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PubMed2001

Characterization of clinical isolates of Klebsiella pneumoniae from 19 laboratories using the National Committee for Clinical Laboratory Standards extended-spectrum beta-lactamase detection methods.

Extended-spectrum beta-lactamases (ESBLs) are enzymes found in gram-negative bacilli that mediate resistance to extended-spectrum cephalosporins and aztreonam. In 1999, the National Committee for Clinical Laboratory Standards (NCCLS) published methods for screening and confirming the presence of ESBLs in Klebsiella pneumoniae, Klebsiella oxytoca, and Escherichia coli. To evaluate the confirmation protocol, we tested 139 isolates of K. pneumoniae that were sent to Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) from 19 hospitals in 11 U.S. states. Each isolate met the NCCLS screening criteria for potential ESBL producers (ceftazidime [CAZ] or cefotaxime [CTX] MICs were > or =2 microg/ml for all isolates). Initially, 117 (84%) isolates demonstrated a clavulanic acid (CA) effect by disk diffusion (i.e., an increase in CAZ or CTX zone diameters of > or =5 mm in the presence of CA), and 114 (82%) demonstrated a CA effect by broth microdilution (reduction of CAZ or CTX MICs by > or =3 dilutions). For five isolates, a CA effect could not be determined initially by broth microdilution because of off-scale CAZ results. However, a CA effect was observed in two of these isolates by testing cefepime and cefepime plus CA. The cefoxitin MICs for 23 isolates that failed to show a CA effect by broth microdilution were > or =32 microg/ml, suggesting either the presence of an AmpC-type beta-lactamase or porin changes that could mask a CA effect. By isoelectric focusing (IEF), 7 of the 23 isolates contained a beta-lactamase with a pI of > or =8.3 suggestive of an AmpC-type beta-lactamase; 6 of the 7 isolates were shown by PCR to contain both ampC-type and bla(OXA) genes. The IEF profiles of the remaining 16 isolates showed a variety of beta-lactamase bands, all of which had pIs of < or =7.5. All 16 isolates were negative by PCR with multiple primer sets for ampC-type, bla(OXA), and bla(CTX-M) genes. In summary, 83.5% of the K. pneumoniae isolates that were identified initially as presumptive ESBL producers were positive for a CA effect, while 5.0% contained beta-lactamases that likely masked the CA effect. The remaining 11.5% of the isolates studied contained beta-lactamases that did not demonstrate a CA effect. An algorithm based on phenotypic analyses is suggested for evaluation of such isolates.

Algorithms↗
PubMed1997

Antimicrobial susceptibility of flavobacteria as determined by agar dilution and disk diffusion methods.

A total of 106 clinical isolates of flavobacteria, including 41 isolates of Flavobacterium meningosepticum, 59 of Flavobacterium indologenes, and 6 of Flavobacterium odoratum were collected from January 1992 to December 1995 from patients in Taiwan. The in vitro activities of antimicrobial agents were determined concomitantly by the standard agar dilution and disk diffusion methods. More than 90% of the flavobacterial isolates were resistant to cephalothin, cefotaxime, ceftriaxone, moxalactam, aztreonam, imipenem, aminoglycosides, erythromycin, and glycopeptides. The majority of F. meningosepticum isolates were susceptible to piperacillin and to minocycline but resistant to ceftazidime, with MICs at which 90% of the isolates are inhibited being 8, 4, and > 128 microg/ml, respectively. Approximately half of the F. indologenes isolates were susceptible to piperacillin, cefoperazone, ceftazidime, and minocycline, with MICs at which 50% of the isolates are inhibited being 4, 16, 8, and 4 microg/ml, respectively. The majority of F. odoratum isolates were resistant to all the antimicrobial agents tested except minocycline, to which five of six isolates were susceptible. With least-squares regression analysis and error rate-bounded analysis methods, the following resistant and susceptible zone diameter breakpoints were established: < or = 12 and > or = 17 mm, respectively, for piperacillin against F. meningosepticum and F. indologenes; < or = 13 and > or = 18 mm, respectively, for ceftazidime against F. meningosepticum and F. indologenes, < or = 17 and > or = 21 mm, respectively, for ofloxacin against F. indologenes; < or = 16 and > or = 20 mm, respectively, for ciprofloxacin against F. meningosepticum. Valid breakpoints for the disk diffusion method could not be established for cefoperazone and ofloxacin against F. meningosepticum and for minocycline against F. meningosepticum and F. indologenes due to a poor correlation coefficient for the regression line or for cefoperazone and ciprofloxacin against F. indologenes due to the presence of remarkable error rates.

Anti-Bacterial Agents↗
PubMed1997

Microbiologic and pharmacodynamic principals applied to the antimicrobial susceptibility testing of ampicillin/sulbactam: analysis of the correlations between in vitro test results and clinical response.

The correlation between various ampicillin/sulbactam in vitro antimicrobial susceptibility test results and the clinical outcome of patients treated with this agent have been examined. A survey of over 29,000 clinical isolates of the family Enterobacteriaceae found that the proportion of susceptible pathogens as assessed by current susceptibility testing interpretive guidelines (NCCLS) for disk diffusion and dilution (MIC) assays was significantly less than the proportion of patients cured or clinically improved in ampicillin/sulbactam clinical trials. Also, the results of two NCCLS methods differ greatly in the perceived percentages of susceptible strains (63.9% versus 72.2%; unacceptable variation). Furthermore, the current interpretive criteria resulted in high false-susceptible (4.2%) and total (19.7%) error rates. When proposed interpretive guidelines were applied, approximately 73 to 87% of the Enterobacteriaceae strains were observed to be susceptible, the variation between methods was minimized, and the error rates were reduced. A retrospective analysis of data from clinical trials with ampicillin/sulbactam indicated that the proportion of patients who were cured or clinically improved and bacterially eradicated was not appreciably different in patients having baseline Enterobacteriaceae pathogens with MICs of 16 or 32 micrograms/ml (ampicillin MIC component) as compared to those with pathogens having MICs of < or = 8 micrograms/ml. Studies in animals, in vitro models, and pharmacokinetic considerations indicate that a change in the MIC breakpoint for ampicillin/sulbactam should be considered. The proposed interpretive guideline revisions for ampicillin/sulbactam susceptibility testing of the Enterobacteriaceae were 1) use current diagnostic reagents with criteria of < or = 16/8 micrograms/ml (> or = 14 mm) as susceptible and > or = 64/32 micrograms/ml (< or = 10 mm) as resistant; e.g., 75.9 to 76.0% spectrum and 1.3% false-susceptible error; 2) use alternative diagnostic reagents (1:1 ratio MIC; 20/20 micrograms disks) with criteria of < or = 8/8 micrograms/ml (> or = 18 mm) as susceptible and > or = 32/32 micrograms/ml (< or = 14 mm) as resistant; e.g., 73.3 to 76.9% spectrum and 1.8% false-susceptible error; or 3) use alternative diagnostic reagents with criteria of < or = 16/16 micrograms/ml (> or = 14 mm) as susceptible and > or = 64/64 micrograms/ml (< or = 10 mm) as resistant; e.g., 84.7 to 86.9% spectrum and 1.3% false-susceptible error. Data from a comprehensive in vitro survey of clinical isolates, retrospective analyses of clinical trials, and studies of animal models support the modification of contemporary interpretive guidelines for ampicillin/sulbactam antimicrobial susceptibility tests. The best short-term criteria would apply current in vitro diagnostic reagents and a modified susceptible breakpoint (< or = 16/8 micrograms/ml as susceptible; option 1 above) until new diagnostic reagents can be qualified by means of studies needed for quality assurance of standardized methods (NCCLS M23-A and FDA procedures). These changes would provide a better in vitro prediction of ampicillin/sulbactam efficacy in clinical practice.

Ampicillin↗
PubMedDate not supplied

Detection of antimicrobial resistance and extended-spectrum beta-lactamase production in Klebsiella pneumoniae strains from infected neonates.

The present study was designed to determine the antimicrobial resistance and extended-spectrum beta-lactamase (ESBL) activities of Klebsiella pneumoniae strains isolated from the neonatal intensive care unit of Atatürk University Hospital, Erzurum, Turkey. Antibiotic susceptibility of 40 isolates was detected by the standard disk diffusion method according to the National Committee for Clinical Laboratory Standards Guidelines. The double-disk synergy method was used to determine ESBL activity, which is associated with resistance to beta-lactam antibiotics. Twenty-four (60%) of 40 K. pneumoniae strains were found to produce ESBL. Of the antibiotics tested, meropenem was found to be the most effective (100%), and ampicillin the least effective (0%). With the increasing incidence of antimicrobial resistance, which poses a clinically significant risk to vulnerable patients, it is important that clinical microbiology laboratories have accurate and timely information concerning the strains of bacteria present to enable them to predict which antibiotics are likely to be effective in treating the infections they may cause.

Ampicillin↗
PubMed2006

Clonality and antimicrobial resistance gene profiles of multidrug- resistant Salmonella enterica serovar infantis isolates from four public hospitals in Rio de Janeiro, Brazil.

In Brazil, Salmonella enterica serovar Infantis resistant to various antimicrobials, including cephalosporins, has been identified as an etiological agent of severe gastroenteritis in hospitalized children since 1994. In this study, 35 serovar Infantis strains, isolated from children admitted to four different Rio de Janeiro, Brazil, hospitals between 1996 and 2001, were characterized by pulsed-field gel electrophoresis (PFGE) and antimicrobial susceptibility testing in order to determine their genetic relatedness and antimicrobial resistance profiles. Thirty-four serovar Infantis strains were resistant to at least two antibiotic classes, and all 35 strains were susceptible to fluoroquinolones, cephamycin, and carbapenem. Extended-spectrum beta-lactamase (ESBL) screening by double-disk diffusion indicated that 32 serovar Infantis strains (91.4%) produced beta-lactamases that were inhibited by clavulanic acid. Antimicrobial resistance gene profiles were determined by PCR for a subset of 11 multidrug-resistant serovar Infantis strains, and putative ESBLs were detected by isoelectric focusing. Ten serovar Infantis strains carried bla(TEM), catI, ant(3")Ia and/or ant(3")Ib, sulI and/or sulII, and tet(D) genes as well as an integron-associated aac(6')-Iq cassette. Eight strains possessed at least four different beta-lactamases with pI profiles that confirmed the presence of both ESBLs and non-ESBLs. Our PFGE profiles indicated that 33 serovar Infantis strains isolated from Rio de Janeiro hospitals came from the same genetic lineage.

Anti-Bacterial Agents↗
PubMed2003

[Antimicrobial activity of hinokitiol against Legionella pneumophila].

Antimicrobial activity of hinokitiol (beta-thujaplicin), which is a major component of the essential oil of Chamaecyparis obtuse, against Legionella pneumophila was investigated experimentally. The quantitative antibacterial assay of hinokitiol was carried out by the disk-diffusion method. The test concentrations of hinokitiol were 0.39 to 25.0 micrograms/disk, and the lowest concentrations of hinokitiol that showed growth inhibition against L. pneumophila were 1.56 micrograms/disk on B-SYE agar and 0.39 microgram/disk on B-SYE agar without iron.

Anti-Bacterial Agents↗
PubMed2004

Vancomycin-resistant Staphylococcus aureus isolate from a patient in Pennsylvania.

A vancomycin-resistant Staphylococcus aureus (VRSA) isolate was obtained from a patient in Pennsylvania in September 2002. Species identification was confirmed by standard biochemical tests and analysis of 16S ribosomal DNA, gyrA, and gyrB sequences; all of the results were consistent with the S. aureus identification. The MICs of a variety of antimicrobial agents were determined by broth microdilution and macrodilution methods following National Committee for Clinical Laboratory Standards (NCCLS) guidelines. The isolate was resistant to vancomycin (MIC = 32 micro g/ml), aminoglycosides, beta-lactams, fluoroquinolones, macrolides, and tetracycline, but it was susceptible to linezolid, minocycline, quinupristin-dalfopristin, rifampin, teicoplanin, and trimethoprim-sulfamethoxazole. The isolate, which was originally detected by using disk diffusion and a vancomycin agar screen plate, was vancomycin susceptible by automated susceptibility testing methods. Pulsed-field gel electrophoresis (PFGE) of SmaI-digested genomic DNA indicated that the isolate belonged to the USA100 lineage (also known as the New York/Japan clone), the most common staphylococcal PFGE type found in hospitals in the United States. The VRSA isolate contained two plasmids of 120 and 4 kb and was positive for mecA and vanA by PCR amplification. The vanA sequence was identical to the vanA sequence present in Tn1546. A DNA probe for vanA hybridized to the 120-kb plasmid. This is the second VRSA isolate reported in the United States.

Anti-Bacterial Agents↗
PubMed1995

Emerging multiply resistant enterococci among clinical isolates. I. Prevalence data from 97 medical center surveillance study in the United States. Enterococcus Study Group.

To assess the evolving problem of therapeutic drug resistances among enterococci, we organized a comprehensive national (United States) surveillance trial using 99 recruited microbiology laboratories in 48 of the 49 contiguous states or districts. All but two sites completed the protocol that generated information from nearly 2000 enterococci, usually isolated from blood cultures. All strains were speciated by the same method (API 20S) and were susceptibility tested by three methods (broth microdilution, disk diffusion, and Etest) against ampicillin, penicillin, vancomycin, teicoplanin, gentamicin, and streptomycin. Strains resistant to a glycopeptide or penicillin, or possessing high-level aminoglycoside resistance were referred to the monitor's laboratory for validation and additional susceptibility testing against other alternative antimicrobial agents. The most common species were Enterococcus faecalis and Enterococcus faecium. However, antimicrobial resistance occurred most often among the E. faecium isolates. Twenty-three percent of participant centers (22 sites) reported 87 vancomycin-resistant isolates, which accounts for 4.4% of the isolates evaluated. A recent audit (March 1994) of the laboratories not reporting vancomycin resistance during the study interval (October-December 1992) revealed that 61% of sites have now recognized these strains, a threefold increase in 12-15 months. Teicoplanin remained active against 28% (Van B phenotype) of vancomycin-resistant enterococci (10 E. faecalis, 13 E. faecium, and one Enterococcus spp.). Ampicillin-resistant beta-lactamase-positive strains were found only at one medical center (two strains, 0.2% of referred or validated strains); however, ampicillin-resistant strains represented 12% of all enterococcal, but nearly 60% of E. faecium strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-Bacterial Agents↗
PubMedDate not supplied

Antimicrobial susceptibility tests with cefotaxime and correlation with clinical bacteriologic response.

Cefotaxime is a new cephalosporin with a wide spectrum of antimicrobial activity. For broth and agar dilution tests, the MIC breakpoints suggested for defining bacterial susceptibility are less than or equal to 8 microgram/ml for susceptible, 16-32 microgram/ml for moderately susceptible (indeterminate), and greater than 32 microgram/ml for resistant. For disk diffusion tests, the zone diameter breakpoints suggested are greater than or equal to 23 mm for susceptible, 15-22 mm for moderately susceptible, and less than or equal to 14 mm for resistant. Examination of the bacteriologic outcome of infection for 1,440 patients treated with cefotaxime shows that the results of disk diffusion tests correlated well with clinical response. The organisms that had cefotaxime disk zones of greater than or equal to 23 mm were eradicated in 89% of cases. Clinical response of urinary tract infections due to Pseudomonas aeruginosa was unrelated to disk results for bacterial isolates; the rate of bacteriologic cure was 55%-64% regardless of category. It is further recommended that cefotaxime and other cephalosporins not be tested against the enterococci.

Acinetobacter↗
PubMed2005

Investigation of falsely reported resistance of Streptococcus equi subsp. zooepidemicus isolates from horses to trimethoprim-sulfamethoxazole.

The objective of this study was to investigate the perceived increase in resistance of Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) isolated from the lower respiratory tract of horses to trimethoprim-sulfamethoxazole (SXT). The recorded SXT-susceptibility results of 50 S. zooepidemicus isolates from the tracheal wash fluid of equine patients examined at Colorado State University Veterinary Teaching Hospital from each of 2 time periods (1987-1990 and 1997-2001) were compared and statistically analyzed using a cross-sectional study design. There was a statistically significant difference between the documented resistance of S. zooepidemicus isolated in the 1987-1990 time period (8%), using quantitative microbroth dilution, and the resistance reported for isolates from the 1997-2001 time period (42%), using Kirby-Bauer agar disk diffusion. Laboratory investigation revealed inadequate quality control of media and subsequent falsely reported resistance of S. zooepidemicus from 1997 to 2001 time period. This study demonstrates how minor deviations from prescribed laboratory-testing guidelines can have a major effect on antimicrobial susceptibility test results. The study also underscores the need for regular surveillance and monitoring of trends in antimicrobial susceptibility to detect and correct such problems. In addition, epidemiologists and others collecting data from laboratories should be cautioned to interact with the laboratory regarding interpretation of results of various testing methods to ensure accurate analysis and conclusions.

Animals↗
PubMedDate not supplied

[Clonal analysis of some multiple antibiotic resistant E. coli strains isolated from river and polluted waters].

Several multiple antibiotic resistant E. coli strains isolated from river and polluted waters were compared for their genetic relatedness. Antibiotic susceptibility testing was performed for gentamycin, kanamycin, ampicillin, tetracycline, chloramphenicol, ceftazidime and cefotaxime, as described by Kirby-Bauer disk diffusion method following NCCLS recommendations. Minimum inhibitory concentrations (MICs) were performed using dilution method in Mueller-Hinton broth with a 0.06-64 micrograms/ml concentration range for all antimicrobials and bacterial inoculum of about 1-2 x 10(8) CFU/ml. For the data analysis NCCLS breakpoints for resistance and sensitivity were used. Genomic DNA was isolated from E. coli strains by CTAB method and digested to completion with HindIII enzyme. Genetic characterization was performed by agarose gel electrophoresis and spectrophotometric analysis. Genetic similarity and clustering were calculated using NISIS program. All E. coli strains isolated from river and polluted waters show a high incidence of multiple antibiotic resistance phenotype, 16% of them being resistant to 7, 6 and 4 antibiotics, 40% to 5 and 8% to 2 antibiotics, respectively. A moderate resistance was observed to kanamycin (higher than 30%) and cefotaxime (68%). The percentage of resistant E. coli strains ranged from 76% (to ampicillin, gentamicin and chloramphenicol) to 85% (to ceftazidime). The best results (resistance about 99%) were obtained with tetracycline. Screening for plasmids relieved the presence into 4 E. coli strains of several plasmids ranging from 3.8 kpb to more than 50 kpb. The number of fragments produced by HindIII digestion of genomic DNA ranged from 11 to 25, with sizes of approximately 22 to more than 750 kb. The phenotypic data shows the dynamic flow of multiple antibioresistant E. coli strains in aquatic media (river and polluted waters). Electrophoretic patterns analysis reflects the incidence and diversity of analyzed plasmids. DNA fingerprinting with genomic DNA RE suggested that, depending of the isolation source, E. coli strains could be grouped in two distinct populations with a different plasmid diversity.

Aminoglycosides↗
PubMed2001

Method preferences and test accuracy of antimicrobial susceptibility testing: updates from the College of Amercian Pathologists Microbiology Surveys Program.

OBJECTIVE: To summarize the antimicrobial susceptibility testing results from the College of American Pathologists (CAP) Microbiology Surveys Program for 2000. Specifically, the frequency of tests used and the quantitative and qualitative (susceptibility category) accuracy were assessed. DESIGN: The CAP Microbiology Surveys challenged subscribers in 2000 with 3 well-characterized organisms for antimicrobial susceptibility testing in pure culture. Each laboratory was to use the test method and reporting procedures routinely applied to patient samples. The strains were National Committee for Clinical Laboratory Standards (NCCLS) quality control organisms with precisely defined antimicrobial susceptibility patterns and reproducibility. Results reported by participants (2685-2979/sample) were graded for categorical accuracy and quantitative performance by comparing reported minimal inhibitory concentrations (microg/mL) or zone diameters (mm) against quality control ranges published by the NCCLS. The appropriateness of reported drugs was determined in the context of the type and anatomic location of the infection. RESULTS: The tests most often used varied by the species of the organism and growth characteristics of the isolated strains. Nonfastidious, rapid-growing Surveys unknowns (Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853) were most often tested with commercial systems (MicroScan, 42.0%-42.4%; Vitek, 41.5%-43.0%) or with the standardized disk diffusion method (12.8%-13.9%). In contrast, fastidious species, such as Streptococcus pneumoniae (ATCC 49619), were predominantly tested by Etest (40.3%), followed by disk diffusion (27.6%) and MicroScan (23.2%). Categorical accuracy was essentially equal between dilution (98.9%) and diffusion (99.0%) methods. Among the minimal inhibitory concentration methods used to test penicillin against S pneumoniae, Etest method quantitative accuracy (96.3%) was greater than that of MicroScan (92.4%). Quantitative accuracy was greatest for dilution minimal inhibitory concentration methods, with more than 90% of results within NCCLS quality control ranges for nearly all reported antimicrobials. Reevaluations of quality control ranges may be needed for 4 to 7 agents, depending on method. Reporting errors were also detected in 2 areas: (1) reporting results for drugs not active at the site of infection and (2) reporting results for drugs tested with suboptimal methods without published NCCLS interpretive criteria. CONCLUSIONS: Antimicrobial susceptibility testing methods used in US laboratories were dominated by commercial products with relatively high accuracy (qualitative and quantitative). As available methods have become better suited to both fastidious and rapid-growing species, reporting errors have assumed a higher level of concern to the CAP Surveys in an effort to minimize prescription errors.

Anti-Bacterial Agents↗
PubMed2000

Synthesis and biological activity of alpha-methylene-gamma-lactones as new aroma chemicals.

Seven kinds of alpha-methylene-gamma-lactones with an alkyl group at the C-4 position were synthesized according to a previously described method, with yields of 28-34%. These alpha-methylene-gamma-lactones had characteristic and unique odors. All alpha-methylene-gamma-lactones added a roast-like odor to materials. The antimicrobial effects of alpha-methylene-gamma-lactones were investigated by using a paper disk diffusion method. The results showed the alpha-methylene-gamma-lactones inhibited the growth of three bacteria (Staphylococcus aureus, Escherichia coli, and Pseudomonas fluorescens) and two fungi (Saccharomyces cerevisiae and Aspergillus niger). In particular, alpha-methylene-gamma-undecalactone and alpha-methylene-gamma-dodecalactone exhibited potent inhibition of the growth of these microorganisms compared to butyl p-hydroxybenzoate as standard antibiotic. The umu test revealed that the alpha-methylene-gamma-lactones suppressed the SOS-inducing activity of three mutagens, furylfuramide, UV irradiation, and Trp-P-1, respectively. The antimicrobial effects and the suppressive effects of SOS induction by alpha-methylene-gamma-lactones had a tendency to intensify as the number of carbons in the side chain increased.

Alkanes↗
PubMed1998

Provisional quality control parameters and interpretive criteria for testing susceptibility of Streptococcus pneumoniae and Haemophilus influenzae to quinupristin/dalfopristin (RP59500). Antimicrobial Testing Quality Control Group.

Studies were undertaken to select tentative criteria for susceptibility testing of quinupristin/dalfopristin against Streptococcus pneumoniae and Haemophilus influenzae. Against 612 isolates of Streptococcus pneumoniae, MICs of quinupristin/dalfopristin were < or = 1.0 microg/ml for all but one strain. With a tentative MIC breakpoint of either < or = 1.0 microg/ml or < or = 2.0 microg/ml for susceptible, a disk diffusion zone diameter breakpoint of > or = 19 mm embraced all but two of the susceptible pneumococci; > or = 16 mm included all strains. For Haemophilus influenzae, MICs of quinupristin/dalfopristin clustered near the tentative breakpoints; 91.5% of the MICs were 2.0 to 8.0 microg/ml. This precluded satisfactory performance of the disk diffusion test in discriminating between resistant and susceptible isolates unless MIC breakpoints are modified for this species: clinical experience will be needed before that can be justified. Based on data from a multilaboratory study, the following quality control limits are proposed for Streptococcus pneumoniae ATCC 49619 when testing quinupristin/dalfopristin: 0.25 to 1.0 microg/ml for broth microdilution tests and 19 to 24 mm for disk diffusion tests. For tests of Haemophilus influenzae ATCC 29247, MIC limits are 2.0 to 16 microg/ml; disk tests were very reproducible but are not yet recommended.

Anti-Bacterial Agents↗
PubMed2001

Antimicrobial activity of BMS 284756 (T-3811) against Neisseria gonorrhoeae tested by three methods.

The potency of BMS 284756, a novel des-F(6)-quinolone, was tested against 137 clinical isolates of Neisseria gonorrhoeae including 50 strains observed to be resistant to ciprofloxacin and other newer quinolones. The gonococci were tested using NCCLS methods (agar dilution, disk diffusion) and Etest. BMS 284756 potency versus N. gonorrhoeae was generally two- to four-fold greater than ciprofloxacin. Penicillin resistance in the absence of ciprofloxacin resistance did not affect BMS 284756 activity. However, elevated ciprofloxacin MICs were associated with higher BMS 284756 MIC results as follows (BMS 284756 MIC(50)/MIC range in mg/l): ciprofloxacin-susceptible strains (0.016 or 0.03/0.004-0.06), ciprofloxacin-intermediate strains (0.06 or 0.12/0.008-0.25) and ciprofloxacin-resistant strains (0.12 or 0.5/0.12-1). Etest MICs were routinely lower than those produced by the reference agar dilution method, but the correlation coefficient remained acceptable (r = 0.87). Similarly acceptable correlation was achieved with 5 microg disk zone diameters (r = 0.78), where all zones were > or = 28 mm (MIC < or = 1 mg/l). In conclusion, BMS 284756 was very active against N. gonorrhoeae (MIC(50) 0.03 mg/l overall) including ciprofloxacin-resistant strains and could be considered as a single-dose therapeutic option for gonorrhoea.

Anti-Infective Agents↗
PubMed1983

Tentative interpretive standards for disk diffusion susceptibility testing with norfloxacin (MK-0366, AM-715).

Norfloxacin is a new orally absorbed quinoline derivative structurally related to nalidixic acid but showing an expanded antibacterial spectrum which includes Enterobacteriaceae, Pseudomonas aeruginosa, Streptococcus faecalis, and staphylococci, among other susceptible bacterial species. The application of the regression line and error rate-bounded methods of analysis to the minimal inhibitory concentration and zone size data collected on 413 clinical isolates favored the selection of a 10-micrograms disk content and the adoption of the following interpretive zone size breakpoints for antimicrobial susceptibility testing with norfloxacin: greater than or equal to 17 mm for susceptible, 13 to 16 mm for intermediate, and less than or equal to 12 mm for resistant categories. It is proposed that isolates with minimal inhibitory concentrations of less than or equal to 16 and greater than or equal to 32 micrograms/ml be considered susceptible and resistant to norfloxacin, respectively. Differences in the antibiotic disk contents and in vitro antibacterial spectra and pharmacokinetic properties, together with the much lower rates of cross-resistance reported between norfloxacin and related drugs, strongly argue against the use of the "class disk" concept in this instance and suggest that the 10-micrograms norfloxacin susceptibility disk should be tested separately.

Anti-Bacterial Agents↗
PubMed1989

Wide variability in Pseudomonas aeruginosa aminoglycoside results among seven susceptibility testing procedures.

Seven commonly used antimicrobial susceptibility testing methods were used to test the susceptibility of 150 isolates of Pseudomonas aeruginosa against gentamicin, tobramycin, amikacin, carbenicillin, and piperacillin. Results were compared with respect to the susceptibility characteristics of the population of isolates as defined by each method. Conventional methods included agar disk diffusion and agar dilution, carried out in accordance with current recommendations of the National Committee for Clinical Laboratory Standards, as well as broth microdilution testing with cation-supplemented Mueller-Hinton broth (CSMHB). Methods in which instrumentation was used for result determination included the Autobac I, Avantage, Sensititre Autoreader (using a breakpoint panel at 18 h of incubation), and Vitek (AMS-240, using the GNS susceptibility card). When necessary for comparison, MIC data were converted to categorical interpretations (susceptible, intermediate, and resistant). With respect to gentamicin, no significant differences were noted among the results of disk diffusion, broth microdilution, Sensititre Auto breakpoint, or Vitek methods which characterized 60 to 67% of isolates as susceptible, 16 to 22% as intermediate, and 13 to 17% as resistant. In contrast, agar dilution, Autobac, and Avantage, although yielding gentamicin results similar to those of one another, were each significantly different in result reporting from the other four methods above for gentamicin results, and they characterized the Pseudomonas population largely as susceptible (88 to 97%), with 0 to 6% intermediate and only 3% to 6% resistant. More isolates were characterized as being resistant to gentamicin in the Avantage test if an assay broth supplemented with increased amounts of calcium was used. Cation impregnation of Autobac disks did not appreciably change Autobac results. The geometric mean MIC of gentamicin was 4 micrograms/ml lower in the agar dilution method than in the CSMHB microdilution method, despite monitoring of the agar for cation content through performance disk diffusion testing with P. aeruginosa ATCC 27853. Tobramycin activity was greater than gentamicin activity, and susceptibility to tobramycin ranged from 89 to 97%, with few statistically significant differences noted among the seven methods studied. Differences in MIC distribution and geometric mean MIC between agar dilution and CSMHB microdilution testing were minimal and suggested less of a cation influence on tobramycin than gentamicin results. Although amikacin was also more active than gentamicin (83 to 99% of isolates were susceptible), differences in the amikacin results among methods tended to reflect the same trends in reporting as seen with gentamicin testing, with the exception that results of Avantage testing were similar to those of disk diffusion, CSMHB microdilution, Sensititre, and Vitek. A difference in geometric mean MIC of 5 micrograms/ml between CSMHB testing and agar dilution testing suggested the influence of divalent cations on amikacin results. Few highly significant differences were noted among methods when isolates were tested against carbenicillin and piperacillin, except that Avantage piperacillin results (66% susceptible) and Autobac piperacillin results (98% susceptible) were noticeably different from the percent piperacillin susceptibility (range, 85 to 92%) measured by the other methods. Method-dependent variability among aminoglycoside susceptibility results, particularly when testing gentamicin, prevents meaningful comparison of Pseudomonas susceptibility trends among hospitals when different methods are used and promotes confusion and frustration among clinical microbiologists and clinicians owing to the uncertainties of clinical meaning of these data.

Aminoglycosides↗