Biomedical subjects

C N Baker

Publications and source records attributed to C N Baker.

At least 19 recordsLinked to original sources

PubMed2000

A simplified method for testing Bordetella pertussis for resistance to erythromycin and other antimicrobial agents.

Present methods of antimicrobial susceptibility testing of Bordetella pertussis are time consuming and require specialized media that are not commercially available. We tested 52 isolates of B. pertussis for resistance to erythromycin, trimethoprim-sulfamethoxazole, chloramphenicol, and rifampin by agar dilution with Bordet-Gengou agar (BGA) containing 20% horse blood (reference method), Etest using BGA and Regan-Lowe agar without cephalexin (RL-C), and disk diffusion using BGA and RL-C. The organisms tested included four erythromycin-resistant isolates of B. pertussis from a single patient, a second erythromycin-resistant strain of B. pertussis from an unrelated patient in another state, and 47 nasopharyngeal surveillance isolates of B. pertussis from children in the western United States. The results of agar dilution testing using direct inoculation of the organisms suspended in Mueller-Hinton broth were within +/-1 dilution of those obtained after overnight passage of the inoculum in Stainer-Scholte medium, which is the traditional method of testing B. pertussis. The Etest method produced MICs similar to those of the agar dilution reference method for three of the four antimicrobial agents tested; the trimethoprim-sulfamethoxazole results were lower with Etest, particularly when the direct suspension method was used. Most of the Etest MICs, except for that of erythromycin, were on scale. Disk diffusion testing using RL-C medium was helpful in identifying the erythromycin-resistant strains, which produced no zone of inhibition around the disk; susceptible isolates produced zones of at least 42 mm. Thus, the antimicrobial susceptibility testing of B. pertussis can be simplified by using the Etest or disk diffusion on RL-C to screen for erythromycin-resistant isolates of B. pertussis.

Animals↗

PubMed1997

Development of provisional disc diffusion breakpoints for testing quinupristin/dalfopristin.

Quinupristin/dalfopristin is an injectable streptogramin with broad activity against many Gram-positive bacteria, including Streptococcus pneumoniae, Staphylococcus aureus and Enterococcus faecium. Although a number of studies have reported the MICs of this compound against a variety of bacteria, there are no published reports of disc diffusion testing. We tested total disc masses of 7.5 and 15 micrograms with varying ratios of the component compounds, quinupristin and dalfopristin, combined in the following quinupristin: dalfopristin ratios (in microgram): 7.5:0, 0:7.5, 10:5, 5:10, 5:2.5 and 2.5:5. Zone diameters and MICs were determined in parallel for 44 isolates of staphylococci, 47 isolates pneumococci and 64 isolates of enterococci. Control strains were included for each species tested, including a strain of Enterococcus faecalis with known resistance to quinupristin/dalfopristin. Using tentative definitions for quinupristin/dalfopristin of < or = 2 mg/L as susceptible and > or = 4 mg/L as resistant, each of the four discs containing ratios of both component compounds separated presumptive susceptible organisms from resistant ones better than either quinupristin or dalfopristin alone. The best correlation of zone sizes and MICs for predicting susceptibility to quinupristin/dalfopristin at an MIC of < or = 2 mg/L was achieved with the 5 micrograms:10 micrograms disc and a zone diameter of > or = 18 mm. These criteria may be useful for identifying organisms that are presumptively susceptible to quinupristin/dalfopristin.

Anti-Bacterial Agents↗

PubMed1996

Evaluation of commercial methods for determining antimicrobial susceptibility of Streptococcus pneumoniae.

Seven commercial systems for antimicrobial susceptibility testing of Streptococcus pneumoniae were evaluated by using a challenge set of 55 pneumococcal isolates with a variety of resistance phenotypes and genotypes. Overall, the results produced by the Pasco and Etest methods were found to be acceptable for all drugs tested except for trimethoprim-sulfamethoxazole testing by the Etest. The Just One system for penicillin MIC testing was also judged to be acceptable (minor error rate, 5.5%). Although the Sensititre and MicroTech methods both produced 12.7% minor errors with penicillin, the Sensititre method classified penicillin-intermediate strains as resistant or vice versa, while four of MicroTech's errors were among intermediate strains that were classified as susceptible. The MicroMedia (minor error rate, 16.4%) and MicroScan Rapid (minor error rate, 63.6%) methods produced unacceptably high levels of errors when testing penicillin. Minor error rates for cefotaxime and ceftriaxone ranged from a low of 12.7% (Etest and Sensititre) to a high of 28% (MicroMedia). Error rates were low for erythromycin, tetracycline, and chloramphenicol by most methods with the exception of the MicroScan method, which had a high very major error rate for erythromycin (34.6%). For testing of beta-lactam drugs, the Pasco, Etest, and Just One tests for penicillin are the most accurate methods; the Sensititre method also provided acceptable results.

Anti-Bacterial Agents↗

PubMed1996

Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci.

We compared the results of the Alamar broth microdilution susceptibility testing method with the results of the National Committee for Clinical Laboratory Standards reference broth microdilution method for 119 gram-positive organisms. The strains were tested for their susceptibilities to 20 antimicrobial agents. Only appropriate antimicrobial agents were evaluated for each species of bacteria. Absolute categorical agreement between the reference method and the test method was 91.5% for enterococci, 99.8% for oxacillin-susceptible staphylococci, and 97.4% for oxacillin-resistant staphylococci. Essential agreement (percent complete agreement plus percent minor errors) was > 99% for all organisms tested. The results for enterococci showed no very major errors, one major error with ofloxacin, and numerous minor errors with the quinolones. However, all except one of the minor errors were within +/- 1 log2 dilution of the reference result. For staphylococci, only 2 very major errors (one each with chloramphenicol and oxacillin), 1 major error (chloramphenicol), and 15 minor errors (multiple drugs) were observed. The Alamar colorimetric system was easy to use and the results were easy to read. It appears to be an acceptable method for antimicrobial susceptibility testing of staphylococci and enterococci.

Colorimetry↗

PubMed1995

Genotypic and phenotypic characterization of Helicobacter cinaedi and Helicobacter fennelliae strains isolated from humans and animals.

By DNA-DNA hybridization, we classified 26 human strains, 4 dog and cat strains, and 4 hamster strains putatively identified as Helicobacter cinaedi as well as 2 human strains and 2 animal strains of Helicobacter fennelliae. All but one human strain belonged to the same hybridization group as the type strain of H. cinaedi. The animal strains also appeared to belong to this hybridization group. Both human strains of H. fennelliae were shown to be H. fennelliae by DNA-DNA hybridization, but both animal strains were less than 15% related to the type strain. All strains were also characterized by plasmid profiles and ribotyping. Plasmids were found in 23% of the human strains, 100% of the hamster strains, and 33% of the dog and cat strains. Human strains were essentially identical by ribotyping, but were clearly differentiated from the hamster and dog and cat strains. Some strains may be difficult to culture on primary isolation; we found that our strains grew well on anaerobic CDC agar, brucella agar, and tryptic soy agar II. Our H. cinaedi and H. fennelliae strains differed from those previously described because some were resistant to cephalothin: some H. cinaedi strains were also resistant to nalidixic acid. All isolates were also characterized by antimicrobial susceptibility testing. We found that human strains of H. cinaedi were more resistant to clindamycin and erythromycin than were animal isolates; 19% of the human strains were resistant to ciprofloxacin. Therefore, we recommend that antimicrobial susceptibility results be obtained before initiating therapy for H. cinaedi and H. fennelliae infections.

Animals↗

PubMed1995

Phenotypic characteristics, fatty acid composition, and isoprenoid quinone content of CDC group IIg bacteria.

Eleven strains of eugonic, nonoxidative, gram-negative rods isolated from clinical specimens formed a distinct group that was designated CDC group IIg. Five of the 11 isolates were from wounds. The phenotypic characteristics of CDC group IIg were most similar to those of Weeksella species, with the major difference being that CDC group IIg strains grew on MacConkey agar in 1 to 2 days, did not hydrolyze gelatin, and did not produce urease. All 11 strains of CDC group IIg possessed a distinct fatty acid profile that was characterized by large amounts (19 to 29%) of 18:1 omega 7c, 16:0, and 16:1 omega 7c, moderate amounts (6 to 10%) of 3-OH-14:0 and 14:0, and smaller amounts (1 to 2%) of 18:2, 18:0, and 3-OH-16:0. This fatty acid profile differs from those of Weeksella species by the absence of branched-chain fatty acids. CDC group IIg contains ubiquinone-8, as opposed to menaquinone-6 in Weeksella species. The isolates were susceptible to a variety of antimicrobial agents, including the aminoglycosides, tetracyclines, quinolones, sulfonamides, and polymyxin B.

Fatty Acids↗

PubMed1994

Helicobacter cinaedi-associated bacteremia and cellulitis in immunocompromised patients.

OBJECTIVE: To define the clinical spectrum of illness associated with Helicobacter cinaedi infection in the United States and to determine associated epidemiologic risk factors and optimal laboratory methods for recovery of H. cinaedi. DESIGN: A retrospective epidemiologic study of 23 patients with H. cinaedi-associated illness. PATIENTS: 23 patients with H. cinaedi infection identified between January 1982 and August 1990. Most isolates (22 of 23) were from blood; one was from stool. RESULTS: Ages ranged from 24 to 84 years (mean, 44 years). Eighty-three percent of patients were men; 17% were women. Clinical and laboratory data were obtained from 21 patients. Eighteen patients were febrile (15 required hospitalization); cellulitis was reported in 9 patients. Sixty percent were immunocompromised; 45% were reported to be seropositive for human immunodeficiency virus (HIV). For bacteremic patients, positive blood cultures were detected by a slightly elevated growth index in an automated blood culture system; many hospital laboratories had difficulty isolating the organism. CONCLUSIONS: Helicobacter cinaedi appears to cause recurrent cellulitis with fever and bacteremia in immunocompromised hosts. Blood cultures from immunocompromised patients with these symptoms may need special handling to isolate H. cinaedi.

Adult↗

PubMed1994

Optimizing testing of methicillin-resistant Staphylococcus species.

Selection of the appropriate NaCl concentration for test medium for oxacillin susceptibility testing of Staphylococcus aureus and coagulase-negative staphylococci has been problematic when using different antimicrobial susceptibility testing methods. Broth microdilution, using cation-adjusted Mueller-Hinton broth + 2% NaCl, is the currently recommended reference method. There is currently no recommendation for the addition of NaCl to agar for dilution susceptibility tests when Staphylococcus species are tested with oxacillin. We examined the effects of adding 0, 2%, 4%, and 5% NaCl to Mueller-Hinton agar and broth for agar dilution, Etest, and broth microdilution tests. The results of these tests were compared with the reference broth microdilution results and with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positive and had oxacillin minimum inhibitory concentrations (MICs) > or = 4 micrograms/ml. Seven strains of S. aureus were mec probe negative but were oxacillin resistant. Seven coagulase-negative strains (three S. epidermidis, one S. haemolyticus, and three S. simulans) were mec probe positive and were oxacillin susceptible. The MICs for oxacillin-resistant strains increased two- to fourfold with the addition of 2% NaCl, but the MICs for oxacillin-susceptible strains were unchanged. Major and very major interpretative rates ranged from 18.2% to 20.2% for agar dilution and Etest without NaCl added to the medium, and these rates decreased to < 1% with the addition of 2% NaCl to the medium. The addition of 4% or 5% NaCl caused major error rates of > 17% for all test methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Culture Media↗

PubMed1994

Evaluation of Alamar colorimetric MIC method for antimicrobial susceptibility testing of gram-negative bacteria.

The Alamar (Alamar Biosciences, Inc., Sacramento, Calif.) colorimetric antimicrobial susceptibility testing method is a new approach to the determination of broth microdilution MICs. The method uses a color indicator to detect growth of microorganisms within the wells of a microdilution tray. The color changes can be read visually or with a fluorometer. The system contains growth and sterility control wells and 20 antimicrobial agents per MIC tray with eight twofold dilutions for each antimicrobial agent. We tested 186 multiresistant, gram-negative bacterial isolates against 33 antimicrobial agents and compared the results to those obtained by agar dilution. Categorical agreement for all agents was 90.9% and ranged from 78.2% for ampicillin-sulbactam to 98.1% for amikacin. Percent agreement for MIC results (within +/- 1 log2 dilution) was 91.0% for all agents and ranged from 69.1% for gentamicin to 97.9% for ciprofloxacin. Most of the disagreements were with the penicillins and cephalosporins for beta-lactamase-producing strains. The Alamar MIC system is very easy to read visually and appears to be a satisfactory addition to currently used MIC determination methods.

Agar↗

PubMed1993

Two percent sodium chloride is required for susceptibility testing of staphylococci with oxacillin when using agar-based dilution methods.

The need to add NaCl to agar media to ensure accuracy of results when testing staphylococci with oxacillin was investigated. The results of four antimicrobial susceptibility testing methods (agar and broth dilution, E test, and disk diffusion) in which the growth medium contained 0, 2, 4, or 5% NaCl were compared with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positive. A total of 7 of the 128 positive strains were coagulase-negative staphylococci with 24-h oxacillin MICs of < or = 2 micrograms/ml. Ninety-five isolates were mec gene negative, including seven strains of Staphylococcus aureus with oxacillin MICs of > or = 4 micrograms/ml. The oxacillin MICs for mec gene-positive, oxacillin-resistant strains of staphylococci increased two- to fourfold with the addition of NaCl to the test medium, while the MICs for mec gene-negative strains did not change in the presence of added salt. Very major error rates for the agar dilution and E test methods in the absence of salt ranged from 18.2 to 20.2%. Major error rates for mec gene-negative S. aureus isolates were > 17% for all test methods when 4 or 5% NaCl was added to the test medium. The addition of 2% NaCl to Mueller-Hinton agar for testing of oxacillin resulted in very major error rates of < 1% for the agar dilution and E test methods although the major error rates for the two methods with added NaCl were 8.5 and 6.9%, respectively. The disk diffusion test did not perform well in this study, showing essential error rates of > or = 18.3%. We recommended the addition of 2% NaC1 to Mueller-Hinton agar when testing staphylococci with oxacillin by either the agar dilution or E test method. NaC1 should not be added for the disk diffusion test.

Culture Media↗

PubMed1993

Tentative criteria for confirming the in vitro susceptibilities of Haemophilus influenzae and Neisseria gonorrhoeae to two fluoroquinolones (sparfloxacin and levofloxacin), including quality control parameters.

Sparfloxacin and levofloxacin were evaluated against 150 Haemophilus influenzae isolates and 149 Neisseria gonorrhoeae isolates in order to define susceptibility testing parameters. Sparfloxacin-susceptible H. influenzae strains were defined as those for which the MICs were < or = 0.25 microgram/ml and the zones were > or = 30 mm, and N. gonorrhoeae susceptible strains were those for which the MICs were < or = 0.03 microgram/ml and the zones were > or = 39 mm (5-micrograms disks). Levofloxacin-susceptible strains of H. influenzae included those for which the MICs were < or = 0.12 microgram/ml and the zones were > or = 32 mm and N. gonorrhoeae susceptible strains were those for which the MICs were < or = 0.12 microgram/ml and the zones were > or = 37 mm (5-micrograms disks). Criteria for a resistant category cannot yet be defined for either quinolone. In multilaboratory studies with different lots of Haemophilus Test Medium, replicate tests with the standard control strain of H. influenzae (ATCC 49247) were evaluated. For sparfloxacin disk tests, the proposed zone size limits were 33 to 42 mm and broth microdilution MIC limits were 0.004 to 0.016 microgram/ml, whereas for levofloxacin tests, zone size limits were 32 to 41 mm and broth microdilution MIC limits were 0.008 to 0.03 microgram/ml. Other multilaboratory studies evaluated tests with supplemented GC agar and N. gonorrhoeae ATCC 49226; for both drugs, zone size limits were 44 to 52 mm and agar dilution MIC limits were 0.004 to 0.016 microgram/ml.

Anti-Infective Agents↗

PubMed1992

The E-Test and Campylobacter jejuni.

The E-Test is a recently introduced method for performing antimicrobial susceptibility tests. We compared the E-Test to the broth microdilution test and to the standard agar dilution test by using five antimicrobial agents tested against 55 clinical isolates of Campylobacter jejuni from 11 locations in USA (group 1). Later, we selected 30 strains (group 2), which were more resistant than the original survey isolates. Erythromycin, tetracycline, and ciprofloxacin were tested on both groups of organisms. When the three test methods were compared with each other at +/- 1 log2 dilution, the E-test gave the best overall agreement, with 85% of all strains being within acceptable limits. Category interpretation of erythromycin (drug of choice) results was a problem using current NCCLS guideline breakpoints. For the E-Test and the agar dilution method, 82% of the strains were in the intermediate category; but with the broth microdilution method, only 16.4% of the isolates were interpreted as intermediate. If the susceptible category breakpoint was raised to less than or equal to 2 micrograms/ml, then only 3% of the C. jejuni isolates would be interpreted as intermediate by any of the three methods. Our preference for antimicrobial susceptibility testing of C. jejuni is either E-Test or agar dilution at 42 degrees C for 16 hr.

Anti-Bacterial Agents↗

PubMed1992

In vitro susceptibilities of aerotolerant Campylobacter isolates to 22 antimicrobial agents.

We evaluated the in vitro activities of 22 antimicrobial agents against 78 human and animal isolates belonging to two aerotolerant Campylobacter species, C. cryaerophila and C. butzleri, using a broth microdilution technique. An additional 10 antimicrobial agents were included at concentrations found in selective Campylobacter media. Strains of C. cryaerophila belonged to two DNA hybridization groups: DNA hybridization group 1A, which includes the type strain of C. cryaerophila, and DNA hybridization group 1B. The aminoglycosides, fluoroquinolones, and one tetracycline (minocycline) demonstrated the most activity against all DNA hybridization groups (C. cryaerophila DNA groups 1A and 1B and C. butzleri). Most isolates were resistant to cephalosporin antibiotics, with the exception of cefotaxime, and were variably susceptible to trimethoprim-sulfamethoxazole. C. cryaerophila DNA hybridization group 1A isolates were generally susceptible to the tetracyclines, chloramphenicol, nalidixic acid, azithromycin, erythromycin, and roxithromycin and moderately susceptible to clindamycin, trimethoprim-sulfamethoxazole, ampicillin, and ampicillin-sulbactam. The MICs of tetracyclines were higher for C. butzleri and C. cryaerophila DNA hybridization group 1B isolates than for C. cryaerophila DNA hybridization group 1A isolates, but most strains were still susceptible to doxycycline and tetracycline; all isolates were susceptible to minocycline. C. butzleri and C. cryaerophila DNA hybridization group 1B isolates were generally resistant to the macrolide antibiotics (including erythromycin), chloramphenicol, clindamycin, nalidixic acid, ampicillin, and trimethoprim-sulfamethoxazole. Differences in antimicrobial susceptibility between aerotolerant Campylobacter species and more common Campylobacter species, e.g., C. jejuni, suggest that different treatment strategies may be necessary. Strains of all three DNA hybridization groups of aerotolerant Campylobacter isolates were susceptible to colistin, polymyxin B, and rifampin at concentrations commonly used in selective media. These results suggest that primary isolation methods for Campylobacter species may need to be modified to include aerotolerant Campylobacter strains.

Animals↗

PubMed1992

Interpretive criteria and quality control parameters for testing susceptibility of Haemophilus influenzae to enoxacin, ofloxacin, and temafloxacin.

Haemophilus influenzae isolates were uniformly susceptible to enoxacin, ofloxacin, and temafloxacin. Zone diameter and MIC interpretive criteria were proposed to define susceptible populations so that mutants with diminished susceptibility might be detected when and if they appear in clinical specimens. Additional collaborative quality control studies defined MIC and zone size limits for tests with H. influenzae ATCC 49247.

Anti-Infective Agents↗

PubMed1992

Accuracy of the E test for determining antimicrobial susceptibilities of staphylococci, enterococci, Campylobacter jejuni, and gram-negative bacteria resistant to antimicrobial agents.

We compared the results of the E test MIC method with the results of agar dilution susceptibility testing for 18 antimicrobial agents against 324 strains of gram-positive and gram-negative bacteria, including 99 strains of staphylococci, 101 strains of antimicrobial-resistant gram-negative bacteria, 40 strains of enterococci, and 84 isolates of Campylobacter jejuni. Overall agreement of MICs (+/- 1 log2 dilution) was 97.3% for staphylococci, 94.6% for gram-negative bacilli, and 100.0% for enterococci. The MIC results for C. jejuni showed an overall agreement of only 82.9%. This was due primarily to a number of offscale values that limited the number of comparisons with clindamycin, trimethoprim-sulfamethoxazole, and tetracycline. Interpretative criteria for the results of the two test methods, however, were similar. Overall, the E test produced MIC results comparable to those of agar dilution when multiresistant organisms were tested. However, it was necessary to add 2% NaCl to the agar when testing oxacillin against staphylococci for both the E test and agar dilution to obtain results comparable to those of the broth microdilution method.

Bacteria↗

PubMed1992

Haemophilus influenzae ATCC 49766, an alternative quality control strain for monitoring broth microdilution susceptibility tests with selected beta-lactams.

A beta-lactamase-negative, ampicillin-resistant strain of Haemophilus influenzae is currently used for quality control of broth microdilution tests performed with Haemophilus test medium (HTM). Studies with eight lots of HTM broth documented the fact that MIC limits for some antimicrobial agents are unrealistically stringent; i.e., only three of eight lots of HTM broth were satisfactory for testing cefaclor. An alternative, ampicillin-susceptible strain of H. influenzae (ATCC 49766) was found to provide much more reproducible results with five problematic drugs (cefaclor, cefuroxime, cefamandole, loracarbef, and cefonicid). Multilaboratory studies defined MIC control limits for both control strains tested against 12 antimicrobial agents.

Ampicillin↗

PubMed1992

Proposed interpretive criteria and quality control parameters for testing susceptibility of Neisseria gonorrhoeae to beta-lactam-clavulanate combinations.

To support future clinical studies, in vitro susceptibility tests were examined to determine whether Neisseria gonorrhoeae could be tested reliably against two beta-lactam-clavulanate combinations. All isolates that were tested appeared to be susceptible to amoxicillin and ticarcillin in combination with clavulanic acid. In the absence of resistant isolates, only a breakpoint for a susceptible category could be defined for agar dilution tests with amoxicillin-clavulanic acid (MIC of less than or equal to 2.0/1.0 micrograms/ml is tentatively proposed). For disk diffusion tests, a corresponding breakpoint zone diameter of greater than or equal to 28 mm is suggested. The validity of the breakpoints for penicillinase-negative penicillin-resistant strains awaits clinical data. Proposed quality control limits for testing amoxicillin-clavulanic acid by agar dilution and disk diffusion methods are a MIC of 0.25/0.125 to 1.0/0.5 micrograms/ml and zones of 30 to 40 mm in diameter for N. gonorrhoeae ATCC 49226, a MIC of 0.125/0.06 to 0.5/0.25 micrograms/ml for Staphylococcus aureus ATCC 29213, and zones of 30 to 38 mm for S. aureus ATCC 25923. Ticarcillin-clavulanate is currently tested against other species by preparing doubling dilutions of ticarcillin with a constant 2 micrograms of clavulanate per ml. By that method, all gonococci were susceptible to low concentrations. However, the amount of clavulanic acid that is included (2 micrograms/ml) will, by itself, inhibit many strains of N. gonorrhoeae. Consequently, the role of ticarcillin in the combination cannot be determined, and such tests are not recommended.

Amoxicillin↗

PubMed1991

Meningococcal disease in the United States--1986. Meningococcal Disease Study Group.

Active surveillance for invasive meningococcal disease was conducted during 1986 and 1987 in six areas of the United States with a total population of approximately 34 million persons. The incidence of meningococcal disease was 1.3:10(5). The highest incidence of disease among the surveillance areas was in Los Angeles County (1.65:10(5). Neisseria meningitidis serogroups B and C caused about equal amounts of disease, which reflects a recent increase in the incidence of group C disease. Group C caused more than half of the cases of meningococcal disease in Los Angeles and Tennessee but less than one-third of the cases in Missouri and Oklahoma. Multilocus enzyme electrophoresis demonstrated that a group of closely related isolates of N. meningitidis was prevalent in Los Angeles during the surveillance period and was associated with an increased incidence of meningococcal disease there.

Adolescent↗