scholarly journals Towards a Standardized Method for Broth Microdilution Susceptibility Testing of Haemophilus parasuis

2016 ◽  
Vol 55 (1) ◽  
pp. 264-273 ◽  
Author(s):  
Sandra Prüller ◽  
Conny Turni ◽  
Patrick J. Blackall ◽  
Martin Beyerbach ◽  
Günter Klein ◽  
...  
Keyword(s):  
Antimicrobial Agents ◽  
Susceptibility Testing ◽  
Haemophilus Parasuis ◽  
Broth Microdilution ◽  
Content Type ◽  
Medium Components ◽  
Mueller Hinton ◽  
Significant Difference ◽  
Visible Growth ◽  
Mueller Hinton Broth

ABSTRACTCurrently, there is no agreed method available for broth microdilution susceptibility testing ofHaemophilus parasuis, one of the most important bacterial pathogens in pig production. Therefore, the aim of this study was to develop a method that could be easily performed by diagnostic laboratories and that appears suitable for a harmonized susceptibility testing. Growth determinations using one type strain and three field isolates revealed no visible growth ofH. parasuisin media which have proven to be suitable for susceptibility testing of fastidious organisms. Therefore, a new medium, cation-adjusted Mueller-Hinton broth (CAMHB) plus NADH and sterile filtered heat-inactivated chicken serum, was developed. The reproducibility of MICs obtained in this medium was evaluated and statistically analyzed, considering a model with two different variables (precondition of five identical MICs and MIC mode accepting a deviation of ±1 dilution step, respectively). No significant differences for both variables were seen between two time points investigated and between results obtained with the recently proposed test medium broth (TMB). Nearly all MICs of quality control strains were in the acceptable range. Subsequently, 47H. parasuisisolates representing 13 serovars were tested with the newly developed medium and TMB. Statistical analysis of all isolates and 15 antimicrobial agents and antimicrobial combinations showed no significant difference between MICs obtained in supplemented CAMHB and TMB. Because of a simplified implementation in routine diagnostic and a lower chance of interference between medium components and antimicrobial agents, supplemented CAMHB is recommended with an incubation time of 24 h.

scholarly journals Evaluation of cefiderocol activity for Gram-negative bacteria and performance of cefiderocol disk diffusion testing using multiple brands of Mueller Hinton Agar

2021 ◽  
Vol 156 (Supplement_1) ◽  
pp. S5-S5
Author(s):  
Robert Potter ◽  
Meghan Wallace ◽  
Carey-Ann Burnham
Keyword(s):  
Susceptibility Testing ◽  
Disk Diffusion ◽  
Resistant Bacteria ◽  
Clinical Microbiology Laboratory ◽  
Gram Negative Bacteria ◽  
Broth Microdilution ◽  
Gram Negative ◽  
Mueller Hinton ◽  
Significant Difference ◽  
Mueller Hinton Agar

Abstract Cefidericol is a cephalosporin-siderophore antibiotic for the treatment of multidrug resistant Gram-negative bacteria. Similar to other cephalosporin antibiotics, the lethal mechanism of action is due to inhibition of penicillin binding proteins leading to lysis of the bacteria. However, unlike previously developed antibiotics, the siderophore portion of cefidericol is able to bind iron and then be actively transported into the periplasmic space. To ascertain the feasibility of cefidericol antibiotic susceptibility testing in the Barnes-Jewish Clinical Microbiology Laboratory, we collected a cohort of multidrug Enterobacteriacae (5 Enterobacter cloace, 8 Escherichia coli, 12 Klebsiella pneumoniae), Pseudomonas aeruginosa (n=23), Stenotrophomonas maltophila (n=24), and Acinetobacter baumannii (n=25). We evaluated activity of cefidericol on these strains, and the performance of disk diffusion using three different brands of Mueller-Hinton Agar (BD, Hardy, and Remel). The reference method for comparison was an FDA-cleared broth microdilution panel containing cefidericol (ThermoFisher Scientific). Using CLSI breakpoints, we found that disk diffusion with BD agar had 96% categorical agreement for Enterobacterales, 100% for P. aeruginosa, 92% for A. baumannii, 96% for S. maltophila. We found that Hardy had 96% categorical agreement for Enterobacterales, 92% for P. aeruginosa, 92% for A. baumannii, 96% for S. maltophila. Finally, we found that Hardy had 96% categorical agreement for Enterobacterales, 92% for P. aeruginosa, 92% for A. baumannii, 96% for S. maltophila. Minor errors on any media never exceed 4% and there were no very major errors. Resistance to cefidericol within our cohort of selected antibiotic resistant bacteria was rare, one E. coli isolate and two P. aeruginosa isolates had minimal inhibitory concentrations (MICs) > 32 μg/mL. The highest MICs for one isolate of A. baumannii and one isolate S. maltophila was 8 μg/mL and 4 μg/mL, respectively, both of which were intermediate. There was no difference in the distribution of zone disk diffusion diameter for A. baumannii or Enterobacterales. However, there was a significant difference in the distribution of zone disk diffusion diameters for P. aeruginosa and S. maltophila on BD vs Hardy agar. The median for P. aeruginosa on BD is 25 mm while it is 29 mm on Hardy. The trend for S. maltophila is the opposite as the median for BD was 31.5 mm and 28.5 mm for Hardy. Use of FDA vs CLSI vs EUCAST breakpoints significantly changes outcome of susceptibility testing for broth microdilution and disk diffusion. As one example for broth microdilution of A. baumannii, we had one isolate intermediate using CLSI breakpoints, 4 resistant using EUCAST breakpoints, and 4 resistant and 3 intermediate isolates using FDA breakpoints. Our work demonstrates that cefedericol testing can be performed in a routine format, with certain organismal differences on Mueller-Hinton agar, and that different interpretative criteria significantly change outcomes.

scholarly journals Simple Phenotypic Tests To Improve Accuracy in Screening Chromosomal and Plasmid-Mediated Colistin Resistance in Gram-Negative Bacilli

2020 ◽  
Vol 59 (1) ◽  
pp. e01701-20
Author(s):  
Fernando Pasteran ◽  
Diego Danze ◽  
Alejandra Menocal ◽  
Carla Cabrera ◽  
Ignacio Castillo ◽  
...  
Keyword(s):  
Susceptibility Testing ◽  
Agar Plate ◽  
Systematic Screening ◽  
Gram Negative ◽  
Content Type ◽  
Mueller Hinton ◽  
Improve Accuracy ◽  
Mueller Hinton Agar ◽  
Phenotypic Tests ◽  
Mueller Hinton Broth

ABSTRACTCLSI and EUCAST recommend that only broth microdilution (BMD) should be used for routine colistin susceptibility testing; however, this technique can be difficult to perform in resource-poor settings. The purpose of this study was to evaluate the accuracy of a colistin agar spot test (COL-AS) and a colistin drop test (COL-DT) compared to BMD. COL-AS and COL-DT were assessed with a collection of 271 Gram-negative bacilli clinical isolates: 195 Enterobacterales (including 63 mcr-1 positive strains), 37 Acinetobacter spp., and 39 Pseudomonas aeruginosa. For COL-AS, 3.0 μg/ml (final concentration) of colistin was added to a Mueller-Hinton agar plate and subsequently swabbed with a 0.5 McFarland standard suspension of the tested strain within a 1 cm2 spot. For COL-DT, 10 μl of a 16 μg/ml colistin solution was dripped on the surface of a Mueller-Hinton agar plate, previously inoculated with a lawn of the tested strain (0.5 McFarland standard). Colistin solution was made either by dissolving powder or by disk elution in cation-adjusted Mueller-Hinton broth (CA-MHB). Overall, 141/271 (52%) isolates were categorized as colistin resistant by reference BMD. COL-AS yielded a categorical agreement (CA) of 95.5% compared to BMD, with 0.7% very major errors and 3.8% major errors. COL-DT yielded a CA of 96.2% compared to BMD, with 0.7% and 0% very major errors and 3.1% and 3.8% major errors, for colistin powder and disk elution solutions, respectively. Most major errors occurred for mcr-1 strains with MICs that fluctuated from 2 to 4 μg/ml according to the method used. In conclusion, we developed and validated methods suited to the systematic screening of resistance to colistin in Gram-negative bacilli.

scholarly journals In Vitro Activity of the Siderophore Cephalosporin, Cefiderocol, against Carbapenem-Nonsusceptible and Multidrug-Resistant Isolates of Gram-Negative Bacilli Collected Worldwide in 2014 to 2016

2017 ◽  
Vol 62 (2) ◽  
Author(s):  
Meredith A. Hackel ◽  
Masakatsu Tsuji ◽  
Yoshinori Yamano ◽  
Roger Echols ◽  
James A. Karlowsky ◽  
...  
Keyword(s):  
Burkholderia Cepacia ◽  
Antimicrobial Agents ◽  
Multidrug Resistant ◽  
Clinical Isolates ◽  
Content Type ◽  
Microdilution Method ◽  
Mueller Hinton ◽  
Broth Microdilution Method ◽  
Mueller Hinton Broth

ABSTRACT The in vitro activity of the investigational siderophore cephalosporin, cefiderocol (formerly S-649266), was determined against a 2014–2016, 52-country, worldwide collection of clinical isolates of carbapenem-nonsusceptible Enterobacteriaceae (n = 1,022), multidrug-resistant (MDR) Acinetobacter baumannii (n = 368), MDR Pseudomonas aeruginosa (n = 262), Stenotrophomonas maltophilia (n = 217), and Burkholderia cepacia (n = 4) using the Clinical and Laboratory Standards Institute (CLSI) standard broth microdilution method. Iron-depleted cation-adjusted Mueller-Hinton broth (ID-CAMHB), prepared according to a recently approved (2017), but not yet published, CLSI protocol, was used to test cefiderocol; all other antimicrobial agents were tested using CAMHB. The concentration of cefiderocol inhibiting 90% (MIC90) of isolates of carbapenem-nonsusceptible Enterobacteriaceae was 4 μg/ml; cefiderocol MICs ranged from 0.004 to 32 μg/ml, and 97.0% (991/1,022) of isolates demonstrated cefiderocol MICs of ≤4 μg/ml. The MIC90s for cefiderocol for MDR A. baumannii, MDR P. aeruginosa, and S. maltophilia were 8, 1, and 0.25 μg/ml, respectively, with 89.7% (330/368), 99.2% (260/262), and 100% (217/217) of isolates demonstrating cefiderocol MICs of ≤4 μg/ml. Cefiderocol MICs for B. cepacia ranged from 0.004 to 8 μg/ml. We conclude that cefiderocol demonstrated potent in vitro activity against a 2014–2016, worldwide collection of clinical isolates of carbapenem-nonsusceptible Enterobacteriaceae, MDR A. baumannii, MDR P. aeruginosa, S. maltophilia, and B. cepacia isolates as 96.2% of all (1,801/1,873) isolates tested had cefiderocol MICs of ≤4 μg/ml.

scholarly journals Effect of In Vitro Testing Parameters on Ceftazidime-Avibactam Minimum Inhibitory Concentrations

2015 ◽  
Vol 2015 ◽  
pp. 1-6 ◽  
Author(s):  
Tiffany R. Keepers ◽  
Marcela Gomez ◽  
Donald Biek ◽  
Ian Critchley ◽  
Kevin M. Krause
Keyword(s):  
Susceptibility Testing ◽  
Ambient Air ◽  
Medium Composition ◽  
In Vitro Testing ◽  
Acidic Ph ◽  
Broth Microdilution ◽  
Mueller Hinton ◽  
Horse Blood ◽  
Mueller Hinton Broth

Effects of varying in vitro susceptibility testing parameters of the broth microdilution assay on ceftazidime-avibactam MICs were determined and compared to meropenem and piperacillin-tazobactam for 9 Enterobacteriaceae and 4 Pseudomonas aeruginosa isolates. The effect of varying incubation conditions (ambient air or 5% CO2), pH of medium, medium composition (cation-adjusted Mueller Hinton Broth with and without laked horse blood and Haemophilus Test Medium), cation content of the medium, and inoculum density were tested. Most variations had no effect on ceftazidime-avibactam MIC values (no more than a 2-fold change). However, acidic pH or high inoculum resulted in 4- to 16-fold changes in MIC, which was similar to those observed for meropenem and piperacillin-tazobactam under these conditions. Overall, this study shows that slight variations in testing parameters during routine MIC testing will likely have no significant effect on ceftazidime-avibactam MIC values.

scholarly journals Verification of an Automated, Digital Dispensing Platform for At-Will Broth Microdilution-Based Antimicrobial Susceptibility Testing

2016 ◽  
Vol 54 (9) ◽  
pp. 2288-2293 ◽  
Author(s):  
Kenneth P. Smith ◽  
James E. Kirby
Keyword(s):  
Antimicrobial Agents ◽  
Susceptibility Testing ◽  
Multidrug Resistant ◽  
Resistant Bacteria ◽  
Reference Laboratory ◽  
Broth Microdilution ◽  
Content Type ◽  
Susceptibility Data ◽  
Stock Solutions ◽  
At Will

With rapid emergence of multidrug-resistant bacteria, there is often a need to perform susceptibility testing for less commonly used or newer antimicrobial agents. Such testing can often be performed only by using labor-intensive, manual dilution methods and lies outside the capacity of most clinical labs, necessitating reference laboratory testing and thereby delaying the availability of susceptibility data. To address the compelling clinical need for microbiology laboratories to perform such testing in-house, we explored a novel, automated, at-will broth microdilution-based susceptibility testing platform. Specifically, we used the modified inkjet printer technology in the HP D300 digital dispensing system to dispense, directly from stock solutions into a 384-well plate, the 2-fold serial dilution series required for broth microdilution testing. This technology was combined with automated absorbance readings and data analysis to determine MICs. Performance was verified by testing members of theEnterobacteriaceaefor susceptibility to ampicillin, cefazolin, ciprofloxacin, colistin, gentamicin, meropenem, and tetracycline in comparison to the results obtained with a broth microdilution reference standard. In precision studies, essential and categorical agreement levels were 96.8% and 98.3%, respectively. Furthermore, significantly fewer D300-based measurements were outside ±1 dilution from the modal MIC, suggesting enhanced reproducibility. In accuracy studies performed using a panel of 80 curated clinical isolates, rates of essential and categorical agreement and very major, major, and minor errors were 94%, 96.6%, 0%, 0%, and 3.4%, respectively. Based on these promising initial results, it is anticipated that the D300-based methodology will enable hospital-based clinical microbiology laboratories to perform at-will broth microdilution testing of antimicrobials and to address a critical testing gap.

scholarly journals Performance of Vitek 2 for Antimicrobial Susceptibility Testing of Enterobacteriaceae with Vitek 2 (2009 FDA) and 2014 CLSI Breakpoints

2014 ◽  
Vol 53 (3) ◽  
pp. 816-823 ◽  
Author(s):  
April M. Bobenchik ◽  
Eszter Deak ◽  
Janet A. Hindler ◽  
Carmen L. Charlton ◽  
Romney M. Humphries
Keyword(s):  
Antimicrobial Susceptibility ◽  
Antimicrobial Agents ◽  
Susceptibility Testing ◽  
Test System ◽  
Broth Microdilution ◽  
Antimicrobial Susceptibility Test ◽  
Content Type ◽  
Carbapenem Resistant ◽  
Clinical Laboratories ◽  
Vitek 2

Vitek 2 (bioMérieux Inc., Durham, NC) is a widely used commercial antimicrobial susceptibility test system. We compared the MIC results obtained using the Vitek 2 AST-GN69 and AST-XN06 cards to those obtained by CLSI broth microdilution (BMD) for 255 isolates ofEnterobacteriaceae, including 25 isolates of carbapenem-resistantEnterobacteriaceae. In total, 25 antimicrobial agents were examined. For 10 agents, the MIC data were evaluated using two sets of breakpoints: (i) the Vitek 2 breakpoints, which utilized the 2009 FDA breakpoints at the time of the study and are equivalent to the 2009 CLSI M100-S19 breakpoints, and (ii) the 2014 CLSI M100-S24 breakpoints. There was an overall 98.7% essential agreement (EA). The categorical agreement was 95.5% (CA) using the Vitek 2 breakpoints and 95.7% using the CLSI breakpoints. There was 1 very major error (VME) (0.05%) observed using the Vitek 2 breakpoints (cefazolin) and 8 VMEs (0.5%) using the CLSI breakpoints (2 each for aztreonam, cefepime, and ceftriaxone, and 1 for cefazolin and ceftazidime). Fifteen major errors (MEs) (0.4%) were noted using the Vitek 2 breakpoints and 8 (0.5%) using the CLSI breakpoints. Overall, the Vitek 2 performance was comparable to that of BMD for testing a limited number ofEnterobacteriaceaecommonly isolated by clinical laboratories. Ongoing studies are warranted to assess performance in isolates with emerging resistance.

scholarly journals In Vitro Activities of Daptomycin against 2,789 Clinical Isolates from 11 North American Medical Centers

2001 ◽  
Vol 45 (6) ◽  
pp. 1919-1922 ◽  
Author(s):  
Arthur L. Barry ◽  
Peter C. Fuchs ◽  
Steven D. Brown
Keyword(s):  
Clinical Isolates ◽  
In Vitro Activity ◽  
Broth Microdilution ◽  
In Vitro Susceptibility ◽  
Medical Centers ◽  
Mueller Hinton ◽  
Calcium Cations ◽  
Important Exception ◽  
Mueller Hinton Broth

ABSTRACT The in vitro activity of daptomycin is affected by the concentration of calcium cations in the test medium. Mueller-Hinton broth is currently adjusted to contain 10 to 12.5 mg of magnesium per liter and 20 to 25 mg of calcium per liter, but for testing of daptomycin, greater concentrations of calcium (50 mg/liter) are recommended to better resemble the normal concentration of ionized calcium in human serum. Two levels of calcium were used for broth microdilution tests of 2,789 recent clinical isolates of gram-positive bacterial pathogens. MICs of daptomycin were two- to fourfold lower when the broth contained additional calcium. For most species, however, the percentages of strains that were inhibited by 2.0 μg of daptomycin per ml were essentially identical with the two broth media. Enterococci were the important exception; i.e., 92% were inhibited when tested in calcium-supplemented broth but only 35% were inhibited by 2.0 μg/ml without the additional calcium. This type of information should be considered when selecting criteria for defining in vitro susceptibility to daptomycin.

scholarly journals An Improved Medium for Colistin Susceptibility Testing

2018 ◽  
Vol 56 (5) ◽  
Author(s):  
Konrad Gwozdzinski ◽  
Saina Azarderakhsh ◽  
Can Imirzalioglu ◽  
Linda Falgenhauer ◽  
Trinad Chakraborty
Keyword(s):  
Susceptibility Testing ◽  
Acquired Resistance ◽  
Multidrug Resistant ◽  
Colistin Resistance ◽  
Reliable Determination ◽  
Content Type ◽  
E Coli ◽  
Resistant Species ◽  
Mueller Hinton ◽  
Serovar Typhimurium

ABSTRACTThe plasmid-located colistin resistance genemcr-1confers low-level resistance to colistin, a last-line antibiotic against multidrug-resistant Gram-negative bacteria. Current CLSI-EUCAST recommendations require the use of a broth microdilution (BMD) method with cation-adjusted Mueller-Hinton (CA-MH) medium for colistin susceptibility testing, but approximately 15% of all MCR-1 producers are classified as sensitive in that broth. Here we report on an improved calcium-enhanced Mueller-Hinton (CE-MH) medium that permits simple and reliable determination ofmcr-1-containingEnterobacteriaceae. Colistin susceptibility testing was performed for 50mcr-1-containingEscherichia coliandKlebsiella pneumoniaeisolates, 7 intrinsically polymyxin-resistant species,K. pneumoniaeandE. coliisolates with acquired resistance to polymyxins due tomgrBandpmrBmutations, respectively, and 32mcr-1-negative, colistin-susceptible isolates ofAcinetobacter baumannii,Citrobacter freundii,Enterobacter cloacae,E. coli,K. pneumoniae, andSalmonella entericaserovar Typhimurium. A comparison of the colistin MICs determined in CA-MH medium and those obtained in CE-MH medium was performed using both the BMD and strip-based susceptibility test formats. We validated the data using an isogenic IncX4 plasmid lackingmcr-1. Use of the CE-MH broth provides clear separation between resistant and susceptible isolates in both BMD and gradient diffusion assays; this is true for bothmcr-1-containingEnterobacteriaceaeisolates and those exhibiting either intrinsic or acquired colistin resistance. CE-MH medium is simple to prepare and overcomes current problems associated with BMD and strip-based colistin susceptibility testing, and use of the medium is easy to implement in routine diagnostic laboratories, even in resource-poor settings.

scholarly journals Agreement Assessment of Tigecycline Susceptibilities Determined by the Disk Diffusion and Broth Microdilution Methods among Commonly Encountered Resistant Bacterial Isolates: Results from the TigecyclineIn VitroSurveillance in Taiwan (TIST) Study, 2008 to 2010

2011 ◽  
Vol 56 (3) ◽  
pp. 1414-1417 ◽  
Author(s):  
Jien-Wei Liu ◽  
Wen-Chien Ko ◽  
Cheng-Hua Huang ◽  
Chun-Hsing Liao ◽  
Chin-Te Lu ◽  
...  
Keyword(s):  
Susceptibility Testing ◽  
Total Error ◽  
Error Rates ◽  
Resistant Bacteria ◽  
Drug Resistant ◽  
Broth Microdilution ◽  
Content Type ◽  
E Coli ◽  
Drug Resistant Bacteria

ABSTRACTThe TigecyclineIn VitroSurveillance in Taiwan (TIST) study, initiated in 2006, is a nationwide surveillance program designed to longitudinally monitor thein vitroactivity of tigecycline against commonly encountered drug-resistant bacteria. This study compared thein vitroactivity of tigecycline against 3,014 isolates of clinically important drug-resistant bacteria using the standard broth microdilution and disk diffusion methods. Species studied included methicillin-resistantStaphylococcus aureus(MRSA;n= 759), vancomycin-resistantEnterococcus faecium(VRE;n= 191), extended-spectrum β-lactamase (ESBL)-producingEscherichia coli(n= 602), ESBL-producingKlebsiella pneumoniae(n= 736), andAcinetobacter baumannii(n= 726) that had been collected from patients treated between 2008 and 2010 at 20 hospitals in Taiwan. MICs and inhibition zone diameters were interpreted according to the currently recommended U.S. Food and Drug Administration (FDA) criteria and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria. The MIC90values of tigecycline against MRSA, VRE, ESBL-producingE. coli, ESBL-producingK. pneumoniae, andA. baumanniiwere 0.5, 0.125, 0.5, 2, and 8 μg/ml, respectively. The total error rates between the two methods using the FDA criteria were high: 38.4% for ESBL-producingK. pneumoniaeand 33.8% forA. baumannii. Using the EUCAST criteria, the total error rate was also high (54.6%) forA. baumanniiisolates. The total error rates between these two methods were <5% for MRSA, VRE, and ESBL-producingE. coli. For routine susceptibility testing of ESBL-producingK. pneumoniaeandA. baumanniiagainst tigecycline, the broth microdilution method should be used because of the poor correlation of results between these two methods.

scholarly journals In VitroSpectrum of Pexiganan Activity When Tested against Pathogens from Diabetic Foot Infections and with Selected Resistance Mechanisms

2015 ◽  
Vol 59 (3) ◽  
pp. 1751-1754 ◽  
Author(s):  
Robert K. Flamm ◽  
Paul R. Rhomberg ◽  
Katie M. Simpson ◽  
David J. Farrell ◽  
Helio S. Sader ◽  
...  
Keyword(s):  
Diabetic Foot ◽  
Antimicrobial Agents ◽  
Resistance Mechanisms ◽  
Surveillance Program ◽  
Content Type ◽  
Diabetic Foot Infections ◽  
Antimicrobial Surveillance ◽  
Mueller Hinton ◽  
The Family ◽  
Infection Types

ABSTRACTPexiganan, a 22-amino-acid synthetic cationic peptide, is currently in phase 3 clinical trials as a topical antimicrobial agent for the treatment of mild infections associated with diabetic foot ulcers. Bacterial isolates from the 2013 SENTRY Antimicrobial Surveillance Program designated as pathogens from diabetic foot infections (DFI) and Gram-negative and -positive pathogens from various infection types that harbored selected resistance mechanisms/phenotypes were tested against pexiganan in reference cation-adjusted Mueller-Hinton broth. The MIC50and MIC90against all organisms tested from DFI were 16 and 32 μg/ml, respectively.Escherichia coli,Klebsiella pneumoniae,Citrobacter koseri,Enterobacter cloacae,Acinetobacterspecies, andPseudomonas aeruginosaMIC values ranged from 8 to 16 μg/ml. Pexiganan MIC values amongStaphylococcus aureus(methicillin-resistantS. aureus[MRSA] and methicillin-susceptibleS. aureus[MSSA]), beta-hemolytic streptococci, andEnterococcus faeciumranged from 8 to 32 μg/ml. Pexiganan activity was not adversely affected for members of the familyEnterobacteriaceaeorP. aeruginosathat produced β-lactamases or resistance mechanisms to other commonly used antimicrobial agents. Decreased susceptibility to vancomycin did not affect pexiganan activity againstS. aureusorE. faecium.Enterococcus faecalisappears to be intrinsically less susceptible to pexiganan (MIC, 32 to 256 μg/ml). The “all organism” MIC90of 32 μg/ml for pexiganan in this study was >250-fold below the pexiganan concentration in the cream/delivery vehicle being developed for topical use.

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