PubMed Health⌕ Search

Biomedical subjects

S A Marshall

Publications and source records attributed to S A Marshall.

At least 19 recordsLinked to original sources
PubMed2001

Achieving stability and conformational specificity in designed proteins via binary patterning.

We have developed a method to determine the optimal binary pattern (arrangement of hydrophobic and polar amino acids) of a target protein fold prior to amino acid sequence selection in protein design studies. A solvent accessible surface is generated for a target fold using its backbone coordinates and "generic" side-chains, which are constructs whose size and shape are similar to an average amino acid. Each position is classified as hydrophobic or polar according to the solvent exposure of its generic side-chain. The method was tested by analyzing a set of proteins in the Protein Data Bank and by experimentally constructing and analyzing a set of engrailed homeodomain variants whose binary patterns were systematically varied. Selection of the optimal binary pattern results in a designed protein that is monomeric, well-folded, and hyperthermophilic. Homeodomain variants with fewer hydrophobic residues are destabilized, while additional hydrophobic residues induce aggregation. Binary patterning, in conjunction with a force field that models folded state energies, appears sufficient to satisfy two basic goals of protein design: stability and conformational specificity.

Animals↗
PubMed2001

Comparison of two commercial systems (Vitek and MicroScan-WalkAway) for antimicrobial susceptibility testing of Pseudomonas aeruginosa isolates from cystic fibrosis patients.

Antimicrobial susceptibility testing of cystic fibrosis (CF) isolates of Pseudomonas aeruginosa is difficult because the organisms are often mucoid and slow-growing. This study of 498 CF strains examined the correlation of results derived from two commonly used commercial systems (Vitek, MicroScan-WalkAway) with a reference method for 10 antimicrobials. Correlation to reference results was unacceptably low for all agents and both commercial systems had a high rate of very major (false-susceptible) errors. Although mucoid strains produced a 4.8% greater intermethod error, it was not markedly different than non-mucoid strains for the Vitek System. Overall, these tested commercial systems performed poorly for CF isolates in contrast to earlier reported, high correlations with the reference methods (broth microdilution frozen panels and agar dilution) of the National Committee for Clinical Laboratory Standards, the standardized disk diffusion test, and the Etest (AB BIODISK, Solna, Sweden).

Aminoglycosides↗
PubMed2001

Comparison of the Vitek Gram-Positive Susceptibility 106 card and the MRSA-screen latex agglutination test for determining oxacillin resistance in clinical bloodstream isolates of Staphylococcus aureus.

The Vitek automated susceptibility testing system with a modified Gram-Positive Susceptibility (GPS) 106 Card (bioMerieux Vitek, Inc., Hazelwood, Mo.) and a rapid slide latex agglutination test (MRSA-Screen; Denka Seiken Co., Ltd., Tokyo, Japan) were evaluated for their ability to detect oxacillin resistance in Staphylococcus aureus. The oxacillin-salt agar screen (OS) test, the reference broth microdilution method, and the detection of the mecA gene by PCR were compared with the commercial products. A total of 200 contemporary (1999) bloodstream infection isolates were collected from the SENTRY Antimicrobial Surveillance Program, representing diverse geographic areas throughout the world. Among the 99 mecA-positive isolates, 3 isolates were found negative by the MRSA-Screen. Another two isolates did not grow on OS plates and had MICs of 0.5 and 2 microg/ml with the Vitek GPS card. All 101 mecA-negative isolates were also found negative by the MRSA-Screen and were categorized as susceptible by the GPS card. Overall, the MRSA-Screen, GPS card, and OS test had sensitivities of 96.9, 98.0, and 98.0% and specificities of 100.0, 100.0, and 98.0%, respectively. MRSA-Screen was a rapid (</=15 min) and simple test to perform, and the GPS card provided results in <8 h. Both methods were sensitive and specific for detecting staphylococcal oxacillin resistance in the clinical microbiology laboratory.

Bacteremia↗
PubMed2000

Evaluation of quinupristin/dalfopristin (Synercid) and RPR 106972 stability in susceptibility testing media.

In response to conflicting reports on the chemical stability of quinupristin/dalfopristin, a study was designed to assess the in vitro longevity and effects of media and storage conditions on this streptogramin combination. Broth microdilution trays containing parenteral (quinupristin/dalfopristin) and oral (RPR 106972) streptogramin combinations as well as pristinomycin components (P-I and P-II) were preincubated at 35 degrees C for 12-72 h before inoculation with control strains (Streptococcus pneumoniae ATCC 49619, Haemophilus influenzae ATCC 49247, Enterococcus faecalis ATCC 29212, Staphylococcus aureus ATCC 29213) and five clinical isolates with various drug resistance phenotypes. Overall, the mean quinupristin/dalfopristin activity loss was 24%/12 h, 41%/18 h, 43%/24 h, 69%/48 h and 79%/72 h with no detected loss of potency when measured by E. faecalis until 18 h. RPR 106972 mean activity loss was 6%/12 h, 19%/18 h, 19%/24 h, 56%/48 h and 71%/72 h with no loss of potency as measured by S. aureus until 48 h. Overall, P-I components had greater stability as compared with P-II for both drug combinations. Bioassays showed similar trends in decreased activity. Bioassay differences among media types were only significant (> 3 mm; greater loss of potency) for haemophilus test media for both P-II components at 72 h. The presence of an organism in the medium had no effect on stability assay results. The effect of storage temperature (4, 25 degrees C) on quinupristin/dalfopristin and RPR 106972 stability was also detrimental to drug potency indicating the requirement for rigid quality assurance for streptogramin diagnostic reagents when determining activity by reference or standardized susceptibility tests.

Anti-Bacterial Agents↗
PubMed2000

Comparison of agar diffusion methodologies for antimicrobial susceptibility testing of Pseudomonas aeruginosa isolates from cystic fibrosis patients.

Pseudomonas aeruginosa is the most common pathogen infecting the lungs of patients with cystic fibrosis (CF). Improved antimicrobial chemotherapy has significantly increased the life expectancy of these patients. However, accurate susceptibility testing of P. aeruginosa isolates from CF sputum may be difficult because the organisms are often mucoid and slow growing. This study of 597 CF isolates of P. aeruginosa examined the correlation of disk diffusion and Etest (AB BIODISK, Solna, Sweden) results with a reference broth microdilution method. The rates of interpretive errors for 12 commonly used antipseudomonal antimicrobials were determined. The disk diffusion method correlated well (zone diameter versus MIC) for all of the agents tested. However, for mucoid isolates, correlation coefficients (r values) for piperacillin, piperacillin-tazobactam, and meropenem were <0.80. The Etest correlation with reference broth microdilution results (MIC versus MIC) was acceptable for all of the agents tested, for both mucoid and nonmucoid isolates. Category interpretation errors were similar for the disk diffusion and Etest methods with 0.4 and 0.1%, respectively, very major errors (false susceptibility) and 1.1 and 2.2% major errors (false resistance). Overall, both agar diffusion methods appear to be broadly acceptable for routine clinical use in susceptibility testing of CF isolates of P. aeruginosa.

Agar↗
PubMed1999

Spectrum and antimicrobial activity of alexomycin (PNU-82, 127), a peptide compound projected for use in animal health.

Alexomycin (PNU-82, 127) is a thiopeptide antimicrobial complex intended for veterinary practice that belongs to a series of cyclic peptides produced by Streptomyces arginensis. MICs against selected routine and fastidious clinical isolates of animal and human origin were determined by broth microdilution or agar dilution reference methods. Alexomycin was active against Gram-positive pathogens such as oxacillin-susceptible and -resistant Staphylococcus aureus and coagulase-negative staphylococci (260 strains; MIC90, 0.5 microgram/mL), as well as Enterococcus species (95 strains; MIC90, 0.25 to 0.5 microgram/mL), and generally inactive against Gram-negative aerobes. Alexomycin had more potent activity against Streptococcus bovis (MIC90, 0.12 microgram/mL), S. agalactiae (MIC90, 0.12 microgram/mL), Corynebacterium species (MIC90, 0.06-0.12 microgram/mL), and Listeria monocytogenes (MIC90, 0.5 microgram/mL). Alexomycin activity was limited against Bacillus species (MIC90, 1 microgram/mL), Neisseria meningiditis (MIC90, 2 micrograms/mL), Haemophilus influenzae (MIC90, 8 micrograms/mL), Moraxella catarrhalis (MIC90, 16 micrograms/mL), and Campylobacter jejuni (MIC90, 32 micrograms/mL). This thiopeptide complex was also found to be stable at low concentrations (0.015-32 micrograms/mL) in Mueller-Hinton broth for up to 24 h, possesses static antimicrobial activity and did not produce resistant mutants after multiple passages at subinhibitory drug concentrations. Alexomycin seems to have potential for use as a feed additive to increase feed efficiency and promote growth in poultry and swine as well as other applications against Gram-positive pathogens.

Animals↗
PubMed1999

Antimicrobial activity and spectrum of SCH27899 (Ziracin) tested against gram-positive species including recommendations for routine susceptibility testing methods and quality control. Quality Control Study Group.

SCH27899 is an oligosaccharide, everninomicin antibiotic with activity primarily against Gram-positive pathogens. The activity of SCH27899 was evaluated against 360 routine clinical isolates by the broth microdilution (BMD), agar dilution (AD), disk diffusion (DD), and Etest (AB BIODISK, Solna, Sweden) methods. In addition, results from a nine center SCH27899 quality control (QC) trial were used to establish QC ranges. SCH27899 MICs for 330 Gram-Positive strains, including multiply-resistant staphylococci and enterococci, ranged from 0.015 to 1 microgram/ml with MIC90s of 0.12 to 0.5 microgram/ ml. SCH27899 had no measurable activity against the 30 selected Gram-negative strains tested (MICs, > 256 micrograms/ml), with the exception of Moraxella catarrhalis MICs, 0.12 microgram/ ml). Etest MICs for SCH27899 correlated well with AD and BMD results with > 90% of MICs within +/- one log2 dilutions of the reference test results. Three disk concentrations (2.5-, 5-, 10-microgram) of SCH27899 were evaluated, but minimal difference of zone diameters between disk drug contents was observed (+/- 2 mm). SCH27899 disk zone diameters correlated poorly with reference MICs due to small zone diameters (range, 11 to 22 mm) attributed to poor diffusion through agar mediums, a product of this compound's high molecular weight and solubility. The use of the DD method for SCH27899 was not recommended. The proposed MIC quality assurance limits for SCH27899 using Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 29212 was 0.06 to 0.25 microgram/ml for both QC strains and methods. SCH27899 appears to be a eveminomicin-derivative widely active against important Gram-positive cocci, and in vitro dilution testing methods would be preferred for clinical use, validated by the recommended MIC control ranges cited in this report.

Aminoglycosides↗
PubMed1999

Antimicrobial activity of SCH27899 (Ziracin), a novel everninomicin derivative, tested against Streptococcus spp.: disk diffusion/etest method evaluations and quality control guidelines. The Quality Control Study Group.

To combat the increasing rates of penicillin resistance among pneumococci and viridans group streptococci, new Gram-positive active agents are needed to avoid the overuse of vancomycin. SCH27899 is an everninomicin derivative with strong activity against glycopeptide-resistant enterococci, oxacillin-resistant staphylococci, and penicillin-resistant streptococci. This study tests the in vitro activity of SCH27899 against 304 strains of streptococci and evaluates the quality of the agar dilution, broth microdilution, disk diffusion, and Etest methods for this antimicrobial agent. Quality-control (QC) ranges for SCH27899 are also proposed. SCH27899 broth microdilution MICs among the penicillin-susceptible and -resistant streptococci tested ranged from < or = 0.008-0.5 microgram/mL. Organism groups with their respective MIC90s were as follows: Streptococcus pneumoniae (100 strains) and beta-haemolytic streptococci (70 strains), 0.12 microgram/mL; Streptococcus bovis (10 strains), 0.25 microgram/mL; and viridans group streptococci (124 strains), 0.5 microgram/mL. Etest SCH27899 MICs correlated well with broth microdilution MICs (92% +/- one log2 dilution, 98% +/- two log2 dilutions). Agar dilution SCH27899 MICs correlated well with broth microdilution MICs, but a shift toward slightly higher agar dilution MICs was attributed to difficulties in reading trailing endpoints with this method. Three concentrations (2.5, 5, and 10 micrograms) of SCH27899 were used for the disk diffusion method with small inhibition zone diameters (range, 11 to 19 mm) and limited variation between diameters (+/- 2 mm) as a result, both products of this compound's high molecular weight and poor diffusion through agar mediums. Proposed control ranges for SCH27899 when testing S. pneumoniae ATCC 49619 from a nine-center (30 tests per center) quality-control trial are < or = 0.016 to 0.032 microgram/mL for Etest, and 0.008 to 0.032 microgram/mL for broth microdilution tests from an earlier study. Because of the limited diffusion ability and bacteriostatic nature of SCH27899, MICs should be read at 80% of inhibition with agar in vitro systems (Etest, agar dilution), and the disk diffusion method is not recommended.

Aminoglycosides↗
PubMed1999

Energy functions for protein design.

Recent successes in protein design have illustrated the promise of computational approaches. These methods rely on energy expressions to evaluate the quality of different amino acid sequences for target protein structures. The force fields optimized for design differ from those typically used in molecular mechanics and molecular dynamics calculations.

Chemical Phenomena↗
PubMed1999

Comparison of activities of broad-spectrum beta-lactam compounds against 1,128 gram-positive cocci recently isolated in cancer treatment centers.

We report the in vitro activities of broad-spectrum beta-lactam antimicrobials tested against 1,128 gram-positive pathogens recently isolated from cancer patients. Cefepime and imipenem were more active than ceftazidime and ceftriaxone against these organisms. Only vancomycin demonstrated reliable activity against oxacillin-resistant staphylococci, Enterococcus spp., and Corynebacterium spp. The spectrum of gram-positive organisms against which cefepime and imipenem have activity provides an advantage over ceftazidime as empiric therapy for cancer patients, potentially reducing the need for the empiric addition of glycopeptides.

Anti-Bacterial Agents↗
PubMed1999

Evaluation of reference dilution test methods for antimicrobial susceptibility testing of Pseudomonas aeruginosa strains isolated from patients with cystic fibrosis.

The development of multidrug-resistant Pseudomonas aeruginosa in patients with cystic fibrosis (CF) is most likely a consequence of increasing life expectancy and more prolonged exposure to antibiotics. The optimal method for antibiotic susceptibility testing of CF strains, particularly mucoid P. aeruginosa strains, is unknown. Antimicrobial susceptibilities of 48 CF strains (25 mucoid) and 50 non-CF strains to 12 anti-Pseudomonas agents were tested by both agar dilution and commercially custom-prepared broth microdilution plates (PML Microbiologicals, Portland, Oreg.) in three laboratories simultaneously to determine if broth microdilution could substitute for agar dilution as the reference method in subsequent studies. Comparison of MICs generated by agar dilution and broth microdilution demonstrated correlation coefficients (r) exceeding 0.85 for all agents tested; correlation was excellent for aminoglycosides (r >/= 0.92) and very good for beta-lactam agents including agents paired with a beta-lactamase inhibitor (r >/= 0.87) and for ciprofloxacin (r = 0.86). Correlation was not improved by 48-h readings, but correlation between 24- and 48-h readings ranged between 0.91 and 0.98 for both methods. Interlaboratory variations were minimal, as the percentage of acceptable variations was 94% for both methods, and serious discords were infrequent (<2% of comparisons). However, CF strains were more likely to have serious discords than were non-CF strains (P < 0. 0001), although mucoid strains were not more likely to have serious discords than were nonmucoid strains. In this study, MICs determined by custom-prepared broth microdilution compared favorably with MICs determined by agar dilution. Thus, this broth microdilution assay can serve as a reference method and facilitate future studies to determine the optimal method for antibiotic susceptibility testing of CF strains.

Cystic Fibrosis↗
PubMed1998

In vitro activity of cefepime and other broad-spectrum beta-lactams tested against 129 mec A-negative Staphylococcus spp. isolates: a multicenter sample.

The in vitro activity of cefepime was compared to that of penicillin, piperacillin/tazobactam, ceftazidime, ceftriaxone, imipenem, vancomycin, and teicoplanin by using the broth microdilution method against 129 isolates of Staphylococcus [50 S. aureus and 79 coagulase-negative staphylococci (CoNS], selected for their lack of the mec A gene as determined by the polymerase chain reaction. These isolates were obtained from a recent (1995-1996) surveillance of nearly 5000 nosocomial blood stream isolates from more than 40 geographically diverse U.S. medical centers. These results were compared to CoNS results from the same collection selected for their phenotypic susceptibility to oxacillin (OS; MIC < or = 2 micrograms/ml) regardless of their med A genotype. Cefepime, as well as piperacillin/ tazobactam, ceftriaxone, and imipenem, showed 100% susceptibility against OS and mec A-negative staphylococci. Ceftazidime showed relative resistance (30.2% resistant) against CoNS classified as OS based on phenotypic characteristics (MIC < or = 2 micrograms/mL) as compared to strains of mec A-negative CoNS (5.1% resistant). Accurate phenotypic detection of mec A-positive staphylococci by simple standardized in vitro susceptibility tests becomes very important to guide empirical use of beta-lactams for therapy. Furthermore, previously published MIC90 and range data for broad-spectrum beta-lactams versus OS have been falsely elevated by the presence of mec A-positive strains. The greater use of these potent beta-lactams against true mec A-negative staphylococci should enhance clinical outcomes and reduce the need for vancomycin.

Anti-Bacterial Agents↗
PubMed1998

Staphylococcus aureus and coagulase-negative staphylococci from blood stream infections: frequency of occurrence, antimicrobial susceptibility, and molecular (mecA) characterization of oxacillin resistance in the SCOPE program.

Staphylococci are major causes of nosocomial blood stream infection. The recently completed SCOPE Surveillance Program found that coagulase-negative staphylococci (CoNS) and Staphylococcus aureus were the first and second most common etiologic agents, respectively, causing nosocomial blood stream infection in the USA. The frequency of oxacillin resistance was 68% among 1553 strains of CoNS and 26% among 787 strains of S. aureus in this study. Extended susceptibility profiles were generated for a subset of 150 S. aureus and 300 CoNS against 16 antimicrobial agents. Oxacillin-susceptible strains of both CoNS and S. aureus were uniformly susceptible to beta-lactam agents with the exception of ampicillin and penicillin. Oxacillin-susceptible S. aureus were also highly susceptible to the fluoroquinolones, aminoglycosides, and trimethoprim/sulfamethoxazole. The oxacillin-susceptible CoNS were less susceptible to these agents, and only glycopeptides were reliably active against oxacillin-resistant strains. PCR detection of the mecA gene was used to scrutinize current NCCLS interpretive breakpoint MICs for determining susceptibility or resistance to oxacillin. We found complete concordance between the presence or absence of mecA and the NCCLS oxacillin interpretive breakpoint categories for S. aureus. In contrast, the NCCLS breakpoints for oxacillin significantly underestimate the degree of true oxacillin resistance among CoNS. Using the presence of mecA as the reference standard, we detected 15.7% false susceptibility to oxacillin using a MIC susceptible breakpoint concentration of < or = 2 micrograms/mL. Lowering the oxacillin MIC breakpoint to < or = 0.25 microgram/mL for CoNS would greatly improve the accuracy of the MIC test performance. We found that both the current oxacillin disk test and the 30-microgram ceftizoxime disk test functioned quite well in predicting those strains of CoNS that contain mecA. These studies have demonstrated both a high level of antimicrobial resistance among nosocomial blood stream isolates of staphylococci as well as significant problems with the current NCCLS breakpoints for oxacillin when testing CoNS.

Anti-Bacterial Agents↗
PubMed1998

Activity of a broad-spectrum cephalosporin (Ro 48-8391) alone and in combination with two novel beta-lactamase inhibitors (Ro 48-5545 and Ro 48-8724).

The susceptibility of a group of beta-lactamase-producing and drug-resistant Gram-positive and Gram-negative organisms was tested against a novel cephalosporin (Ro 48-8391) alone and in combination with two bridged carbacephem beta-lactamase inhibitors (Ro 48-5545 or Ro 48-8724) and compared with that of ceftriaxone, ceftazidime, and cefepime (representative "third- and fourth-generation" cephalosporins), imipenem, and a combination of piperacillin and tazobactam. Five hundred and one selected clinical isolates were tested using the reference broth microdilution method (National Committee for Clinical Laboratory Standards). Ro 48-8391 has a spectrum of activity and potency most similar to ceftriaxone but with improved activity against Gram-positive species. The two beta-lactamase inhibitors, Ro 48-5545 and Ro 48-8724, have modest antimicrobial activity. When combined with Ro 48-8391, the beta-lactamase inhibitor Ro 48-8724 was superior to the combination of Ro 48-8391 and Ro 48-5545 in spectrum and enzyme inhibition against extended spectrum beta-lactamase enzyme-producing Escherichia coli and Klebsiella pneumoniae, and against Enterobacteriaceae with "stably derepressed" Bush-Jacoby-Medeiros gr 1 enzymes (ceftazidime-resistant Enterobacter and Citrobacter). Ro 48-5545 and Ro 48-8724 appear to be promising beta-lactamase inhibitors with potential application against chromosomal- and plasmid-mediated enzymes. Ro 48-8391, although superior to some currently available "third-generation" cephems, was not a well-matched active codrug because of limited activity against several commonly isolated species of clinically important bacteria. Further efforts are necessary to find a penicillin or cephem with activity more complementary to that of the tested beta-lactamase inhibitors and the Ro 48-8391 compound could be focused for therapeutic use in serious streptococcal infections.

Anti-Bacterial Agents↗
PubMed1998

Evaluation of the Vitek system to accurately test the susceptibility of Pseudomonas aeruginosa clinical isolates against cefepime.

A total of 300 recent blood stream and pneumonia isolates of Pseudomonas aeruginosa from 32 different medical centers in the United States were used to assess the accuracy of the Vitek System (GNS-107 card) for cefepime, a new "fourth-generation" cephalosporin. The Vitek System test result was compared to the consensus of the three other methods (reference broth microdilution, disk diffusion, Etest (AB BIODISK, Solna, Sweden)); and 271 of 300 consensus category of susceptibility sets were unanimous. The Vitek System produced a 25.3% error rate (5.3% false resistance, 20.0% minor errors). A consistent trend toward higher MIC results with the Vitek System was observed that produced a 15.3 to 21.3% lower susceptible rate compared with the other susceptibility test methods. The consensus cefepime resistance rate was only 4.3% compared to 14.0% for the Vitek GNS-107 card. The error was reproducible on triplicate repeat testing. These results indicate an unacceptable rate of false resistance being produced by the Vitek System when testing cefepime against P. aeruginosa strains. Alternative methods are suggested for this broad-spectrum antipseudomonal cephalosporin.

Bacteremia↗