Comparative assessment of Etest for testing susceptibilities of Neisseria gonorrhoeae to penicillin, tetracycline, ceftriaxone, cefotaxime, and ciprofloxacin: investigation using 510(k) review criteria, recommended by the Food and Drug Administration

Use of genome sequencing to resolve differences in gradient diffusion and agar dilution antimicrobial susceptibility testing performance of Neisseria gonorrhoeae isolates in Alberta, Canada

Agar dilution is the gold standard method for phenotypic antimicrobial susceptibility testing (AST) for Neisseria gonorrhoeae. However, this method is laborious and requires expertise, so laboratories that perform N. gonorrhoeae AST may choose alternative methods such as disk diffusion and gradient diffusion. In this study, we retrospectively compare the performance of gradient diffusion to agar dilution for 2,394 unique N. gonorrhoeae isolates identified in Alberta from 2017 to 2020 against azithromycin, cefixime, ceftriaxone, ciprofloxacin, penicillin, and tetracycline. Genome sequencing was utilized to resolve discrepancies between AST methods, detect antimicrobial resistance markers, and identify trends between error rates and sequence types (STs) of isolates. Over 90% of N. gonorrhoeae isolates were susceptible to azithromycin, cefixime, and ceftriaxone, whereas decreased susceptibility was observed for ciprofloxacin, penicillin, and tetracycline. Categorical (CA) and essential agreement (EA) was poorest between the two methods for penicillin (CA: 86.02%; EA: 77.69%) and tetracycline (CA: 47.22%; EA: 55.96%); however, the low CA was primarily attributed to minor errors. Antimicrobial agents with errors outside of acceptable limits included azithromycin (very major error: 18.42%; major error: 7.73%) and tetracycline (very major error: 6.17%). Genome sequencing on a subset of isolates resolved 30.3% of the azithromycin major errors and confirmed the azithromycin or tetracycline very major errors. Significant associations between certain STs and error types for azithromycin and tetracycline were also identified. Overall, gradient diffusion compared well to agar dilution for cefixime, ceftriaxone, and ciprofloxacin, and genome sequencing was identified as a useful tool to arbitrate discrepant susceptibility testing results between gradient diffusion and agar dilution for N. gonorrhoeae.

Loci for prediction of penicillin and tetracycline susceptibility in Neisseria gonorrhoeae: a genome-wide association study

BACKGROUND

Neisseria gonorrhoeae poses an urgent public health threat because of increasing antimicrobial resistance; however, much of the circulating population remains susceptible to historical treatment regimens. Point-of-care diagnostics that report susceptibility could allow for reintroduction of these regimens, but development of such diagnostics has been restricted to ciprofloxacin, for which susceptibility can be predicted from a single locus. We aimed to define genetic variants associated with susceptibility to penicillin and tetracycline.

METHODS

We collected publicly available global whole-genome sequencing data (n=12 045) from clinical N gonorrhoeae isolates, with phenotypic resistance data for penicillin (n=6935), and tetracycline (n=5727). Using conditional genome-wide association studies, we defined genetic variants associated with susceptibility to penicillin and tetracycline. We excluded isolates that could not be classified as either susceptible or resistant. To validate our results, we assembled 1479 genomes from the US Centers for Disease Control and Prevention (CDC)'s Gonococcal Isolate Surveillance Project, for which urethral specimens are collected at sentinel surveillance sites across the USA. We evaluated the sensitivity and specificity of susceptibility-associated alleles using Clinical & Laboratory Standards Institute breakpoints for susceptibility and non-resistance in both the global and validation datasets.

FINDINGS

In our conditional penicillin genome-wide association study, the presence of a genetic variant defined by a non-mosaic penA allele without an insertion at codon 345 was associated with penicillin susceptibility and had the highest negative effect size (β) of significant variants (p=5·0x10-14, β -2·5). In combination with the absence of blaTEM, this variant predicted penicillin susceptibility with high specificity (99·8%) and modest sensitivity (36·7%). For tetracycline, the wildtype allele at rpsJ codon 57, encoding valine, was associated with tetracycline susceptibility (p=5·6x10-16, β -1·6) after conditioning on the presence of tetM. The combination of rpsJ codon 57 allele and tetM absence predicted tetracycline susceptibility with high specificity (97·2%) and sensitivity (88·7%).

INTERPRETATION

As few as two genetic loci can predict susceptibility to penicillin and tetracycline in N gonorrhoeae with high specificity. Molecular point-of-care diagnostics targeting these loci have the potential to increase available treatments for gonorrhoea.

FUNDING

National Institute of Allergy and Infectious Diseases, the National Science Foundation, and the Smith Family Foundation.

Comparison of Microdilution Method with Agar Dilution Method for Antibiotic Susceptibility Test of Neisseria gonorrhoeae

Introduction

Antimicrobial resistance (AMR) of Neisseria gonorrhoeae (N. gonorrhoeae) becomes a grave public health problem in the world. A strengthened Antimicrobial Resistance Surveillance Program is needed to track the trend of AMR development. However, the lack of a proper antimicrobial susceptibility test (AST) method is a barrier to expand the AMR surveillance in China. Traditional agar dilution (AD) method is laborious and E-test strips have no approval license for clinical use. Herein, a Chinese group modified the microdilution (MD) method for clinical ASTs. The objective of this study is to compare the MD method with the AD method for N. gonorrhoeae AST.

Materials and Methods

A total of 166 clinical isolates were tested for antimicrobial susceptibility of ceftriaxone, spectinomycin, azithromycin, ciprofloxacin, tetracycline, and penicillin using MD and AD method simultaneously. Results of MD method were read manually or automatically. Rates of essential agreement (EA), category agreement (CA), minor error, and very major error were compared.

Results

The total EAs (compared with results read manually) of penicillin, tetracycline, ciprofloxacin, spectinomycin, ceftriaxone, and azithromycin were 90.4%, 97.0%, 85.5%, 100.0%, 94%, and 72.3%; and CAs were 82.5%, 94.0%, 100%, 100%, 95.2%, and 94%, respectively.

Conclusion

We conclude that the MD method might be an alternative for clinical AST of N. gonorrhoeae in China. In particular, MD method has the potency of accurate differentiation of isolates resistant to ceftriaxone or azithromycin, which were empirically recommended for gonococcal treatment, but its quality remained suboptimal, and further improvement is needed for clinical use.

Genotypic and Phenotypic Characterization of Antimicrobial Resistance in Neisseria gonorrhoeae: a Cross-Sectional Study of Isolates Recovered from Routine Urine Cultures in a High-Incidence Setting

The objectives of this study were to perform genomic and phenotypic characterization of antimicrobial resistance in Neisseria gonorrhoeae isolates recovered from urine samples from patients in St. Louis, MO, USA. Sixty-four clinical isolates were banked over a 2-year period and subjected to antimicrobial susceptibility testing (AST) by Kirby-Bauer disk diffusion (penicillin, tetracycline, cefuroxime, and ciprofloxacin) and gradient diffusion (tetracycline, doxycycline, azithromycin, ceftriaxone, cefixime, ciprofloxacin, gemifloxacin, and delafloxacin). The medical records for the patients were evaluated to determine the demographics, location, and prescribed treatment regimen. Isolate draft genomes were assembled from Illumina shotgun sequencing data, and resistance determinants were identified by ResFinder and PointFinder. Of the 64 isolates, 97% were nonsusceptible to penicillin, with resistant isolates all containing the blaTEM-1b gene; 78 and 81% of isolates were nonsusceptible to tetracycline and doxycycline, respectively, with resistant isolates all containing the tet(M) gene. One isolate was classified as non-wild-type to azithromycin, and all isolates were susceptible to ceftriaxone; 89% of patients received this combination of drugs as first-line therapy. Six percent of isolates were resistant to ciprofloxacin, with most resistant isolates containing multiple gyrA and parC mutations. Correlation between disk and gradient diffusion AST devices was high for tetracycline and ciprofloxacin (R2 > 99% for both). The rates of N. gonorrhoeae antibiotic resistance in St. Louis are comparable to current rates reported nationally, except ciprofloxacin resistance was less common in our cohort. Strong associations between specific genetic markers and phenotypic susceptibility testing hold promise for the utility of genotype-based diagnostic assays to guide directed antibiotic therapy.IMPORTANCENeisseria gonorrhoeae causes the sexually transmitted infection gonorrhea, which is most commonly diagnosed using a DNA-based detection method that does not require growth and isolation of N. gonorrhoeae in the laboratory. This is problematic because the rates of antibiotic resistance in N. gonorrhoeae are increasing, but without isolating the organism in the clinical laboratory, antibiotic susceptibility testing cannot be performed on strains recovered from clinical specimens. We observed an increase in the frequency of urine cultures growing N. gonorrhoeae after we implemented a total laboratory automation system for culture in our clinical laboratory. Here, we report on the rates of resistance to multiple historically used, first-line, and potential future-use antibiotics for 64 N. gonorrhoeae isolates. We found that the rates of antibiotic resistance in our isolates were comparable to national rates. Additionally, resistance to specific antibiotics correlated closely with the presence of genetic resistance genes, suggesting that DNA-based tests could also be designed to guide antibiotic therapy for treating gonorrhea.

Comparison of Neisseria gonorrhoeae minimum inhibitory concentrations obtained using agar dilution versus microbroth dilution methods

With increasing antibiotic resistance observed amongst clinical isolates of Neisseria gonorrhoeae, the second most prevalent sexually transmitted bacterial disease in the United States, there is still a need for antimicrobial susceptibility testing (AST). The current method recommended by the Clinical and Laboratory Standards Institute is agar dilution. In this study, we show that a commercially available version of Fastidious Broth is capable of supporting N. gonorrhoeae in the evaluation of minimum inhibitory concentrations of 4 antibiotics (ceftriaxone, azithromycin, ciprofloxacin, and tetracycline), when comparing the agar dilution (AD) versus microbroth dilution (MBD) method and the susceptibilities obtained for 32 N. gonorrhoeae isolates. Herein, 3 out of the 4 antibiotics tested showed 94% or greater essential agreement (EA) and 91% or greater categorical agreement (CA) respectively, when comparing the MBD versus AD methods.

Performance characteristics of newer MIC gradient strip tests compared with the Etest for antimicrobial susceptibility testing of Neisseria gonorrhoeae

For Neisseria gonorrhoeae susceptibility testing, Etest, comparable to agar dilution, is frequently used. In recent years, newer MIC gradient strip tests have been commercialized. However, these tests have not been appropriately evaluated for gonococci. We evaluated the sensitivity, specificity, accuracy, quality, availability of antimicrobials and cost of the MIC Test Strip (Liofilchem), M.I.C.Evaluator (Oxoid) and Ezy MIC Strip (HiMedia), compared to the reference Etest (bioMérieux), for gonococcal susceptibility testing. The MICs of eight antimicrobials in 103 gonococcal international reference strains (n = 29) and clinical isolates (n = 74) were examined. Coefficient of determination (R² ), complete agreement, essential agreement, SIR categorical agreement, sensitivity, specificity and accuracy were calculated. R² of the MICs for the antimicrobials ranged between 0.674-0.996, 0.617-0.993, and 0.643-0.994 for the MIC Test Strip, M.I.C.Evaluator strips and Ezy MIC Strips respectively. The essential agreement (SIR categorical agreement) was 99.6% (88.6%), 100% (87.1%) and 93.0% (83.1%) respectively. M.I.C.Evaluator strips for gonococcal key antimicrobials were lacking and the Ezy MIC Strips showed an inconsistent accuracy, quality and some strips were contaminated. The Liofilchem MIC Test Strips had limitations, but might be relatively accurate alternatives to Etest for gonococci. Strict quality assurance (at manufacturing and testing laboratory), including quality controls, are required.

A new rapid resazurin-based microdilution assay for antimicrobial susceptibility testing of Neisseria gonorrhoeae

Objectives

Rapid, cost-effective and objective methods for antimicrobial susceptibility testing of Neisseria gonorrhoeae would greatly enhance surveillance of antimicrobial resistance. Etest, disc diffusion and agar dilution methods are subjective, mostly laborious for large-scale testing and take ∼24 h. We aimed to develop a rapid broth microdilution assay using resazurin (blue), which is converted into resorufin (pink fluorescence) in the presence of viable bacteria.

Methods

The resazurin-based broth microdilution assay was established using 132 N. gonorrhoeae strains and the antimicrobials ceftriaxone, cefixime, azithromycin, spectinomycin, ciprofloxacin, tetracycline and penicillin. A regression model was used to estimate the MICs. Assay results were obtained in ∼7.5 h.

Results

The EC 50 of the dose-response curves correlated well with Etest MIC values (Pearson's r  = 0.93). Minor errors resulting from misclassifications of intermediate strains were found for 9% of the samples. Major errors (susceptible strains misclassified as resistant) occurred for ceftriaxone (4.6%), cefixime (3.3%), azithromycin (0.6%) and tetracycline (0.2%). Only one very major error was found (a ceftriaxone-resistant strain misclassified as susceptible). Overall the sensitivity of the assay was 97.1% (95% CI 95.2-98.4) and the specificity 78.5% (95% CI 74.5-82.9).

Conclusions

A rapid, objective, high-throughput, quantitative and cost-effective broth microdilution assay was established for gonococci. For use in routine diagnostics without confirmatory testing, the specificity might remain suboptimal for ceftriaxone and cefixime. However, the assay is an effective low-cost method to evaluate novel antimicrobials and for high-throughput screening, and expands the currently available methodologies for surveillance of antimicrobial resistance in gonococci.

Accuracy and reproducibility of the Etest to detect drug-resistant Neisseria gonorrhoeae to contemporary treatment

PURPOSE

Neisseria gonorrhoeae is a sexually transmitted bacterial pathogen that continues to evolve to become resistant to known antibiotics. In preparing for potential emergence, the Centers for Disease Control and Prevention recommends that clinical laboratories maintain or develop protocols to assess antibiotic susceptibly for this organism. This study examines the intra-laboratory variability of using the Etest method to provide consistent MIC values for N. gonorrhoeae and also compared the results of the Etest to known agar dilution MIC values.

METHODOLOGY

Clinical N. gonorrhoeae isolates, 100 paired duplicates, were tested by eight laboratories for antibiotic susceptibility to ceftriaxone, cefixime and azithromycin using Etest strips.Results/Key findings. Overall, >80 % of the paired Etest MIC values were within one log2 dilution of the replicate. When compared to the agar dilution reference method, the cefixime Etest MIC values were consistently underreported by one dilution (seven laboratories) or two dilutions (one laboratory). The azithromycin Etest MIC values agreed 90.7 % with the agar dilution MIC values while the agreement with ceftriaxone was 90.9 %.

CONCLUSION

Overall, the Etest method yielded reproducible MIC values within each laboratory with the azithromycin and ceftriaxone MIC results consistent to the reference agar dilution method while the cefixime result tended to provide a lower MIC value.

Cluster of Neisseria gonorrhoeae Isolates With High-level Azithromycin Resistance and Decreased Ceftriaxone Susceptibility, Hawaii, 2016

BACKGROUND

The Centers for Disease Control and Prevention (CDC) currently recommends dual therapy with ceftriaxone and azithromycin for gonorrhea to ensure effective treatment and slow emergence of antimicrobial resistance. Since 2013, the prevalence of reduced azithromycin susceptibility increased in the United States; however, these strains were highly susceptible to cephalosporins. We identified a cluster of Neisseria gonorrhoeae isolates with high-level azithromycin resistance, several of which also demonstrated decreased ceftriaxone susceptibility.

METHODS

Eight N. gonorrhoeae isolates collected from 7 patients on Oahu, Hawaii, seen 21 April 2016 through 10 May 2016 underwent routine Etest antimicrobial susceptibility testing by the Hawaii Department of Health. All demonstrated elevated azithromycin minimum inhibitory concentrations (MICs) >256 μg/mL and elevated ceftriaxone MICs (≥0.125 μg/mL). Isolates were sent to the University of Washington and CDC for confirmatory agar dilution testing; sequence data were sent to CDC for analysis. All patients were interviewed and treated, and when possible, partners were interviewed, tested, and treated.

RESULTS

All isolates had azithromycin MICs >16 µg/mL and 5 had ceftriaxone MICs = 0.125 µg/mL by agar dilution. All isolates were β-lactamase positive and were resistant to penicillin, tetracycline, and ciprofloxacin. Genomic analysis revealed genetic relatedness. No patients reported recent travel or antibiotic use, and no male patients reported male sex partners. All patients were successfully treated.

CONCLUSIONS

This cluster of genetically related gonococcal isolates with decreased ceftriaxone susceptibility and high-level azithromycin resistance may bring the threat of treatment failure in the United States with the current recommended dual therapy one step closer.

Antimicrobial susceptibility and molecular epidemiology of Neisseria gonorrhoeae in Germany

Antimicrobial drug resistance in Neisseria gonorrhoeae has become an increasing public health problem. Hence, surveillance of resistance development is of crucial importance to implement adequate treatment guidelines. Data on the spread of antibiotic resistance among gonococcal isolates in Germany, however, is scarce. In a resistance surveillance study conducted by the Paul Ehrlich Society for Chemotherapy between October 2010 and December 2011, 23 laboratories all over Germany were requested to send N. gonorrhoeae isolates to the study laboratory in Frankfurt am Main. Species verification was performed biochemically using ApiNH and with Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS). Antimicrobial susceptibility testing was performed using the Etest method. For molecular epidemiological analysis, N. gonorrhoeae strains were genotyped by means of N. gonorrhoeae multi-antigen sequence typing. A total of 213 consecutive gonococcal isolates were analyzed in this nationwide study. Applying EUCAST breakpoints, high resistance rates were found for ciprofloxacin (74%) and tetracycline (41%). Penicillin non-susceptibility was detected in 80% of isolates. The rate of azithromycin resistance was 6%, while all strains were susceptible to spectinomycin, cefixime, and ceftriaxone. Molecular typing of gonococcal isolates revealed a great heterogeneity of 99 different sequence types (ST), but ST1407 predominated (n=39). This is the first comprehensive German multi-centre surveillance study on antibiotic susceptibility and molecular epidemiology of N. gonorrhoeae with implications for antibiotic choice for treatment of gonorrhoea. The World Health Organization supports the concept that an efficacious treatment of gonorrhoea results in at least 95% of infections being cured. Accordingly, as spectinomycin is not available on the German market, only the third generation cephalosporins cefixime and ceftriaxone are regarded as valuable drugs for empirical treatment of gonorrhoea in Germany.

Assessment of Etest as an alternative to agar dilution for antimicrobial susceptibility testing of Neisseria gonorrhoeae

We studied whether the Etest can be used as an alternative to agar dilution to determine antimicrobial susceptibilities of ceftriaxone, cefixime, and cefpodoxime in Neisseria gonorrhoeae surveillance. One hundred fifteen clinical and laboratory isolates of N. gonorrhoeae were tested following the Clinical Laboratory Improvement Amendments (CLIA)-approved CLSI standard agar dilution method and, separately, by the Etest according to the manufacturer's recommendations. The MICs were determined and compared. Ten laboratory-generated mutants were used to simulate substantially nonsusceptible specimens. The Etest and agar dilution methods were well correlated. Statistical tests produced regression R2 values of 88%, 82%, and 85% and Pearson correlation coefficients of 92%, 91%, and 92% for ceftriaxone, cefixime, and cefpodoxime, respectively. When paired comparisons were made, the two tests were 88.7%, 80%, and 87% within 1 log2 dilution from each other for ceftriaxone, cefixime, and cefpodoxime, respectively. The within-2-log2 agreements were 99.1%, 98.3%, and 94.8% for ceftriaxone, cefixime, and cefpodoxime, respectively. Notwithstanding the good correlations and the within-2-log2 general agreement, the Etest results produced slightly lower MICs than the agar dilution results. In conclusion, we found that the Etest can be effectively used as an alternative to agar dilution testing to determine the susceptibility of N. gonorrhoeae to ceftriaxone, cefixime, and cefpodoxime, although we recommend further research into extremely resistant isolates. For isolates within the typical range of clinical MICs, reexamination of the Etest interpretation of susceptible and nonsusceptible categories would likely allow for successful transition from agar dilution to the Etest.

Comparative assessment of CDS, CLSI disc diffusion and Etest techniques for antimicrobial susceptibility testing of Neisseria gonorrhoeae: a 6-year study

BACKGROUND

A variety of techniques are available for antimicrobial susceptibility testing of Neisseria gonorrhoeae.

OBJECTIVE

The aim of this study was to find a cost-effective, reliable and easily applicable microbiological method to detect antimicrobial susceptibilities of N. gonorrhoeae in resource-poor countries.

DESIGN

Prospective study.

SETTING

Male and female STD clinic of Regional STD Teaching, Training and Research Centre, New Delhi, India.

PARTICIPANTS

N. gonorrhoeae isolates from all male and female patients presenting with acute gonococcal urethritis and cervical discharge.

MATERIAL AND METHODS

A total of 295 consecutive N. gonorrhoeae isolates during 2005-2010 was used to compare the Clinical and Laboratory Standards Institute (CLSI) and CDS disc diffusion technique with Etest by performing antimicrobial susceptibility testing in parallel for penicillin, tetracycline, ceftriaxone, ciprofloxacin and spectinomycin. WHO reference strains were used as controls.

RESULTS

CDS disc diffusion zones of inhibition showed that complete percentage agreement for penicillin, ciprofloxacin and tetracycline was high with their analogous Etest minimal inhibitory concentrations in comparison to CLSI disc diffusion technique, that is, 91.5%, 92.9% and 99.3% versus 87.5%, 88.5% and 74.9%, respectively. CDS results had less number of major and minor category discrepancies in comparison to CLSI and CDS method showed excellent correlation coefficient (r=1) with Etest for all five antimicrobial agents tested in comparison to CLSI (r=0.92). It was very poor (r=0.61) by CLSI method for tetracycline. The correlation coefficients between the two methods and the Etest were identical if tetracycline was removed from the CLSI analysis.

CONCLUSIONS

The CDS technique is an attractive alternative for N. gonorrhoeae susceptibility testing and is recommended for monitoring the antimicrobial susceptibility in less developed and resource-poor settings to facilitate enhanced antimicrobial resistance surveillance when the WHO Gonococcal Antimicrobial Surveillance Programme is undergoing expansion to meet the ongoing challenges of surveillance and control of gonococcal antimicrobial resistance.

Rapid detection of ciprofloxacin effects on Fusarium graminearum and F. avenaceum cells in modulating environmental pH using a reactive, non-toxic food-dye indicator

The objective of the study was to assess the effect of ciprofloxacin antibiotic on the physiological or phenotypic characteristics of food-borne toxigenic Fusarium graminearum and F. avenaceum molds under in vitro conditions. In the presence of ciprofloxacin, Fusarium mycelia growth and morphology were altered based on the antibiotic concentration range used. Results showed that ciprofloxacin in concentrations ≥40μg/mL induced chlamydospore formation in Fusaria and as such, this antibiotic should be considered as an important abiotic stress factor and growth inhibitor. A novel method was investigated to correlate chlamydospore formation with the colour changes observed in FD&C Green Number 3, a common water soluble food dye. The antibiotic-treated F. graminearum and F. avenaceum isolates produced chamydospores, which in turn altered environmental pH with concomitant changes in the colour and intensity of the dye. The colour changes observed as a function of environmental pH were supported by instrumental methods (pH meter and spectroscopy), and a commercial pH indicator (thymol blue) results. In conclusion, we propose that FD&C Green Number 3 can be used as an accurate indicator for the rapid assessment of Fusarium molds when grown on ciprofloxacin antibiotic-containing substrate. Special emphasis should be given to an indirect risk assessment of antibiotic effects on toxic molds.

Trends in antimicrobial susceptibility of Neisseria gonorrhoeae in Israel, 2002 to 2007, with special reference to fluoroquinolone resistance

MICs for penicillin, tetracycline, ceftriaxone, ciprofloxacin, spectinomycin, and azithromycin were determined by the Etest method for 406 gonococcal strains obtained in 2002-2007 from an STI clinic and a general microbiology laboratory in Central Israel. A total of 27.1% were resistant to ciprofloxacin, with increasing resistance over the years All isolates remain susceptible to ceftriaxone and spectinomycin.

Antimicrobial susceptibility of Neisseria gonorrhoeae isolates determined by the agar dilution, disk diffusion and Etest methods: comparison of results using GC agar and chocolate agar

Although the use of GC agar for determining Neisseria gonorrhoeae antimicrobial susceptibilities is suggested by Clinical and Laboratory Standard Institute (CLSI) guidelines, chocolate agar is still used in some regions owing to its low cost and availability. To determine the differences in susceptibilities determined using GC and chocolate agars, 163 non-duplicate N. gonorrhoeae isolates were tested. Minimum inhibitory concentrations (MICs) and percent susceptibilities determined using the GC agar dilution method, respectively, were as follows: ceftriaxone, 0.004-0.125 mg/L, 100%; cefixime, 0.002 mg/L to >32 mg/L, 98.2%; and ciprofloxacin, 0.002 mg/L to >32 mg/L, 3.1%. Comparison of ceftriaxone MICs determined by the Etest using GC agar and chocolate agar showed that use of GC agar tended to result in lower MICs than GC agar dilution, whilst use of chocolate agar tended to result in higher MICs (concordance, 55.8% and 82.8%, respectively). Disk inhibition zones obtained using GC agar and chocolate agar (and their correlation coefficients) were, respectively: ceftriaxone, 35-55 mm and 25-50mm (0.46); ciprofloxacin, 6-55 mm and 6-43 mm (0.84); and penicillin, 6-47 mm and 6-50 mm (0.93). Use of chocolate agar with the disk diffusion method for ceftriaxone was associated with a 5.5% false resistance rate. In summary, compared with GC agar, susceptibility testing using chocolate agar tends to yield higher MICs with the Etest and smaller disk inhibition zones with disk diffusion methods. Clinical microbiology laboratories should strictly adhere to CLSI recommendations by using GC agar instead of chocolate agar when performing susceptibility testing for N. gonorrhoeae.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. As resistance continues to emerge among a wide range of clinically relevant bacteria, the complexity of this communication increases. This updated version provides an overview of the important susceptibility concerns for most commonly isolated bacterial pathogens.

Agar and broth dilution methods to determine the minimal inhibitory concentration (MIC) of antimicrobial substances

The aim of broth and agar dilution methods is to determine the lowest concentration of the assayed antimicrobial agent (minimal inhibitory concentration, MIC) that, under defined test conditions, inhibits the visible growth of the bacterium being investigated. MIC values are used to determine susceptibilities of bacteria to drugs and also to evaluate the activity of new antimicrobial agents. Agar dilution involves the incorporation of different concentrations of the antimicrobial substance into a nutrient agar medium followed by the application of a standardized number of cells to the surface of the agar plate. For broth dilution, often determined in 96-well microtiter plate format, bacteria are inoculated into a liquid growth medium in the presence of different concentrations of an antimicrobial agent. Growth is assessed after incubation for a defined period of time (16-20 h) and the MIC value is read. This protocol applies only to aerobic bacteria and can be completed in 3 d.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. The emergence of resistance in nearly all commonly isolated bacterial organisms has highlighted the need for ongoing dialogue between the laboratory and those who use its services.

High prevalence of high-level ciprofloxacin resistance in Neisseria gonorrhoeae in Tel Aviv, Israel: correlation with response to therapy

A survey of the drug susceptibilities of gonococcal isolates from 100 consecutive patients attending clinics in the Tel Aviv area of Israel during the period from February to September 2000 has shown a 61% rate of ciprofloxacin resistance (MICs, > or =1 microg/ml); 51 isolates were highly resistant (MICs, > or =4 microg/ml). Ciprofloxacin-resistant strains were more prevalent among isolates with chromosomally mediated resistance to penicillin and tetracycline. Therapeutic failures with ciprofloxacin correlated with MICs of > or =1 mg/ml (P < 0.01). The high rate of ciprofloxacin resistance coinciding with a sharp rise in the incidence of gonorrhea precludes the use of fluoroquinolone drugs for the empirical treatment of gonococcal infections acquired in Israel.

Antimicrobial susceptibilities of Neisseria gonorrhoeae in Kigali, Rwanda, and trends of resistance between 1986 and 2000

BACKGROUND

Plasmid-mediated and chromosomal-mediated resistance of Neisseria gonorrhoeae to penicillin, tetracycline, thiamphenicol, and trimethoprim-sulfamethoxazole has spread dramatically in Africa. Monitoring of antimicrobial susceptibility is a key element in the control of sexually transmitted diseases.

GOAL

To document antimicrobial susceptibilities of gonococci isolated during the past 15 years in Kigali, Rwanda.

STUDY DESIGN

Minimal inhibitory concentrations of recently collected gonococcal isolates of eight antimicrobials were determined. The results were compared with data collected for isolates obtained since 1986.

RESULTS

In 1986, 35% of the gonococcal isolates were penicillinase-producing N gonorrhoeae. Tetracycline-resistant N gonorrhoeae appeared in 1989. The prevalence of penicillinase-producing N gonorrhoeae and tetracycline-resistant N gonorrhoeae increased significantly to 70.5% and 89.2%, respectively. Chromosomal resistance to penicillin, tetracycline, and thiamphenicol increased temporarily, then decreased significantly. Chromosomal resistance to trimethoprim-sulfamethoxazole appeared in 1988 and increased to 21.6%. All the isolates were susceptible to ceftriaxone, ciprofloxacin, spectinomycin, and kanamycin.

CONCLUSIONS

This study illustrated the rapidly increasing frequencies of penicillinase-producing N gonorrhoeae and tetracycline-resistant N gonorrhoeae. Chromosomal resistance to thiamphenicol and trimethoprim-sulfamethoxazole excludes these drugs as alternative treatment. Programs for antimicrobial susceptibility surveillance of N gonorrhoeae should urgently be established in Africa.

Antimicrobial activity of gatifloxacin tested against Neisseria gonorrhoeae using three methods and a collection of fluoroquinolone-resistant strains

Gatifloxacin, a new 8-methoxy fluoroquinolone, was tested against 131 strains of Neisseria gonorrhoeae by reference agar dilution, disk diffusion, and Etest (AB BIODISK, Solna, Sweden) methods on supplemented GC agar. Gatifloxacin activity was equal to ciprofloxacin (MIC50, 0.008 microgram/mL) against strains fully susceptible to fluoroquinolones, but was generally four-fold more active (MIC90, 0.064-0.094 microgram/mL) against strains with par C or gyr A mutations and resistance to ciprofloxacin. Etest results were comparable to those generated by the agar dilution test [correlation coefficient (r) = 0.97]. Gatifloxacin zone diameters using 5-microgram disks also correlated well (r = 0.86-0.87) with the agar dilution and Etest MIC results. Breakpoints for laboratory testing of Neisseria gonorrhoeae strains await clinical trial outcome correlations, but susceptibility at < or = 0.125 or < or = 0.25 microgram/mL (> or = 34 mm) seems appropriate. All three tests used in this study seem applicable for laboratory testing of isolates from patients with uncomplicated gonorrhoeae receiving therapy with gatifloxacin.

Antimicrobial activity of trovafloxacin tested against ciprofloxacin-susceptible and -resistant Neisseria gonorrhoeae. Interpretive criteria and comparisons with Etest results

Trovafloxacin, a new fluorinated naphthyridine, has enhanced activity against Gram-positive cocci, while retaining an excellent spectrum against Gram-negative pathogens. It has been used successfully in clinical trials for therapy of gonorrhea, and this investigation proposes in vitro susceptibility testing criteria for trovafloxacin. A total of 150 Neisseria gonorrhoeae clinical isolates (50 resistant to ciprofloxacin; MICs > or = 0.12 microgram/mL) were tested by methods recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and the Etest (AB BIODISK, Solna, Sweden). Trovafloxacin was very active against gonococci (MIC90, 0.008 to 0.015 microgram/mL), but was generally eightfold less potent versus ciprofloxacin-resistant strains. Etest results correlated well (r = 0.96; 98% of MICs +/- one log2 dilution) compared to the reference agar dilution test. Reference agar dilution and Etest MICs were compared to disk-diffusion test zones (10-micrograms trovafloxacin disk), and excellent categorical agreement (89.4 to 99.3%) was achieved without significant false-susceptible or -resistant error (< or = 1.3%). Tentative breakpoints were suggested initially to outline the ciprofloxacin-susceptible and trovafloxacin-susceptible as susceptible (MIC, < or = 0.015 microgram/mL; zones > or = 47 mm), and strains with various well-characterized mutations of the gyr A and par C genes as either intermediate or resistant to trovafloxacin. When the results of clinical studies treating ciprofloxacin-resistant N. gonorrhoeae with trovafloxacin become available, the alternative breakpoints could be utilized for resistant MIC breakpoints of > or = 0.06 microgram/mL or > or = 0.5 microgram/mL. Trovafloxacin was stable in supplemented GC agar for 21 days stored at refrigerated temperatures. These in vitro results indicate that trovafloxacin should be a very acceptable agent for therapy of gonorrhea and other common sexually transmitted pathogens.

Validation of Etest for seven antimicrobial agents using regulatory criteria for the assessment of antimicrobial susceptibility devices

Etest, a stable-gradient antimicrobial susceptibility test method (AB Biodisk, Solna, Sweden), was compared to the agar dilution method for tests with amikacin, tobramycin, aztreonam, cefdinir, cefprozil, ceftazidime and sparfloxacin. The study design followed recommended guidelines for 510(k) application by the U.S. Food and Drug Administration (FDA). Results demonstrated Etest accuracy (MIC +/- 1.0 log2 dilutions compared to agar dilution MIC) to be 90.4% with only one of 1150 comparisons showing a reproducible variation of 2 log2 dilutions. Categorical equivalency was > or = 92% for all drugs except ceftazidime (82 to 89%), but false-susceptible error was rare (< 0.1%). Etest was very reproducible (100% of results +/- 1.0 log2 dilution) and quality control results conformed to published MIC ranges. The Etest seems to render accurate results for these seven antimicrobial agents evaluated.