In Vitro Synergy Testing of Gentamicin, an Old Drug Suggested as Future Treatment Option for Gonorrhoea, in Combination With Six Other Antimicrobials Against Multidrug-Resistant Neisseria gonorrhoeae Strains

Emerging threat of antimicrobial resistance in Neisseria gonorrhoeae: pathogenesis, treatment challenges, and potential for vaccine development

The continuous rise of antimicrobial resistance (AMR) is a serious concern as it endangers the effectiveness of healthcare interventions that rely on antibiotics in the long run. The increasing resistance of Neisseria gonorrhoeae, the bacteria responsible for causing gonorrhea, to commonly used antimicrobial drugs, is a major concern. This has now become a critical global health crisis. In the coming years, there is a risk of a hidden epidemic caused by the emergence of gonococcal AMR. This will worsen the global situation. Infections caused by N. gonorrhoeae were once considered easily treatable. However, over time, they have become increasingly resistant to commonly used therapeutic medications, such as penicillin, ciprofloxacin, and azithromycin. As a result, this pathogen is developing into a true "superbug," which means that ceftriaxone is now the only available option for initial empirical treatment. Effective management strategies are urgently needed to prevent severe consequences, such as infertility and pelvic inflammatory disease, which can result from delayed intervention. This review provides a thorough analysis of the escalating problem of N. gonorrhoeae, including its pathogenesis, current treatment options, the emergence of drug-resistant mechanisms, and the potential for vaccine development. We aim to provide valuable insights for healthcare practitioners, policymakers, and researchers in their efforts to combat N. gonorrhoeae antibiotic resistance by elucidating the multifaceted aspects of this global challenge.

In vitro evaluation of antimicrobial resistance selection in Neisseria gonorrhoeae

Gonococcal infections represent an urgent public-health threat as >50% of cases caused by Neisseria gonorrhoeae strains display reduced susceptibility to at least one antimicrobial agent. We evaluated the pharmacodynamics of a number of antimicrobials against N. gonorrhoeae in order to assess the likelihood of mutant selection by these agents. The mutant prevention concentration (MPC) and mutant selection window (MSW) were determined for azithromycin, ceftriaxone, doxycycline, ertapenem, gentamicin, ciprofloxacin, levofloxacin and moxifloxacin against a wild-type strain of N. gonorrhoeae (ATCC 49226) and a gyrA mutant of ATCC 49226. Pharmacokinetic parameters, including peak concentration (C_max), half-life (t_1/2) and area under the plasma concentration-time curve over 24 h (AUC), associated with each agent were used to calculate the time within the MSW (T_MSW, percentage of the dosing interval that antimicrobial concentrations fall within the MSW), C_max/MPC ratio and AUC/MPC ratio for each antimicrobial agent. Concentrations of ceftriaxone (500 mg), ertapenem, ciprofloxacin, levofloxacin and moxifloxacin surpass the MPC for both strains. Results of pharmacodynamic analyses suggest that ertapenem, ciprofloxacin, levofloxacin and moxifloxacin may be most likely to prevent mutant selection in N. gonorrhoeae. Use of ceftriaxone, azithromycin, doxycycline or gentamicin for gonorrhoea is expected to lead to the ongoing emergence of resistance to these agents. There is a clear need to develop novel treatment regimens for gonococcal infections in order to limit the dissemination of resistance in N. gonorrhoeae.

Comparison of gentamicin MICs by agar dilution and Etest for clinical isolates of Neisseria gonorrhoeae

BACKGROUND

In South Africa, Neisseria gonorrhoeae (NG) is the predominant cause of male urethritis syndrome (MUS). The national MUS treatment guidelines recommend gentamicin as salvage therapy for ceftriaxone treatment failures. We ascertained and compared gentamicin MICs obtained by agar dilution and Etest for clinical isolates of NG.

METHODS

Gentamicin MICs for NG culture isolates obtained from 272 MUS cases in 2017 were determined using agar dilution, as per CLSI agar dilution methods, and Etest® (bioMérieux, Marcy-l'Étoile, France). Previously published interpretive criteria were used: MIC ≤4 mg/L, susceptible (S); MIC 8-16 mg/L, intermediately resistant (IR); and MIC ≥32 mg/L, resistant (R). WHO 2008 NG reference strains were used as comparison standards.

RESULTS

Gentamicin agar dilution versus Etest MIC results (mg/L) were as follows: MIC50 = 16 versus 4; MIC90 = 16 versus 8; minimum MIC = 4 versus 1; and maximum MIC = 32 versus 16. Interpretive categories for agar dilution versus Etest were as follows: S, 4.4% versus 86.8%; IR, 86.0% versus 13.4%; and R, 9.6% versus 0%. The gentamicin MIC50 by agar dilution was significantly higher than by Etest (sign test P value <0.001); overall MIC agreement was 7.4% [kappa statistic (κ) = -0.014 (95% CI -0.039 to 0.010)]. Correlation with expected MICs for WHO reference strains was consistently better with Etest than with agar dilution.

CONCLUSIONS

There was a significant discordance between NG gentamicin MICs by agar dilution versus Etest. NG gentamicin AST methodology must be standardized and interpretive criteria established to optimize the monitoring of susceptibility trends.

Antimicrobial susceptibility of Neisseria gonorrhoeae in Barcelona during a five-year period, 2013 to 2017

IntroductionIncreasing rates of antimicrobial resistance in Neisseria gonorrhoeae cause problems for treating gonorrhoea.AimThis observational study aimed to describe isolates from all patients found infected with N. gonorrhoeae, in Barcelona, Spain, between 2013 and 2017, and with available antimicrobial susceptibility data.MethodsMinimum inhibitory concentrations (MICs) of penicillin (PEN), cefixime (CFM), ceftriaxone (CRO), azithromycin (AZM), ciprofloxacin (CIP), spectinomycin (SPT), fosfomycin (FOF) and gentamicin (GEN) were determined by E-test. Susceptibility was assessed using clinical breakpoints from the European Committee on Antimicrobial Susceptibility Testing. Time trends for PEN, CFM, AZM and CIP were investigated using logistic regression.ResultsOf 1,979 patients with infection (2,036 isolates), 1,888 (95.4%) were men. Patient median age was 32 years. The proportions of isolates resistant to extended-spectrum cephalosporins were low, with 0.3% (5/1,982) resistant to CRO and 4.9% (98/1,985) to CFM. AZM resistance prevalence was 2.7% (52/1,981), including 16 isolates detected in 2016 and 2017, with high-level resistance. For CIP, 51.3% (1,018/1,986) of isolates were resistant, and for PEN, 20.1% (399/1,985). All isolates were susceptible to SPT. MIC50 and MIC90 values of GEN were 4 and 6 mg/L and of FOF 12 and 24 mg/L, respectively. Between 2013 and 2017, PEN and CFM resistance rates each decreased from 28.1% (92/327) to 12.2% (70/572) and from 8.3% (27/327) to 4.4% (25/572) (p ≤ 0.0073). In contrast, AZM resistance prevalence appeared to increase from 1.5% in 2014 (5/340) to 3.0% (17/572) in 2017. No trend was identified for CIP.ConclusionAntimicrobial susceptibility surveillance is important to timely detect new phenotypes and trends.

Evaluation of alternative antibiotics for susceptibility of gonococcal isolates from China

The efficacy of the currently recommended first-line treatments for gonococcal infections - ceftriaxone monotherapy or ceftriaxone/azithromycin dual therapy - is waning rapidly, and efficient alternative antimicrobials are needed urgently to ensure that future treatment of gonorrhoea remains available. As such, the aim of this study was to screen alternative clinically approved antimicrobials for in-vitro activity against Neisseria gonorrhoeae. The susceptibility levels of 504 clinical isolates from Zhejiang Province, China to ertapenem, tigecycline, gentamicin, fosfomycin, gemifloxacin, doxycycline and rifampicin were investigated using the agar dilution method. The presence of resistance determinants was identified by polymerase chain reaction and sequencing. The minimum inhibitory concentration inhibiting 90% of growth (MIC₉₀) was 0.06 mg/L for ertapenem, 0.25 mg/L for tigecycline, 16 mg/L for doxycycline, 4 mg/L for gemifloxacin, 16 mg/L for gentamicin, 32 mg/L for fosfomycin and 128 mg/L for rifampicin. All strains appeared to be susceptible to tigecycline (MIC ≤0.5 mg/L), while a poor correlation between tigecycline and tetracycline susceptibility was observed, indicating that tetracycline resistance determinants have little impact on tigecycline susceptibility. For ertapenem, 30 isolates showed an MIC >0.125 mg/L, but the correlation between ertapenem and ceftriaxone susceptibility was low and only two strains showed an MIC >0.125 mg/L for both antibiotics. Therefore, it appeared that most ceftriaxone-resistant isolates were still susceptible to ertapenem. In conclusion, tigecycline and ertapenem showed good activity against N. gonorrhoeae and limited cross-resistance with previously used antibiotics. Therefore, they might be interesting candidates for further evaluation of their suitability as alternative antigonococcal therapies.

In vitro assessment of gentamicin and azithromycin-based combination therapy against Neisseria gonorrhoeae isolates in India

PURPOSE

The public health burden of infections caused by Neisseria gonorrhoeae is magnified due to high rates of resistance to traditional antimicrobials. The aim of this study was to evaluate the in vitro efficacy of an alternative dual therapy comprising gentamicin and azithromycin.

METHODOLOGY

The E-test method was used to determine the minimum inhibitory concentrations (MICs) of gentamicin and azithromycin individually prior to testing in combination using the cross or 90^o angle formation method. A total of 70 clinical isolates of N.gonorrhoeae displaying varying ceftriaxone MICs along with 2 reference strains (WHO K and P) and 1 ceftriaxone-resistant QA isolate were examined. The fractional inhibitory concentration index (FICI) was calculated and the results were interpreted using the following criteria: synergy, FICI ≤0.5; indifference or additive, FICI >0.5 to ≤4.0; and antagonism, FICI >4.0.

RESULTS

A total of 54 (77.1 %) isolates displayed indifference, while 16 (22.9 %) demonstrated synergy. When azithromycin was tested alone, the MICs ranged from 0.016 to 2 µg ml^-1 . However, in combination with gentamicin, the mean MIC value of all isolates decreased from 0.275 µg ml^-1 to 0.090 µg ml^-1 (P=0.05).When gentamicin was tested alone, the MICs ranged from 0.25 to 8 µg ml^-1, with a mean MIC of 4.342 µg ml^-1, whereas in combination with azithromycin it decreased significantly to 2.042 µg ml^-1 (P=0.04).

CONCLUSION

No antagonism was observed in this combination, suggesting that it could be a future treatment option as we prepare for a post-cephalosporin era. However, comprehensive in vivo evaluations are warranted and recommendations should be made based on clinical trials.

Future Prospects for Neisseria gonorrhoeae Treatment

Gonorrhea is a sexually transmitted disease with a high morbidity burden. Incidence of this disease is rising due to the increasing number of antibiotic-resistant strains. Neisseria gonorrhoeae has shown an extraordinary ability to develop resistance to all antimicrobials introduced for its treatment. In fact, it was recently classified as a &ldquo;Priority 2&rdquo; microorganism in the World Health Organization (WHO) Global Priority List of Antibiotic-Resistant Bacteria to Guide Research, Discovery and Development of New Antibiotics. Seeing as there is no gonococcal vaccine, control of the disease relies entirely on prevention, diagnosis, and, especially, antibiotic treatment. Different health organizations worldwide have established treatment guidelines against gonorrhea, mostly consisting of dual therapy with a single oral or intramuscular dose. However, gonococci continue to develop resistances to all antibiotics introduced for treatment. In fact, the first strain of super-resistant N.gonorrhoeae was recently detected in the United Kingdom, which was resistant to ceftriaxone and azithromycin. The increase in the detection of resistant gonococci may lead to a situation where gonorrhea becomes untreatable. Seeing as drug resistance appears to be unstoppable, new treatment options are necessary in order to control the disease. Three approaches are currently being followed for the development of new therapies against drug-resistant gonococci: (1) novel combinations of already existing antibiotics; (2) development of new antibiotics; and (3) development of alternative therapies which might slow down the appearance of resistances. N. gonorrhoeae is a public health threat due to the increasing number of antibiotic-resistant strains. Current treatment guidelines are already being challenged by this superbug. This has led the scientific community to develop new antibiotics and alternative therapies in order to control this disease.

In vitro efficacy of 21 dual antimicrobial combinations comprising novel and currently recommended combinations for treatment of drug resistant gonorrhoea in future era

BACKGROUND

Recent WHO guidelines recommend dual therapy with ceftriaxone or cefixime plus azithromycin for gonorrhea. Azithromycin in combination with gentamicin or spectinomycin has been recommended in treatment failure cases. Due to emergence of multi-drug resistant (MDR) and extensively-drug resistant (XDR) Neisseria gonorrhoeae strains, it is important to look for efficacy of these combinations and also of others that might be used in future. Therefore, we aimed to evaluate in vitro synergy of 21 dual combinations including current and alternative WHO recommended treatment regimens and other dual combinations.

METHODS AND FINDINGS

The potential utility of in-vitro interactions of 21 combinations was investigated against 95 N. gonorrhoeae strains including 79 MDR and one XDR strain collected during March 2013 to July 2017 and fractional inhibitory concentration index (FICI) was calculated. These 21 combinations comprised of two WHO currently recommended (cefixime+azithromycin, ceftriaxone+azithromycin); two WHO recommended in treatment failure cases (azithromycin+gentamicin, spectinomycin+azithromycin) and other 17 combinations.

RESULTS

FICI of the four WHO recommended antimicrobial combinations were higher (>1.0) than the five novel combinationbreeds (FICI range 0.603-0.951) in the study i.e. gentamicin+ertapenem, moxifloxacin+ertapenem, spectinomycin+ertapenem, azithromycin+ moxifloxacin, cefixime+gentamicin. No antagonistic effect of the above four WHO recommended combinations except spectinomycin+azithromycin (FICI = 4.25) was observed for the XDR strain. Out of above five novel combinations, four combinations produced high synergistic effects in overall 95 strains and also for the XDR strain with FICI of 0.13 to 0.38. Antagonistic effects varying from 3.2 to 12.6% were observed for 10 out of 21 tested combinations (azithromycin in combination with gentamicin and spectinomycin; ceftriaxone with moxifloxacin, gentamicin, spectinomycin and ertapenem; spectinomycin with moxifloxacin and gentamicin; cefixime and gentamicin combination with moxifloxacin).

CONCLUSION

WHO recommended cefixime+azithromycin, ceftriaxone+azithromycin combinations having no antagonism indicates their continuing clinical utility. Highest antagonism without any synergistic effect for the WHO recommended spectinomycin+azithromycin in treatment failure cases suggests that this combination should be evaluated further both in vitro and in vivo. Highest synergistic or additive effect without any antagonistic effect of the above five novel combinations suggests that these may be recommended for treatment in future.