Antimicrobial resistance in healthcare, agriculture and the environment: the biochemistry behind the headlines

The crisis of antimicrobial resistance (AMR) is one of the most serious issues facing us today. The scale of the problem is illustrated by the recent commitment of Heads of State at the UN to coordinate efforts to curb the spread of AMR infections. In this review, we explore the biochemistry behind the headlines of a few stories that were recently published in the public media. We focus on examples from three different issues related to AMR: (i) hospital-acquired infections, (ii) the spread of resistance through animals and/or the environment and (iii) the role of antimicrobial soaps and other products containing disinfectants in the dissemination of AMR. Although these stories stem from three very different settings, the underlying message in all of them is the same: there is a direct relationship between the use of antimicrobials and the development of resistance. In addition, one type of antimicrobial could select for cross-resistance to another type and/or for multidrug resistance. Therefore, we argue the case for increased stewardship to not only cover clinical use of antibiotics, but also the use of antimicrobials in agriculture and stewardship of our crucially important biocides such as chlorhexidine.

Frequent Cross-Resistance to Dapivirine in HIV-1 Subtype C-Infected Individuals after First-Line Antiretroviral Therapy Failure in South Africa

A vaginal ring containing dapivirine (DPV) has shown moderate protective efficacy against HIV-1 acquisition, but the activity of DPV against efavirenz (EFV)- and nevirapine (NVP)-resistant viruses that could be transmitted is not well defined. We investigated DPV cross-resistance of subtype C HIV-1 from individuals on failing NVP- or EFV-containing antiretroviral therapy (ART) in South Africa. Plasma samples were obtained from individuals with >10,000 copies of HIV RNA/ml and with HIV-1 containing at least one non-nucleoside reverse transcriptase (NNRTI) mutation. Susceptibility to NVP, EFV, and DPV in TZM-bl cells was determined for recombinant HIV-1_LAI containing bulk-amplified, plasma-derived, full-length reverse transcriptase sequences. Fold change (FC) values were calculated compared with a composite 50% inhibitory concentration (IC₅₀) from 12 recombinant subtype C HIV-1_LAI plasma-derived viruses from treatment-naive individuals in South Africa. A total of 25/100 (25%) samples showed >500-FCs to DPV compared to treatment-naive samples with IC₅₀s exceeding the maximum DPV concentration tested (132 ng/ml). A total of 66/100 (66%) samples displayed 3- to 306-FCs, with a median IC₅₀ of 17.6 ng/ml. Only 9/100 (9%) samples were susceptible to DPV (FC < 3). Mutations L100I and K103N were significantly more frequent in samples with >500-fold resistance to DPV compared to samples with a ≤500-fold resistance. A total of 91% of samples with NNRTI-resistant HIV-1 from individuals on failing first-line ART in South Africa exhibited ≥3-fold cross-resistance to DPV. This level of resistance exceeds expected plasma concentrations, but very high genital tract DPV concentrations from DPV ring use could block viral replication. It is critically important to assess the frequency of transmitted and selected DPV resistance in individuals using the DPV ring.

Field-Evolved Resistance and Cross-Resistance of Brazilian Tuta absoluta (Lepidoptera: Gelechiidae) Populations to Diamide Insecticides

Tuta absoluta (Meyrick), one of the most important tomato pests worldwide, is heavily controlled by the application of insecticides. Diamide insecticides represent a new class of products recently registered to control T. absoluta After 6 yr of use, control failures have been reported in populations of this pest, suggesting a hypothetical resistance development. Therefore, a resistance survey was performed using nine populations of T. absoluta that were collected in open fields, including from areas with reports of a reduced efficacy of diamides in the Northeast and Central regions of Brazil. Initial surveys with diagnostic and label doses proved the reduced efficacy of diamides against most populations. The LC₅₀ values of chlorantraniliprole varied from 0.0044 (Brasília) to 1,263 (América Dourada) mg AI liter^-1 (the resistance ratios [RR₅₀] ranged from 1.0- to 288,995-fold), whereas the LC₅₀ values for cyantraniliprole and flubendiamide, respectively, varied from 0.015 (Brasília) to 281 (América Dourada) mg AI liter^-1 and from 0.038 (Guaraciaba do Norte) to 3,018 (Gameleira 1) mg AI liter^-1 The resistance ratios (RR₅₀) ranged from 1.0- to 18,423-fold for cyantraniliprole and from 1.0- to 80,413-fold for flubendiamide. The log LC₅₀ values of pairwise diamides were strongly and significantly correlated, which denoted cross-resistance among them. Very high resistance to diamides in T. absoluta was observed in this study, suggesting that strategies to mitigate resistance and thereby control the pest must not include only insecticides. Other control tactics must be carefully implemented over time to increase the life span of diamides, including rotational practices with other molecules.

The P450 CYP6Z1 confers carbamate/pyrethroid cross-resistance in a major African malaria vector beside a novel carbamate-insensitive N485I acetylcholinesterase-1 mutation

Carbamates are increasingly used for vector control notably in areas with pyrethroid resistance. However, a cross‐resistance between these insecticides in major malaria vectors such as Anopheles funestus could severely limit available resistance management options. Unfortunately, the molecular basis of such cross‐resistance remains uncharacterized in An. funestus, preventing effective resistance management. Here, using a genomewide transcription profiling, we revealed that metabolic resistance through upregulation of cytochrome P450 genes is driving carbamate resistance. The P450s CYP6P9a,CYP6P9b and CYP6Z1 were the most upregulated detoxification genes in the multiple resistant mosquitoes. However, in silico docking simulations predicted CYP6Z1 to metabolize both pyrethroids and carbamates, whereas CYP6P9a and CYP6P9b were predicted to metabolize only the pyrethroids. Using recombinant enzyme metabolism and inhibition assays, we demonstrated that CYP6Z1 metabolizes bendiocarb and pyrethroids, whereas CYP6P9a and CYP6P9b metabolize only the pyrethroids. Other upregulated gene families in resistant mosquitoes included several cuticular protein genes suggesting a possible reduced penetration resistance mechanism. Investigation of the target‐site resistance in acetylcholinesterase 1 (ace‐1) gene detected and established the association between the new N485I mutation and bendiocarb resistance (odds ratio 7.3; P < 0.0001). The detection of multiple haplotypes in single mosquitoes after cloning suggested the duplication of ace‐1. A TaqMan genotyping of the N485I in nine countries revealed that the mutation is located only in southern Africa with frequency of 10–15% suggesting its recent occurrence. These findings will help in monitoring the spread and evolution of carbamate resistance and improve the design of effective resistance management strategies to control this malaria vector.

Association between clinical antibiotic resistance and susceptibility of Pseudomonas in the cystic fibrosis lung

Background and objectives: Cystic fibrosis patients suffer from chronic lung infections that require long-term antibiotic therapy. Pseudomonas readily evolve resistance, rendering antibiotics ineffective. In vitro experiments suggest that resistant bacteria may be treated by exploiting their collateral sensitivity to other antibiotics. Here, we investigate correlations of sensitivity and resistance profiles of Pseudomonas aeruginosa that naturally adapted to antibiotics in the cystic fibrosis lung.

Methodology: Resistance profiles for 13 antibiotics were obtained using broth dilution, E-test and VITEK mass spectroscopy. Genetic variants were determined from whole-genome sequences and interrelationships among isolates were analyzed using 13 MLST loci.

Result: Our study focused on 45 isolates from 13 patients under documented treatment with antibiotics. Forty percent of these were clinically resistant and 15% multi-drug resistant. Colistin resistance was found once, despite continuous colistin treatment and even though colistin resistance can readily evolve experimentally in the laboratory. Patients typically harbored multiple genetically and phenotypically distinct clones. However, genetically similar clones often had dissimilar resistance profiles. Isolates showed mutations in genes encoding cell wall synthesis, alginate production, efflux pumps and antibiotic modifying enzymes. Cross-resistance was commonly observed within antibiotic classes and between aminoglycosides and β-lactam antibiotics. No evidence was found for consistent phenotypic resistance to one antibiotic and sensitivity to another within one genotype.

Conclusions and implications: Evidence supporting potential collateral sensitivity in clinical P. aeruginosa isolates remains equivocal. However, cross-resistance within antibiotic classes is common. Colistin therapy is promising since resistance to it was rare despite its intensive use in the studied patients.

Parallel and cross-resistances of clinical yeast isolates determined by susceptibility pattern analysis

For calculated initial antifungal therapy, knowledge on parallel and cross-resistances are vitally important particularly in the case of multiresistant isolates. Based on a strain collection of 1,062 yeast isolates from a German/Austrian multicentre study, susceptibility pattern analysis (SPA) was used to determine the proportion of parallel and cross-resistances to eight antifungal agents (AFAs) encompassing flucytosine, amphotericin B, azoles (fluconazole, voriconazole and posaconazole) and echinocandins (caspofungin, micafungin and anidulafungin). A total of 414 (39.0%) isolates were resistant for one or more of the AFAs. Resistance to one AFA was shown for 18.1% of all isolates. For 222 isolates (20.9%), resistance to two to seven AFAs was noted (7.7%; 7.7%; 3.6%; 1.0%; 0.7% and 0.2% to 2, 3, 4, 5, 6 and 7 antifungal compounds, respectively). Partial parallel resistances within the azole and echinocandin classes, respectively, were found for 81 (7.6%) and 70 (6.6%) isolates. Complete parallel resistances for azoles, echinocandins and combined for both classes were exhibited by 93 (8.8%), 18 (1.7%) and 6 (0.6%) isolates, respectively. Isolates displaying cross-resistances between azoles and echinocandins were infrequently found. Highly resistant isolates (resistance to ≥6 AFAs) were almost exclusively represented by Candida albicans. Highly standardized testing of AFAs in parallel and from the same inocula followed by SPA allows detailed insights in the prevalence and distribution of susceptibility patterns of microbial isolates.

Progress in the mechanism and drug development of castration-resistant prostate cancer

Although prostate cancer can initially respond to androgen deprivation therapy, it will inevitably relapse and switch to a castration-resistant state. The progress in understanding the mechanism of castration-resistant prostate cancer (CRPC) has led to the evolution of novel agents, including sipuleucel-T as an immunomodulant agent, enzalutamide as an androgen receptor antagonist, docetaxel as a chemotherapeutic agent and radium-223 as a radiopharmaceutical agent. In this review, we discuss the main mechanisms of CRPC and the development of promising agents along with the novel therapies in the clinic. New therapeutic challenges remain, such as the identification of predictive biomarkers and the optimal combinations of agents. Future investigation is still needed for a better understanding of CRPC.

Cross-resistance and baseline susceptibility of Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) to cyantraniliprole in the south of China

BACKGROUND

The oriental leafworm moth, Spodoptera litura Fab. (Lepidoptera: Noctuidae), is a widely distributed polyphagous insect pest in Asia that has been shown to be resistant to various types of insecticide. The newly registered anthranilic diamide cyantraniliprole provided novel insight and great opportunities to control S. litura.

RESULTS

In this study, the susceptibilities of S. litura collected from South China to cyantraniliprole were measured by standard leaf-disc bioassay, and obvious variation in susceptibility was observed among the 17 field populations, with LC50 values varying from 0.206 to 1.336 mg AI L(-1). Significant correlations were detected between the LC50 values of cyantraniliprole and chlorantraniliprole (P < 0.05). However, no significant correlation (P > 0.05) was observed between the two anthranilic diamides and other insecticides with different action mechanisms (delcamethrin, chlorpyrifos, indoxacarb and emamectin benzoate). Piperonyl butoxide showed obvious synergism in Lab-Sus, ZC14 and cyantraniliprole-resistant strains, while diethyl maleate and S,S,S-tributylphorotrithioate had no obvious synergistic effects in any of the strains tested.

CONCLUSION

These results revealed obvious regional variation in cyantraniliprole susceptibilities among populations of S. litura from different areas, and potential cross-resistance to chlorantraniliprole, which suggested that S. litura could develop resistance to cyantraniliprole. Detoxification enzymes might not be involved in the observed tolerance in field-collected populations and the cyantraniliprole-resistant strain.

Field-Based Assessment of Resistance to Bt Corn by Western Corn Rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of corn and is managed with corn that produces insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt). Beginning in 2009, resistance to Cry3Bb1 corn, and severe injury to Cry3Bb1 corn in the field, was observed in Iowa. However, few data exist on how Cry3Bb1-resistant western corn rootworm interact with various management practices in the field. Using a field experiment, we measured adult emergence and feeding injury to corn roots for both Cry3Bb1-resistant and Cry3Bb1-susceptible populations of western corn rootworm when tested against various Bt corn hybrids and a soil-applied insecticide. Between 2012 and 2013, we evaluated five fields that were associated with greater than one node of feeding injury to Cry3Bb1 corn by western corn rootworm (i.e., problem-field populations), and a laboratory strain that had never been exposed to Bt corn (i.e., control population). Adult emergence for western corn rootworm and root injury to corn were significantly higher in problem-field populations than control populations for both Cry3Bb1 corn and mCry3A corn. By contrast, corn with Cry34/35Ab1, either alone or pyramided with Cry3Bb1, significantly reduced adult emergence and root injury in both problem fields and control fields. In problem fields, application of the soil-applied insecticide to Cry3Bb1 corn significantly reduced root injury, but not adult emergence. Our results are discussed in terms of developing strategies for managing western corn rootworm with resistance to Cry3Bb1 and mCry3A, and delaying the additional evolution of Bt resistance by this pest.

Induced Bacterial Cross-Resistance toward Host Antimicrobial Peptides: A Worrying Phenomenon

Bacterial resistance to conventional antibiotics has reached alarming levels, threatening to return to the pre-antibiotic era. Therefore, the search for new antimicrobial compounds that overcome the resistance phenomenon has become a priority. Antimicrobial peptides (AMPs) appear as one of the most promising antibiotic medicines. However, in recent years several AMP-resistance mechanisms have been described. Moreover, the AMP-resistance phenomenon has become more complex due to its association with cross-resistance toward AMP effectors of the host innate immune system. In this context, the use of AMPs as a therapeutic option could be potentially hazardous, since bacteria could develop resistance toward our innate immune system. Here, we review the findings of major studies that deal with the AMP cross-resistance phenomenon.

Prognostic parameters for response to enzalutamide after docetaxel and abiraterone treatment in metastatic castration-resistant prostate cancer patients; a possible time relation

BACKGROUND

Abiraterone Acetate (AA) and Enzalutamide (Enz) are effective hormonal treatments in mCRPC patients. Retrospective studies suggested clinical cross-resistance between Enz and AA. However, 12.8-39.1% of patients previously treated with docetaxel (Doc) and AA do respond to Enz. These responders have not been characterized.

METHODS

102 Enz treated mCRPC patients after AA and Doc treatment were included in this study. Differences in patient characteristics and previous treatment outcomes between PSA responders and non-responders on Enz were evaluated.

RESULTS

Median Progression-Free Survival was 12.2 weeks (95%CI 11.7-14.3) and Overall Survival 43.5 weeks (95%CI 37.4-61.2). There were 26 (25%) Enz-responders and 76 (75%) non-responders. Significant higher percentages of Gleason scores ≥ 8 and PSA doubling times (PSA-DT) <3 months were found in Enz responders than in non-responders. The interval between end of AA and start of Enz treatment (IAE) for responders was 24.6 weeks (IQR 4.0-48.1) and 8.9 weeks for non-responders (IQR 3.7-25.9) (P = 0.08). In an IAE <40 days subgroup (34 patients), Enz responses were related to AA non-responsiveness, while univariate and logistic regression analysis of baseline criteria of a subgroup of patients with an IAE ≥ 40 (68 patients) revealed significant differences in baseline PSA levels, PSA-DT <3 months, Gleason scores ≥ 8 and IAE's between Enz responders and non-responders.

CONCLUSIONS

PSA response to Enz after previous AA and Doc treatment was associated with a longer IAE, a higher Gleason score and a PSA-DT <3 months. Identification of these patients might be of value for sequencing of treatment options.

Resistance Management for San Jose Scale (Hemiptera: Diaspididae)

The San Jose scale Diaspidiotus perniciosus Comstock is one of the most important pests of deciduous fruit trees. The major cause of recent outbreaks in apple orchards is thought to be the development of insecticide resistance, specifically organophosphates. The first report was given in North America, and now, in Chile. In the present study, San Jose scale populations collected from two central regions of Chile were checked for their susceptibility to different mode of action insecticides in order to establish alternatives to manage this pest. No evidence of cross resistance between organophosphates insecticides and acetamiprid, buprofezin, pyriproxyfen, spirotetramat, sulfoxaflor, or thiacloprid was found. Baselines of LC50-LC95 for different life stages of San Jose scale are given, as reference to future studies of resistance monitoring. The systemic activity of acetamiprid, spirotetramat, and thiacloprid was higher than the contact residue effect of these compounds. For sulfoxaflor, both values were similar. Program treatments including one or more of these compounds are compared in efficacy and impact on resistance ratio values. In order to preserve new insecticides as an important tool to control San Jose scale, resistance management programs should be implemented, considering insecticide mode of action classes alternated or mixed.

Susceptibility to sulfuryl fluoride and lack of cross-resistance to phosphine in developmental stages of the red flour beetle, Tribolium castaneum (Coleoptera: Tenebrionidae)

BACKGROUND

Our aim was to ascertain the potential of sulfuryl fluoride (SF) as an alternative fumigant to manage phosphine-resistant pests. We tested the susceptibility of all life stages of red flour beetle, Tribolium castaneum (Herbst), to SF and assessed the presence of cross-resistance to this fumigant in phosphine-resistant strains of this species.

RESULTS

Analysis of dose-response data indicated that the egg was the stage most tolerant to SF under a 48 h exposure period. At LC50 , eggs were 29 times more tolerant than other immature stages and adults, and required a relatively high concentration of 48.2 mg L(-1) for complete mortality. No significant differences in tolerance to SF were observed among the three larval instars, pupae and adults, and all of these stages were controlled at a low concentration of 1.32 mg L(-1) . Phosphine-resistant strains did not show cross-resistance to SF.

CONCLUSION

Our research concluded that the current maximum registered rate of SF, 1500 gh m(-3) , is adequate to control all the post-embryonic life stages of T. castaneum over a 48 h fumigation period, but it will fail to achieve complete mortality of eggs, indicating the risk of some survival of eggs under this short exposure period. As there is no cross-resistance to SF in phosphine-resistant insects, it will play a key role in managing phosphine resistance in stored-grain insect pests.

Increasing antibiotic resistance in preservative-tolerant bacterial strains isolated from cosmetic products

To ensure the microbiological quality, consumer safety and organoleptic properties of cosmetic products, manufacturers need to comply with defined standards using several preservatives and disinfectants. A drawback regarding the use of these preservatives is the possibility of generating cross-insusceptibility to other disinfectants or preservatives, as well as cross resistance to antibiotics. Therefore, the objective of this study was to understand the adaptive mechanisms of Enterobacter gergoviae, Pseudomonas putida and Burkholderia cepacia that are involved in recurrent contamination in cosmetic products containing preservatives. Diminished susceptibility to formaldehyde-donors was detected in isolates but not to other preservatives commonly used in the cosmetics industry, although increasing resistance to different antibiotics (β-lactams, quinolones, rifampicin, and tetracycline) was demonstrated in these strains when compared with the wild-type strain. The outer membrane protein modifications and efflux mechanism activities responsible for the resistance trait were evaluated. The development of antibiotic-resistant microorganisms due to the selective pressure from preservatives included in cosmetic products could be a risk for the emergence and spread of bacterial resistance in the environment. Nevertheless, the large contribution of disinfection and preservation cannot be denied in cosmetic products.

Cross-resistance and Inheritance of Resistance to Emamectin Benzoate in Spodoptera exigua (Lepidoptera: Noctuidae)

Beet armyworm, Spodoptera exigua (Hübner), is a worldwide pest of many crops. Chemical insecticides are heavily used for its control in China, and serious resistance has been evolved in the field to a variety of insecticides including emamectin benzoate. Through repeated backcrossing to a susceptible strain (WH-S) and selection with emamectin benzoate, the trait conferring resistance to emamectin benzoate in a field-collected population of S. exigua (moderately resistant to emamectin benzoate and strongly resistant to pyrethroids and indoxacarb) was introgressed into WH-S to generate a near-isogenic resistant strain (WH-EB). Compared with WH-S, the WH-EB strain developed a 1,110-fold resistance to emamectin benzoate and a high level of cross-resistance to abamectin (202-fold), with low levels of cross-resistance to cypermethrin (10-fold) and chlorfluazuron (7-fold), but no cross-resistance to representatives of another six different classes of insecticides (chlorantraniliprole, chlorfenapyr, indoxacarb, spinosad, tebufenozide, and chlorpyrifos). Resistance to emamectin benzoate in WH-EB was autosomal, incompletely dominant, and polygenic. Limited cross-resistance in WH-EB indicates that emamectin benzoate can be rotated with other classes of insecticides to which it does not show cross-resistance to delay the evolution of resistance in S. exigua. The incompletely dominant nature of resistance in S. exigua may explain the rapid evolution of resistance to emamectin benzoate in the field, and careful deployment of this chemical within a resistance management program should be considered.

Sugarcane Aphid (Hemiptera: Aphididae): Host Range and Sorghum Resistance Including Cross-Resistance From Greenbug Sources

The graminous host range and sources of sorghum [Sorghum bicolor (L.) Moench.] plant resistance, including cross-resistance from greenbug, Schizaphis graminum (Rondani), were studied for the newly emerging sugarcane aphid, Melanaphis sacchari (Zehntner), in greenhouse no-choice experiments and field evaluations. The sugarcane aphid could not survive on field corn, Zea mays (L.), Teff grass, Eragrostis tef (Zucc.), proso millet, Panicum miliaceum L., barley, Hordeum vulgare L., and rye, Secale cereale L. Only sorghum genotypes served as hosts including Johnsongrass, Sorghum halepense (L.), a highly suitable noncrop host that generates high numbers of sugarcane aphid and maintains moderate phenotypic injury. The greenbug-resistant parental line RTx2783 that is resistant to greenbug biotypes C and E was resistant to sugarcane aphid in both greenhouse and field tests, while PI 55607 greenbug resistant to biotypes B, C, and E was highly susceptible. PI 55610 that is greenbug resistant to biotypes B, C, and E maintained moderate resistance to the sugarcane aphid, while greenbug-resistant PI 264453 was highly susceptible to sugarcane aphid. Two lines and two hybrids from the Texas A&M breeding program B11070, B11070, AB11055-WF1-CS1/RTx436, and AB11055-WF1-CS1/RTx437 were highly resistant to sugarcane aphid, as were parental types SC110, SC170, and South African lines Ent62/SADC, (Macia/TAM428)-LL9, (SV1*Sima/IS23250)-LG15. Tam428, a parental line that previously showed moderate resistance in South Africa and India, also showed moderate resistance in these evaluations. Overall, 9 of 20 parental sorghum entries tested for phenotypic damage in the field resulted in good resistance to the sugarcane aphid and should be utilized in breeding programs that develop agronomically acceptable sorghums for the southern regions of the United States.

Collateral sensitivity of antibiotic-resistant microbes

Understanding how evolution of microbial resistance towards a given antibiotic influences susceptibility to other drugs is a challenge of profound importance. By combining laboratory evolution, genome sequencing, and functional analyses, recent works have charted the map of evolutionary trade-offs between antibiotics and have explored the underlying molecular mechanisms. Strikingly, mutations that caused multidrug resistance in bacteria simultaneously enhanced sensitivity to many other unrelated drugs (collateral sensitivity). Here, we explore how this emerging research sheds new light on resistance mechanisms and the way it could be exploited for the development of alternative antimicrobial strategies.

Construction and characterisation of near-isogenic Plutella xylostella (Lepidoptera: Plutellidae) strains resistant to Cry1Ac toxin

BACKGROUND

Resistance to insecticidal Bacillus thuringiensis (Bt) toxins has arisen in multiple populations of the worldwide Brassica pest Plutella xylostella (L.). To help elucidate the mechanism of resistance to Bt Cry1Ac toxin in a population from Florida, two pairs of near-isogenic lines (NILs) were developed.

RESULTS

NILs were generated using either backcross or recombinant inbred line methodologies and evaluated for near-isogenicity with inter-simple-sequence-repeat (ISSR) markers. Backcross line BC6F4 maintained a similar level of Cry1Ac resistance to parental strain DBM1Ac-R (>5000-fold) yet showed 98.24% genetic similarity to the susceptible parental strain DBM1Ac-S. Single-pair backcrosses between DBM1Ac-S and BC6F4 revealed that Cry1Ac resistance was controlled by one recessive autosomal locus. BC6F4 exhibited high levels of cross-resistance to Cry1Ab and Cry1Ah but not to Cry1Ca or Cry1Ie.

CONCLUSION

Near-isogenic strains were constructed to provide a reliable biological system to investigate the mechanism of Cry1Ac resistance in P. xylostella. These data suggest that resistance to Cry1Ac, Cry1Ab and Cry1Ah is probably caused by the alteration of a common receptor not recognised by Cry1Ca or Cry1Ie. Understanding Bt toxin cross-resistance provides valuable information to consider when developing pest control strategies to delay resistance evolution. © 2014 Society of Chemical Industry.

Dominant fitness costs of abamectin resistance in Plutella xylostella

BACKGROUND

The TH-Abm strain of Plutella xylostella, exhibiting 23 670-fold resistance to abamectin, was selected from a field-evolved multiresistant population. By repeated backcrossing to a susceptible strain (Roth) and selection with abamectin, the resistance trait of TH-Abm was introgressed into Roth to generate a near-isogenic strain (Roth-Abm). Fitness costs associated with abamectin resistance were examined in Roth-Abm.

RESULTS

Compared with Roth, Roth-Abm obtained 11 500-fold resistance to abamectin and 364 000-, 12- and 12-fold cross-resistance to emamectin benzoate, spinosad and fipronil respectively. Roth-Abm has a significantly longer pupal development time, lesser female pupal weight and lower larval survival than Roth. Female fecundity and egg viability are significantly lower in Roth-Abm than in Roth. All of the above fitness components of the F1 progeny from Roth × Roth-Abm are similar to those of Roth-Abm and are significantly lower than those of Roth. By comparing with the net replacement rate (R0 ) of Roth, the fitness of Roth-Abm, F1a (Roth male × Roth-Abm) and F1b (Roth female × Roth-Abm) are 0.50, 0.50 and 0.53 respectively.

CONCLUSION

Abamectin resistance in Roth-Abm results in significant fitness costs, and the fitness costs are autosomal and dominant. Rotation of abamectin with other insecticides without cross-resistance could be especially useful for delaying abamectin resistance in P. xylostella.

Clinical activity of enzalutamide in Docetaxel-naïve and Docetaxel-pretreated patients with metastatic castration-resistant prostate cancer

BACKGROUND

Two randomized clinical trials have demonstrated a survival advantage with enzalutamide over placebo in both docetaxel (D)-pretreated and D-naïve metastatic castration-resistant prostate cancer (mCRPC) patients. Cross-resistance between androgen receptor-directed therapies and taxanes has been suggested, possibly leading to lower efficacy of enzalutamide in the post-D setting.

METHODS

We aimed to examine the impact of prior D treatment on the clinical activity of enzalutamide in patients with mCRPC. We retrospectively reviewed an institutional database to identify men with mCRPC treated with standard-of-care enzalutamide. Patients were classified as D-naïve or D-pretreated. The efficacy end points were prostate-specific antigen (PSA) response rates (≥ 50% PSA decline), time to PSA progression (TTPP) and clinical/radiographic progression-free survival (PFS) in response to enzalutamide. Differences between groups (D-naïve and D-pretreated) were assessed by univariate and multivariable analyses using logistic and Cox regression models.

RESULTS

One-hundred-seven (107) consecutive patients were included: 60 were D-pretreated and 47 were D-naïve. PSA responses were 43.2% in D-naïve patients and 25.4% in D-pretreated patients (P = 0.089). Median TTPP was 7.2 months (95% CI = 4.5 - 17.2) in the D-naïve group versus 2.6 mo (95% CI = 1.9 - 3.5) in the D-pretreated group (P < 0.0001). Median PFS was not reached for D-naïve men and was 3.3 mo (95% CI = 2.5 - 4.8) for D-pretreated men (P < 0.0001). After adjusting for potential confounders including prior abiraterone use, differences remained statistically significant for TTPP (HR = 2.32; 95% CI = 1.19 - 4.50; P = 0.013) and marginally significant for PFS (HR = 1.90; 95% CI = 0.94 - 3.84; P = 0.073) in multivariable analyses. Among patients who achieved a PSA response to enzalutamide (n = 34), results suggested a trend towards shorter duration of response in D-pretreated patients.

CONCLUSIONS

The clinical activity of enzalutamide appears to be blunted in patients who have previously received docetaxel chemotherapy. These results support the concept of cross-resistance between these two agents.

Toxicity and Cross-Resistance of Insecticides to Cry2Ab-Resistant and Cry2Ab-Susceptible Helicoverpa armigera and Helicoverpa punctigera (Lepidoptera: Noctuidae)

Since 2004-2005 cotton expressing Cry1Ac and Cry2Ab insecticidal proteins from the bacterium Bacillus thuringiensis has been commercially available in Australia to manage the target pests Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren). In both target species, the frequency of alleles conferring resistance to Cry2Ab is unexpectedly high in field populations. A significant challenge for managing these pests would occur if resistance to Cry2Ab toxins inadvertently selected for resistance to other insecticides used to control them. Dose-response bioassays were performed to measure the toxicity of currently registered insecticide sprays on isogenic strains of Cry2Ab-resistant and Cry2Ab-susceptible H. armigera and H. punctigera. Within-species comparisons of Cry2Ab-resistant and Cry2Ab-susceptible strains of H. armigera and H. punctigera indicate no cross-resistance with pyrethroid insecticides. Additionally, Cry2Ab-resistant strains were not cross-resistant to the following selective insecticides: indoxacarb, chlorantraniliprole, and avermectins. In both H. armigera and H. punctigera, Cry2Ab-resistant colonies exhibited a small, but significant, degree of enhanced susceptibility in response to chlorpyrifos and methomyl. We report higher tolerance to conventional insecticides in H. armigera compared with H. punctigera. Our results indicate that there is no significant interplay between Cry2Ab resistance frequencies in H. armigera and H. punctigera and frequencies of resistance to a range of insecticide sprays currently registered for cotton. Therefore, we conclude that any increases in frequencies of the common Cry2Ab resistance phenotypes identified in Australian populations of Helicoverpa spp. are unlikely to increase resistance risk for the indoxacarb, chlorantraniliprole, or avermectin classes of insecticide.

Inheritance patterns, dominance and cross-resistance of Cry1Ab- and Cry1Ac-selected Ostrinia furnacalis (Guenée)

Two colonies of Asian corn borer, Ostrinia furnacalis (Guenée), artificially selected from a Bt-susceptible colony (ACB-BtS) for resistance to Cry1Ab (ACB-AbR) and Cry1Ac (ACB-AcR) toxins, were used to analyze inheritance patterns of resistance to Cry1 toxins. ACB-AbR and ACB-AcR evolved significant levels of resistance, with resistance ratios (RR) of 39-fold and 78.8-fold to Cry1Ab and Cry1Ac, respectively. The susceptibility of ACB-AbR larvae to Cry1Ac and Cry1F toxins, which had not previously been exposed, were significantly reduced, being >113-fold and 48-fold, respectively. Similarly, susceptibility of ACB-AcR larvae to Cry1Ab and Cry1F were also significantly reduced (RR > nine-fold, RR > 18-fold, respectively), indicating cross-resistance among Cry1Ab, Cry1Ac, and Cry1F toxins. However, ACB-AbR and ACB-AcR larvae were equally susceptible to Cry1Ie as were ACB-BtS larvae, indicating no cross-resistance between Cry1Ie and Cry1Ab or Cry1Ac toxins; this may provide considerable benefits in preventing or delaying the evolution of resistance in ACB to Cry1Ab and Cry1Ac toxins. Backcrossing studies indicated that resistance to Cry1Ab toxin was polygenic in ACB-AbR, but monogenic in ACB-AcR, whilst resistance to Cry1Ac toxin was primarily monogenic in both ACB-AbR and ACB-AcR, but polygenic as resistance increased.

Cross-resistance between cyenopyrafen and pyridaben in the twospotted spider mite Tetranychus urticae (Acari: Tetranychidae)

BACKGROUND

Cyenopyrafen is an inhibitor of complex II of the mitochondrial electron transport chain. It has a molecular structure that shares some common features with frequently used complex I inhibitors such as pyridaben. To evaluate whether this similarity in structure poses a cross-resistance risk that might complicate resistance management, we selected for pyridaben and cyenopyrafen resistance in the laboratory and characterized resistance.

RESULTS

The selection for cyenopyrafen conferred cross-resistance to pyridaben and vice versa. Resistance towards these both acaricides was incompletely dominant in adult females. However, in eggs maternal effects were observed in pyridaben resistance, but not in the cyenopyrafen-resistance (completely dominant). In the cyenopyrafen resistant strain, the LC50 of eggs remained lower than the commercially recommended concentration. The common detoxification mechanisms by cytochrome P450 was involved in resistance to these acaricides. Carboxyl esterases were also involved in cyenopyrafen resistance as a major factor.

CONCLUSIONS

Although cross-resistance suggests that pyridaben resistance would confer cyenopyrafen cross-resistance, susceptibility in eggs functions to delay the development of cyenopyrafen resistance.

Strength of selection pressure is an important parameter contributing to the complexity of antibiotic resistance evolution

Revealing the genetic changes responsible for antibiotic resistance can be critical for developing novel antibiotic therapies. However, systematic studies correlating genotype to phenotype in the context of antibiotic resistance have been missing. In order to fill in this gap, we evolved 88 isogenic Escherichia coli populations against 22 antibiotics for 3 weeks. For every drug, two populations were evolved under strong selection and two populations were evolved under mild selection. By quantifying evolved populations’ resistances against all 22 drugs, we constructed two separate cross-resistance networks for strongly and mildly selected populations. Subsequently, we sequenced representative colonies isolated from evolved populations for revealing the genetic basis for novel phenotypes. Bacterial populations that evolved resistance against antibiotics under strong selection acquired high levels of cross-resistance against several antibiotics, whereas other bacterial populations evolved under milder selection acquired relatively weaker cross-resistance. In addition, we found that strongly selected strains against aminoglycosides became more susceptible to five other drug classes compared with their wild-type ancestor as a result of a point mutation on TrkH, an ion transporter protein. Our findings suggest that selection strength is an important parameter contributing to the complexity of antibiotic resistance problem and use of high doses of antibiotics to clear infections has the potential to promote increase of cross-resistance in clinics.

Fitness landscapes emerging from pharmacodynamic functions in the evolution of multidrug resistance

Adaptive evolution often involves beneficial mutations at more than one locus. In this case, the trajectory and rate of adaptation is determined by the underlying fitness landscape, that is, the fitness values and mutational connectivity of all genotypes under consideration. Drug resistance, especially resistance to multiple drugs simultaneously, is also often conferred by mutations at several loci so that the concept of fitness landscapes becomes important. However, fitness landscapes underlying drug resistance are not static but dependent on drug concentrations, which means they are influenced by the pharmacodynamics of the drugs administered. Here, I present a mathematical framework for fitness landscapes of multidrug resistance based on Hill functions describing how drug concentrations affect fitness. I demonstrate that these 'pharmacodynamic fitness landscapes' are characterized by pervasive epistasis that arises through (i) fitness costs of resistance (even when these costs are additive), (ii) nonspecificity of resistance mutations to drugs, in particular cross-resistance, and (iii) drug interactions (both synergistic and antagonistic). In the latter case, reciprocal drug suppression may even lead to reciprocal sign epistasis, so that the doubly resistant genotype occupies a local fitness peak that may be difficult to access by evolution. Simulations exploring the evolutionary dynamics on some pharmacodynamic fitness landscapes with both constant and changing drug concentrations confirm the crucial role of epistasis in determining the rate of multidrug resistance evolution.