected from 33 medical centers. Species identification was confirmed, and all isolates were susceptibility tested using reference broth microdilution methods. Ceftobiprole had high activity against methicillin-susceptible Staphylococcus aureus (MSSA) (100.0% susceptible), methicillin-susceptible coagulase-negative staphylococci (CoNS), beta-hemolytic streptococci, and Streptococcus pneumoniae (99.3% susceptible), with MIC90 values of 0.25, 0.12, ≤0.06, and 0.5 μg/ml, respectively. Ceftobiprole was active against methicillin-resistant S. aureus (MRSA) (98.3% susceptible) and methicillin-resistant CoNS, having a MIC90 of 2 μg/ml. Ceftobiprole was active against Enterococcus faecalis (MIC50/90, 0.5/4 μg/ml) but not against most Enterococcus faecium isolates. Ceftobiprole was very potent against the majority of Enterobacteriaceae (87.3% susceptible), with >80% inhibited at ≤0.12 μg/ml. The potency of ceftobiprole against Pseudomonas aeruginosa (MIC50/90, 2/>8 μg/ml; 64.6% at MIC values of ≤4 μg/ml) was similar to that of ceftazidime (MIC50/90, 2/>16 μg/ml; 75.4% susceptible), but limited activity was observed against Acinetobacter spp. and Stenotrophomonas maltophilia. High activity was also observed against all Haemophilus influenzae (MIC90, ≤0.06 μg/ml) and Moraxella catarrhalis (MIC50/90, ≤0.06/0.25 μg/ml) isolates. Ceftobiprole demonstrated a wide spectrum of antimicrobial activity against this very large longitudinal sample of contemporary pathogens.
In this study, oritavancin had modal MIC, MIC50, and MIC90 values of 0.03, 0.03, and 0.06 μg/ml, respectively, against Staphylococcus aureus. Similar results (MIC50/90, 0.03/0.06 μg/ml) were observed against methicillin-resistant and -susceptible isolates and those demonstrating multidrug-resistant (MDR) and non-MDR phenotypes. When oritavancin (MIC50/90, 0.06/0.12 mg/ml) was tested against S. aureus with elevated MIC values for daptomycin (i.e., 1 to 4 mg/ml) and vancomycin (i.e., 2 mg/ml), it showed MIC results 2-fold higher than those for the more susceptible vancomycin or daptomycin counterparts (MIC50/90, 0.03/0.06 mg/ml), yet it inhibited these isolates at ≤0.25 mg/ml.
Streptococcus pneumoniae isolates (6,958) were collected from patients at 163 U.S. medical centers during 2009 through 2012. Isolates were evaluated for multidrug resistance (MDR) to penicillin, ceftriaxone, erythromycin, tetracycline, trimethoprim-sulfamethoxazole, and levofloxacin. Ceftaroline was 16-fold more potent than ceftriaxone (MIC50/MIC90, ≤0.25/2 μg/ml) against all isolates. For MDR isolates (35.2% of tested strains), ceftaroline (MIC50/MIC90, 0.06/0.25 μg/ml; 100.0% susceptible) was the most active agent tested, being 8-fold more potent than ceftriaxone (MIC50/MIC90, 0.5/2 μg/ml) and 16-fold more potent than penicillin (MIC50/MIC90, 1/4 μg/ml).
Tigecycline was initially approved by the U.S. Food and Drug Administration (FDA) in June 2005. We assessed the evolution of tigecycline in vitro activities since the initial approval of tigecycline for clinical use by analyzing the results of 7 years (2006 to 2012) of data from the SENTRY Antimicrobial Surveillance Program in the United States. We also analyzed trends over time for key resistance phenotypes. The analyses included 68,608 unique clinical isolates collected from 29 medical centers and tested for susceptibility using reference broth microdilution methods. Tigecycline was highly active against Gram-positive organisms, with MIC50 and MIC90 values of 0.12 and 0.25 μg/ml for Staphylococcus aureus (28,278 strains; >99.9% susceptible), 0.06 to 0.12 and 0.12 to 0.25 μg/ml for enterococci (99.3 to 99.6% susceptible), and ≤0.03 and ≤0.03 to 0.06 μg/ml for streptococci (99.9 to 100.0% susceptible), respectively. When tested against 20,457 Enterobacteriaceae strains, tigecycline MIC50 and MIC90 values were 0.25 and 1 μg/ml, respectively (98.3% susceptible using U.S. FDA breakpoints). No trend toward increasing tigecycline resistance (nonsusceptibility) was observed for any species or group during the study period. The prevalence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Enterobacteriaceae increased from 4.4 and 0.5%, in 2006 to 8.5 and 1.5% in 2012, respectively. During the same period, the prevalence of Escherichia coli and Klebsiella spp. with an extended-spectrum β-lactamase (ESBL) phenotype increased from 5.8 and 9.1% to 11.1 and 20.4%, respectively, whereas rates of meropenem-nonsusceptible Klebsiella pneumoniae escalated from 2.2% in 2006 to 10.8% in 2012. The results of this investigation show that tigecycline generally retained potent activities against clinically important organisms isolated in U.S. institutions, including MDR organism subsets of Gram-positive and Gram-negative pathogens.
The post-β-lactamase-inhibitor effect (PBLIE) of tazobactam combined with ceftolozane was evaluated by time-kill assays on two clinical Escherichia coli strains producing CTX-M-15 with or without TEM-1. The organisms were exposed (2 h) to 4 μg/ml/4 μg/ml of ceftolozane-tazobactam (4× MIC), 4 μg/ml of ceftolozane, and medium containing no drug, washed, and resuspended in medium alone or medium containing ceftolozane-tazobactam or ceftolozane. The PBLIE was determined as 1.3 to 2.1 h, and a postantibiotic effect was measured as 0.8 to 0.9 h.
The activities of the novel β-lactam–β-lactamase inhibitor combination ceftazidime-avibactam and comparator agents were evaluated against a contemporary collection of clinically significant Gram-negative bacilli. Avibactam is a novel non-β-lactam β-lactamase inhibitor that inhibits Ambler class A, C, and some D enzymes. A total of 10,928 Gram-negative bacilli—8,640 Enterobacteriaceae, 1,967 Pseudomonas aeruginosa, and 321 Acinetobacter sp. isolates—were collected from 73 U.S. hospitals and tested for susceptibility by reference broth microdilution methods in a central monitoring laboratory (JMI Laboratories, North Liberty, IA, USA). Ceftazidime was combined with avibactam at a fixed concentration of 4 μg/ml. Overall, 99.8% of Enterobacteriaceae strains were inhibited at a ceftazidime-avibactam MIC of ≤4 μg/ml. Ceftazidime-avibactam was active against extended-spectrum β-lactamase (ESBL)-phenotype Escherichia coli and Klebsiella pneumoniae, meropenem-nonsusceptible (MIC ≥ 2 μg/ml) K. pneumoniae, and ceftazidime-nonsusceptible Enterobacter cloacae. Among ESBL-phenotype K. pneumoniae strains, 61.1% were meropenem susceptible and 99.3% were inhibited at a ceftazidime-avibactam MIC of ≤4 μg/ml. Among P. aeruginosa strains, 96.9% were inhibited at a ceftazidime-avibactam MIC of ≤8 μg/ml, and susceptibility rates for meropenem, ceftazidime, and piperacillin-tazobactam were 82.0, 83.2, and 78.3%, respectively. Ceftazidime-avibactam was the most active compound tested against meropenem-nonsusceptible P. aeruginosa (MIC50/MIC90, 4/16 μg/ml; 87.3% inhibited at ≤8 μg/ml). Acinetobacter spp. (ceftazidime-avibactam MIC50/MIC90, 16/>32 μg/ml) showed high rates of resistance to most tested agents. In summary, ceftazidime-avibactam demonstrated potent activity against a large collection of contemporary Gram-negative bacilli isolated from patients in U.S. hospitals in 2012, including organisms that are resistant to most currently available agents, such as K. pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae and meropenem-nonsusceptible P. aeruginosa.
The reference broth microdilution (BMD) antimicrobial susceptibility testing method for telavancin was revised to include dimethyl sulfoxide (DMSO) as a solvent and diluent for frozen-form panel preparation, following the CLSI recommendations for water-insoluble agents. Polysorbate 80 (P-80) was also added to the test medium to minimize proven drug losses associated with binding to plastic surfaces. Four hundred sixty-two Gram-positive isolates, including a challenge set of organisms with reduced susceptibilities to comparator agents, were selected and tested using the revised method for telavancin, and the MIC results were compared with those tested by the previously established method and several Sensititre dry-form BMD panel formulations. The revised method provided MIC results 2- to 8-fold lower than the previous method when tested against staphylococci and enterococci, resulting in MIC50 values of 0.03 to 0.06 μg/ml for staphylococci and 0.03 and 0.12 μg/ml for Enterococcus faecium and Enterococcus faecalis, respectively. Less-significant MIC decreases (1 to 2 log2 dilution steps) were observed when testing streptococci in broth supplemented with blood, which showed similar MIC50 values for both methods. However, Streptococcus pneumoniae had MIC50 results of 0.008 and 0.03 μg/ml when tested by the revised and previous methods, respectively. Highest essential agreement rates (≥94.0%) were noted for one candidate dry-form panel formulation compared to the revised test. The revised BMD method provides lower MIC results for telavancin, especially when tested against staphylococci and enterococci. This is secondary to the use of DMSO for panel production and the presence of P-80, which ensure the proper telavancin testing concentration and result in a more accurate MIC determination. Moreover, earlier studies where the previous method was applied underestimated the in vitro drug potency.
This study summarizes the linezolid susceptibility testing results for 7,429 Gram-positive pathogens from 60 U.S. sites collected during the 2012 sampling year for the LEADER Program. Linezolid showed potent activity when tested against 2,980 Staphylococcus aureus isolates, inhibiting all but 3 at ≤2 μg/ml. Similarly, linezolid showed coverage against 99.5% of enterococci, as well as for all streptococci tested. These results confirm a long record of linezolid activity against U.S. Gram-positive isolates since regulatory approval in 2000.