Comparison of E test with agar dilution for antimicrobial susceptibility testing of Neisseria gonorrhoeae

A guide to therapeutic drug monitoring of β-lactam antibiotics

Therapeutic drug monitoring (TDM) opens the door to personalized medicine, yet it is infrequently applied to β-lactam antibiotics, one of the most commonly prescribed drug classes in the hospital setting. As we continue to understand more about β-lactam pharmacodynamics (PD) and wide inter- and intra-patient variability in pharmacokinetics (PK), the utility of TDM has become increasingly apparent. For β-lactams, the time that free concentrations remain above the minimum inhibitory concentration (MIC) as a function of the dosing interval (%fT>MIC) has been shown to best predict antibacterial effect. Many studies have shown that β-lactam %fT>MIC exposures are often suboptimal across a wide variety of disease states and clinical settings. A limitation to implementing this practice is the general lack of understanding on how to best operationalize this intervention and interpret the results. The instrumentation and expertise needed to quantify β-lactams for TDM is rarely available locally, but certain laboratories advertise these services and perform them regularly. Familiarity with the modalities and nuances of antimicrobial susceptibility testing is crucial to establishing β-lactam concentration targets that meet the relevant exposure thresholds. Evaluation of these concentrations is best accomplished using population PK software and Bayesian modeling, for which a multitude of programs are available. While TDM of β-lactams has shown an ability to increase the rate of target attainment, there is currently limited evidence to suggest that it leads to improved clinical outcomes. Although consensus guidelines for β-lactam TDM do not exist in the United States, guidance would help to promote this important practice and better standardize the approach across institutions. Herein, we discuss the basis for β-lactam TDM, review supporting evidence, and provide guidance for implementation in specific patient populations.

Current and Emerging Methods of Antibiotic Susceptibility Testing

Antibiotic susceptibility testing (AST) specifies effective antibiotic dosage and formulates a profile of empirical therapy for the proper management of an individual patient's health against deadly infections. Therefore, rapid diagnostic plays a pivotal role in the treatment of bacterial infection. In this article, the authors review the socio-economic burden and emergence of antibiotic resistance. An overview of the phenotypic, genotypic, and emerging techniques for AST has been provided and discussed, highlighting the advantages and limitations of each. The historical perspective on conventional methods that have paved the way for modern AST like disk diffusion, Epsilometer test (Etest), and microdilution, is presented. Several emerging methods, such as microfluidic-based optical and electrochemical AST have been critically evaluated. Finally, the challenges related with AST and its outlook in the future are presented.

First description of a cefixime- and ciprofloxacin-resistant Neisseria gonorrhoeae isolate with mutations in key antimicrobial susceptibility-determining genes from the country of Georgia

Antimicrobial resistance in Neisseria gonorrhoeae is a global health problem. Enhanced international collaborative surveillance and disease control are needed to reduce the global burden of this important pathogen. Currently the antimicrobial resistance properties and molecular mechanisms of multidrug-resistant N. gonorrhoeae in the Republic of Georgia represent a significant knowledge gap. Here we report the isolation of a strain of N. gonorrhoeae exhibiting resistance to cefixime and ciprofloxacin with reduced susceptibility to penicillin and tetracycline from a patient being treated at a Georgian medical centre. Notably, this isolate was found to contain a mosaic penA allele and to harbour mutations in genes conferring susceptibility to the β-lactam, cephalosporin, fluoroquinolone, macrolide and penicillin classes of antibiotic. To our knowledge, this is the first report to describe the key mutations conferring the antimicrobial resistance properties of an isolate of N. gonorrhoeae from Georgia.

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.

Comparison of two disc diffusion methods with minimum inhibitory concentration for antimicrobial susceptibility testing of Neisseria gonorrhoeae isolates

Background:

A few studies are available comparing either minimum inhibitory concentration (MIC) values with the Clinical and Laboratory Standards Institute (CLSI) disc diffusion method or MIC with the Australian Gonococcal Surveillance Program (AGSP) method.

Aim:

This study was conducted with the aim to identify the most feasible and cost-effective method for antimicrobial susceptibility testing of Neisseria gonorrhoeae.

Materials and Methods:

Antimicrobial susceptibility testing of N. gonorrhoeae was performed using, in parallel, the E-test for MIC determination and disc diffusion by CLSI and AGSP techniques, and were compared. Susceptibility to penicillin, ciprofloxacin, tetracycline, ceftriaxone and spectinomycin and cefixime were determined by CSLI and AGSP method and Kappa statistics used to analyse the data with SPSS software.

Results:

All isolates were susceptible to ceftriaxone and spectinomycin by three methods. Ninety-nine (99%) strains were resistant to ciprofloxacin, while 1% showed intermediate susceptibility to ciprofloxacin by all methods. Statistically, there was a moderate level of agreement between the methods for penicillin.

Conclusion:

All three methods gave reproducible results. Although the media used in the disc diffusion by the AGSP method is easy and cheap to prepare, the CLSI method of disc diffusion testing is recommended for susceptibility testing of gonococcal isolates because of its feasibility and 100% accuracy, with MIC by E-test as the reference method.

Conjunctivitis caused by Neisseria gonorrhoeae isolates with reduced cephalosporin susceptibility and multidrug resistance

We report two cases of conjunctivitis caused by Neisseria gonorrhoeae with reduced cephalosporin susceptibility. Patients showed no response to cefmenoxime eye drops and intravenous ceftriaxone administration. The patients' condition improved after the addition of oral minocycline. The isolates contained the mosaic penA for reduction of β-lactam susceptibility.

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.

Neisseria gonorrhoeae antimicrobial susceptibility in Lilongwe, Malawi, 2007

BACKGROUND

Malawi adopted syndromic management of sexually transmitted infections in 1993. Based on clinical efficacy and cost, gentamicin 240 mg intramuscularly, and doxycycline 100 mg twice daily x 7 days was selected as the first line regimen to treat urethritis. We sought to establish current laboratory-based Neisseria gonorrhoeae antibiotic susceptibility patterns for Malawi and describe the pattern of susceptibility since syndromic management began.

METHODS

Between May 15 and August 10, 2007, 126 men with urethritis attending the STD clinic at Kamuzu Central Hospital in Lilongwe had history, genital exam, and urethral swabs taken. All were treated with gentamicin and doxycycline in accordance with Malawi guidelines. Gonorrhea was diagnosed by Gram stain and culture. Antimicrobial susceptibility patterns in gonococcal isolates were determined by disk diffusion and E-test minimum inhibitory concentration (MIC) determination and agar dilution MIC determination.

RESULTS

One hundred six isolates were cultured, and MICs were determined for 100. High levels of resistance to tetracycline and penicillin were observed, but isolates were uniformly susceptible to both gentamicin and ciprofloxacin. Susceptibility patterns identified by the agar dilution MIC and E-test MIC agreed.

CONCLUSIONS

The most recent study continues the trend of high susceptibility of gonococcal isolates to gentamicin in Malawi after 14 years of use and suggests agar dilution MICs may be substituted with the simpler E-test methods in future susceptibility testing. However because of the lack of susceptibility criteria for aminoglycosides for N. gonorrhoeae and the difficulty obtaining clinical/in vitro correlates in this setting, caution should be exercised in using these data for modifying treatment regimens.

Rapid detection of a chromosomally mediated penicillin resistance-associated ponA mutation in Neisseria gonorrhoeae using a real-time PCR assay

The recent emergence of a decreased susceptibility of Neisseria gonorrhoeae strains to penicillin in New Caledonia has lead clinicians to operate a change in the treatment strategy. In addition, this important health issue has emphasized the need for a rapid means of detecting penicillin resistance in N. gonorrhoeae in order to select an effective treatment and limit the spread of resistant strains. In recent years, the use of fluorescence resonance energy transfer on the LightCycler has proven to be a valuable tool for the screening of mutations occurring in the genome of various microorganisms. In this study, we developed a real-time PCR assay coupled with a fluorometric hybridization probes system to detect a penicillin resistance-associated mutation on the N. gonorrhoeae ponA gene. Following an extensive evaluation involving 136 isolates, melting curve analysis correctly evidenced a 5 degrees C T(m) shift in all N. gonorrhoeae strains possessing this mutation, as determined by conventional sequencing analysis. Moreover, the mutation profiles obtained with the real-time PCR showed good correlation with the pattern of penicillin susceptibility generated with classical antibiograms. Overall, our molecular assay allowed an accurate and reproducible determination of the susceptibility to penicillin corresponding to a mutation present in all chromosomally mediated resistant strains of N. gonorrhoeae.

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.

A randomized trial of ciprofloxacin versus cefixime for treatment of gonorrhea after rapid emergence of gonococcal ciprofloxacin resistance in The Philippines

From 1994 through 1996-1997, high-level ciprofloxacin resistance (minimum inhibitory concentration [MIC], > or = 4.0 microg/mL) increased from 9% to 49% of gonococcal isolates recovered from consecutive female sex workers in Cebu and Manila, The Philippines (P < .01). During 1996-1997, 105 female sex workers with gonorrhea were prospectively randomized to receive treatment with oral ciprofloxacin, 500 mg, or cefixime, 400 mg, and followed for test of cure. Neisseria gonorrhoeae was reisolated within 28 days after treatment from 1 (3.8%) of 26 women given cefixime versus 24 (32.3%) of 72 women given ciprofloxacin (P < .01). Treatment failure (reisolation of pretreatment auxotype/serovar) occurred in 14 (46.7%) of 30 women infected with strains with MICs of ciprofloxacin > or = 4.0 microg/mL versus 1 (3.6%) of 28 infected by strains with MICs < 4.0 microg/mL (P < .01). High-level, clinically significant gonococcal resistance to ciprofloxacin has rapidly emerged in The Philippines, and spread of fluoroquinolone resistance through commercial sex poses a threat to control of gonorrhea and prevention of human immunodeficiency virus infection and the acquired immunodeficiency syndrome.

Development of gemifloxacin in vitro susceptibility test methods for gonococci including quality control guidelines. The Quality Control Study Group

Gemifloxacin (formerly SB-265805 or LB20304a) is a new fluoronapthyridone with documented activity against Gram-positive and -negative organisms. The activity of gemifloxacin was tested against 150 Neisseria gonorrhoeae strains, using reference agar dilution, standardized disk diffusion, and Etest (AB BIODISK, Solna, Sweden) methods. Gemifloxacin was very potent against ciprofloxacin (CIPRO)-susceptible strains (MIC(90,) 0.008 microg/ml) but was significantly less active against the CIPRO-resistant gonococci (MIC(90,) 0.12 microg/ml). Etest and reference agar dilution MIC results showed excellent correlation (r = 0.96), and 98.7% MICs were within +/- one log(2) dilution. Agar dilution MICs were also compared to zone diameters obtained using gemifloxacin 5-microg disks; and complete intermethod categorical agreement (100%) was achieved applying breakpoints proposed as follows: < or =0.25 microg/ml (zone, > or =25 mm) for susceptible and > or =1 microg/ml (zone, < or =21 mm) for resistant. Gemifloxacin MIC and disk diffusion te quality control (QC) ranges were established for N. gonorrhoeae ATCC 49226. Data were collected from > or = seven laboratories, three GC agar medium lots for both agar MICs and disk methods, and two lots each of the 5- and 10-microg disks. The proposed MIC QC range was 0.002 to 0.016 microg/ml and the calculated mm zone ranges (median +/- 0.5x average mm range) for both disks were similar, but contained only 88.1 to 91.9% of participant results. To achieve the acceptable > or = 95% of all study results within range, a 43 to 54 mm limits (5-microg disks) were necessary. The excellent broad-spectrum activity and a low reported adverse effects profile of gemifloxacin shows a potential for treatment of fluoroquinolone-resistant gonorrhea.

A randomized, community trial of intensive sexually transmitted disease control for AIDS prevention, Rakai, Uganda

OBJECTIVE

To describe the design and first-round survey results of a trial of intensive sexually transmitted disease (STD) control to reduce HIV-1 incidence.

STUDY DESIGN

Randomized, controlled, community-based trial in Rakai District, Uganda.

METHODS

In this ongoing study, 56 communities were grouped into 10 clusters designed to encompass social/sexual networks; clusters within blocks were randomly assigned to the intervention or control arm. Every 10 months, all consenting resident adults aged 15-59 years are visited in the home for interview and sample collection (serological sample, urine, and, in the case of women, self-administered vaginal swabs). Sera are tested for HIV-1, syphilis, gonorrhea, chlamydia, trichomonas and bacterial vaginosis. Following interview, all consenting adults are offered directly observed, single oral dose treatment (STD treatment in the intervention arm, anthelminthic and iron-folate in the control arm). Treatment is administered irrespective of symptoms or laboratory testing (mass treatment strategy). Both arms receive identical health education, condom and serological counseling services.

RESULTS

In the first home visit round, the study enrolled 5834 intervention and 5784 control arm subjects. Compliance with interview, sample collection and treatment was high in both arms (over 90%). Study arm populations were comparable with respect to sociodemographic and behavioral characteristics, and baseline HIV and STD rates. The latter were high: 16.9% of all subjects were HIV-positive, 10.0% had syphilis, and 23.8% of women had trichomonas and 50.9% had bacterial vaginosis.

CONCLUSIONS

Testing the effects of STD control on AIDS prevention is feasible in this Ugandan setting.

Antimicrobial resistance in Neisseria gonorrhoeae in Finland, 1976 to 1995. The Finnish Study Group For Antimicrobial Resistance

BACKGROUND AND OBJECTIVES

The worldwide increase in antimicrobial resistance in Neisseria gonorrhoeae prompted the authors to evaluate the status and course of resistance in gonococci in Finland.

GOALS

The minimal inhibitory concentrations (MIC) of penicillin, tetracycline, spectinomycin, ciprofloxacin, ceftriaxone, and cefixime were tested for 337 consecutive clinical N. gonorrhoeae isolates collected in 19 Finnish microbiology laboratories in 1993.

STUDY DESIGN

The results were compared with data obtained in three Finnish laboratories in 1986 and contrasted with the development of the incidence of gonorrhea and the prevalence of penicillinase-producing N. gonorrhoeae (PPNG) in Finland, 1976 to 1995. The number of strains with an elevated MIC to ciprofloxacin was assessed by questionnaire.

RESULTS

A decrease, from more than 50% in 1986 to 20% in 1993, of strains susceptible to penicillin and tetracycline was observed. The prevalence of PPNG increased from 0% (1976) to 5.7% (1995). In 1995, two strains with a ciprofloxacin MIC of > or = 32 micrograms/ml were reported. No resistance to ceftriaxone or spectinomycin was detected.

CONCLUSIONS

In spite of the rarity of gonorrhea and the availability of efficient antimicrobials in Finland, monitoring of the antimicrobial resistance of N. gonorrhoeae remains important.

Development of an antimicrobial susceptibility surveillance system for Neisseria gonorrhoeae in Malawi: comparison of methods

Susceptibility of Neisseria gonorrhoeae to gentamicin, the primary treatment for gonorrhea in Malawi since 1993, was determined by using agar dilution MICs, E-test MICs, disc diffusion, and clinical cure rate. Agar dilution MICs were slightly higher in 1996 than in 1993 isolates, with a concomitant drop in the clinical cure rate. E-test MICs were substantially lower than agar dilution determinations, with only 77.4% within 1 log2 concentration.

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.

Comparison of E-test with agar dilution methods in testing susceptibility of N. gonorrhoeae to azithromycin

BACKGROUND AND OBJECTIVES

The antimicrobial susceptibility pattern of Neisseria gonorrhoeae varies from one country to another and may also change with time. To monitor these variations and changes, it is desirable to have a method that is simple and reproducible. This study was undertaken to determine the in vitro susceptibility of N. gonorrhoeae to azithromycin and to assess the reliability of results obtained using E-test methodology for determination of the minimum inhibitory concentration (MIC) of azithromycin.

STUDY DESIGN

The MICs for 135 clinical isolates of N. gonorrhoeae were determined by a modified Kirby-Bauer method recommended by the National Committee for Clinical Laboratory Standards against penicillin, cefuroxime, ceftriaxone, norfloxacin, tetracycline, kanamycin, spectinomycin, and azithromycin. The MIC of azithromycin was determined by both the E-test and agar dilution method. All tests were done simultaneously.

RESULTS

The MIC of azithromycin to all 135 isolates ranged from 0.078 to 0.25 microgram/ml with the agar dilution method and from 0.016 to 0.50 microgram/ml with the E-test. The MIC50 and MIC90 of azithromycin were 0.064 microgram/ml and 0.125 microgram/ml, respectively, by the agar dilution method, whereas they are slightly higher by the E-test method. Seventy-six of the isolates were beta-lactamase producers and 69 were high-level tetracycline-resistant N. gonorrhoeae. There was no difference in the MIC50 and MIC90 of azithromycin in these groups of isolates. The percentage agreement within the acceptable +/-1 log2 dilution difference between MICs obtained by E-test and those obtained by the agar dilution method was 97.8%.

CONCLUSIONS

Azithromycin has a very good in vitro antigonococcal activity, and the E-test is a reliable method to determine the MIC of azithromycin against N. gonorrhoeae.

Comparison of Etest and agar dilution for testing the activity of three macrolides against Bordetella parapertussis

Agar dilution and Etest using Mueller-Hinton II agar supplemented with 5% whole defibrinated horse blood were compared for testing the activities of azithromycin, clarithromycin, and erythromycin against 34 clinical isolates of the fastidious species Bordetella parapertussis. There was good overall agreement (100.0% within +/- 1 log2 dilution step) between both methods. The Etest offers an excellent and convenient alternative to agar dilution for testing the activities of macrolides against B. parapertussis.

Antimicrobial resistance in gonococci isolated from patients and from commercial sex workers in Harare, Zimbabwe

The objective is to compare antibiotic resistance amongst gonococci isolated from different patient groups in Harare, Zimbabwe. Antimicrobial susceptibilities of Neisseria. gonorrhoeae were determined by disc sensitivity tests. The MICs for penicillin, kanamycin, ceftriaxone, norfloxacin and ciprofloxacin were determined using E-test strips. There were 147 isolates from symptomatic men, 47 isolates from symptomatic women, 29 isolates from asymptomatic women and 41 isolates from female commercial sex workers. A total of 119 (45%) isolates were PPNG and 23 (16%) non-PPNG isolates had a penicillin MIC > 0.64 mg/l. Over 90% of isolates were resistant to TMP/SMX and 16% were resistant to tetracycline. Resistance was uncommon against kanamycin (6%), erythromycin (2%) or ceftriaxone ( < 1%). For kanamycin, the MIC90 was 32 mg/l, for ceftriaxone the MIC90 was < 0.032 mg/l for non-PPNG and < 0.064 mg/l for PPNG. For norfloxacin and ciprofloxacin the MIC90 was < 0.064 mg/l for both PPNG and non-PPNG. Isolates from the commercial sex workers showed a significantly increased prevalence of PPNG, of penicillin-tolerant non-PPNG and of tetracycline resistance. Four of the 41 isolates from sex workers showed multiple resistance (to penicillin, TMP/SMX, tetracycline and kanamycin) compared to 1/223 isolates from other groups (OR = 24.0). Antimicrobial resistance is common amongst gonococci in Harare, especially with isolates from commercial sex workers. In order for STD treatment to be implemented as an effective strategy in HIV control, continued monitoring of resistance patterns is essential.

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

We evaluated the ability of the Etest (AB Biodisk, Solna, Sweden) method to accurately and reproducibly determine the antimicrobial susceptibility of Neisseria gonorrhoeae. One hundred gonococcal isolates were used to evaluate the diagnostic performance of the Etest compared with the reference agar dilution method for penicillin, tetracycline, ciprofloxacin, and ceftriaxone. Between 92 and 99% of Etest MIC results for all drugs were within +/- 1 log2 dilution of the reference MIC. According to recommended interpretive criteria, ceftriaxone, cefotaxime, and ciprofloxacin had 100% categorical agreement, while penicillin (86%) and tetracycline (85%) categorical agreement percentages were lower because of the large number of strains that were within 0.5 to 1 log2 dilution of the susceptible or resistant breakpoints. Reproducibility data also demonstrated that the Etest was precise (99.1%) when subjected to replicate testing. On the basis of these data, the Etest method provides an effective, simple alternative to the reference agar dilution method for the direct quantification of N. gonorrhoeae susceptibility.

Antimicrobial agents and gonorrhoea: therapeutic choice, resistance and susceptibility testing

INTRODUCTION

Neisseria gonorrhoeae, the causative agent of gonorrhoea is a particularly well adapted pathogen that has continued to evolve mechanisms to evade treatment with antimicrobial agents.

THERAPEUTIC CHOICE

The choice of antibiotic for use in the first-line treatment of gonorrhoea should be made with knowledge of the susceptibility of the isolates of N gonorrhoeae to be encountered.

RESISTANCE

High-level resistance to penicillin and tetracycline in N gonorrhoeae is plasmid-mediated and a major therapeutic problem. Penicillinase-producing N gonorrhoeae, first described in 1976, have now spread worldwide and tetracycline-resistant N gonorrhoeae, described in 1985, are becoming increasingly prevalent. Chromosomal resistance to penicillin is low-level and affects a range of antibiotics. High-level resistance to spectinomycin has been sporadic and has not limited its use whereas the emergence of resistance to ciprofloxacin will have a significant impact on its use for gonorrhoea.

SUSCEPTIBILITY TESTING

A variety of methods are available including disc diffusion, breakpoint agar dilution technique, E-test and determination of the minimum inhibitory concentration (MIC). The choice of methodology will depend on the number and type of isolates and the facilities available for testing.

DISCUSSION

Surveillance programmes to monitor levels of antibiotic resistant isolates are essential to ensure therapeutic success.

Evaluation of in vitro spectra of activity of azithromycin, clarithromycin, and erythromycin tested against strains of Neisseria gonorrhoeae by reference agar dilution, disk diffusion, and Etest methods

The macrolide-azilide susceptibility testing (agar dilution, disk diffusion, Etest) criteria for 105 Neisseria gonorrhoeae strains were evaluated. In addition, the potencies of azithromycin, clarithromycin, and erythromycin were studied. The most active macrolide-azilide agent was azithromycin (MIC at which 90% of the isolates are inhibited [MIC90], 0.5 microgram/ml) compared with clarithromycin (MIC90, 1.5 to 2 micrograms/ml) and erythromycin (MIC90, 2 to 4 micrograms/ml). The Etest (AB Biodisk, Solna, Sweden) was observed to produce MIC results very similar to those of the reference agar dilution test (GC agar base), with 100% of the results within 1 log2 dilution step of the reference MICs. The disk diffusion test zone diameters for all three drugs correlated at an acceptable level (r = -0.81 to -0.92) with the reference agar dilution MICs. Interpretive criteria for susceptibility were proposed for azithromycin at a MIC of < or = 2 micrograms/ml and a disk diffusion test zone of > or = 25 mm. No category for resistance was proposed because of the paucity of strains for which MICs were > 2 micrograms/ml. These tentative criteria should be further validated by correlations with clinical trial data for gonococcal strains (as they emerge) that have azithromycin MICs above the proposed susceptible category range.

Evaluation of E-Test for determination of antimicrobial MICs for Pseudomonas aeruginosa isolates from cystic fibrosis patients

We determined the E-Test and National Committee for Clinical Laboratory Standards standardized agar dilution MICs of ceftazidime, ciprofloxacin, piperacillin, and tobramycin for Pseudomonas aeruginosa during tests of 100 rough and mucoid P. aeruginosa isolates from cystic fibrosis patients. The levels of agreement (+/- 1 log2 dilution) between quantitative E-Test and agar dilution MIC results were 80, 97, 73, and 89% for ceftazidime, ciprofloxacin, piperacillin, and tobramycin, respectively. Comparison of the results after converting the MIC data to qualitative categories (susceptible, intermediate, and resistant) yielded levels of agreement of 84, 96, 88, and 93% for the same agents, respectively. Of the 39 qualitative discrepancies, 36 were minor and 3 were very major. We conclude that use of the E-Test is easier and more practical than use of the agar dilution method for most laboratories and that the E-Test furnishes results which are at least as accurate as those obtained by the agar dilution method. However, the higher cost of the E-Test method would likely discourage most laboratories from selecting it over disk diffusion for routine antimicrobial susceptibility testing of P. aeruginosa isolates from cystic fibrosis patients.