Low Level Of Cross-resistance Research Articles

Application of computational methods in the analysis of pesticide target-site and resistance mechanisms.

Meta-diamide insecticides including broflanilide have a high insecticidal activity by acting on RDL GABA receptors. Both membrane potential assays and docking studies suggest that the target site of meta-diamides is different from that of conventional noncompetitive inhibitors, such as fipronil. In fact, meta-diamides are effective against cyclodiene- and fipronil-resistant pests that carry target-site mutations. Dinotefuran uniquely possesses a tetrahydrofuran ring, whereas other neonicotinoids possess aromatic rings. Moreover, dinotefuran has been reported to be effective against imidacloprid-resistant strains. A docking study predicted the weak binding of dinotefuran to cytochrome P450s which are associated with imidacloprid resistance. Metabolic assays revealed that dinotefuran was not metabolized by these cytochrome P450s. These findings suggest that the lack of metabolic activity of P450s against dinotefuran causes a low level of cross-resistance.

FITNESS COST ASSOCIATED WITH ACARICIDES INHERITANCE RESISTANCE IN THE SPIDER MITE, TETRANYCHUS URTICAE KOCH

Tetranychus urticae caused many problems in field crops in Egypt, despite the use of acaricides that showed loss efficiency against T. urticae so we have studied bioassays tests for certain acaricides on some Egyptian strains of T. urticae. We investigated resistance levels of different acaricides from differ groups against T. urticae. Resistant Ratio (RR), RR50 = 119.62, 19.53, 17.93 and 26.38 fold for fenpyroximate, chlorfenapyr, abamectin and bifenazate, respectively. The strain showed high levels of cross-resistance to abamectin (RR 152), moderate levels of cross resistance to chlorfenapyr (RR 38.08) and low levels of cross-resistance (RR < 10) to bifenzate. Resistance coefficient (RC) values calculated for this active ingredient 2.20 (medium resistance). Sensitivity of the mite to pyrrole and avermectin active ingredient 0.152 and 0.075 (lack of resistance). Carbazate was classified as 1.14 (low resistance). All acaricides shortened the longevity and decreased the fecundity of T. urticae resistance strain compared with the control. Life cycle of the offspring that produced from treated T. urticae females was increased in all treatments compared with the control.

FITNESS COST ASSOCIATED WITH ACARICIDES INHERITANCE RESISTANCE IN THE SPIDER MITE, TETRANYCHUS URTICAE KOCH

tetranychus urticae caused many problems in field crops in Egypt, despite the use of acaricides that showed loss efficiency against T. urticae so we have studied bioassays tests for certain acaricides on some Egyptian strains of T. urticae. We investigated resistance levels of different acaricides from differ groups against T. urticae. Resistant Ratio (RR), RR50 = 119.62, 19.53, 17.93 and 26.38 fold for fenpyroximate, chlorfenapyr, abamectin and bifenazate, respectively. The strain showed high levels of cross-resistance to abamectin (RR 152), moderate levels of cross-resistance to chlorfenapyr (RR 38.08) and low levels of crossresistance (RR < 10) to bifenzate. Resistance coefficient (RC) values calculated for this active ingredient 2.20 (medium resistance). Sensitivity of the mite to pyrrole and avermectin active ingredient 0.152 and 0.075 (lack of resistance). Carbazate was classified as 1.14 (low resistance). All acaricides shortened the longevity and decreased the fecundity of T. urticae resistance strain compared with the control. Life cycle of the offspring that produced from treated T. urticae females was increased in all treatments compared with the control.

Differential metabolism of imidacloprid and dinotefuran by Bemisia tabaci CYP6CM1 variants

Imidacloprid has been used to control one of most serious pests, Bemisia tabaci. However, B. tabaci has developed imidacloprid resistance mainly by over-expressing CYP6CM1. It was reported that imidacloprid-resistant B. tabaci showed no or low level of cross-resistance against dinotefuran. Here, we expressed CYP6CM1 variants using Sf9/baculovirus and/or Drosophila S2 cells and showed that CYP6CM1 variants metabolized imidacloprid but not dinotefuran. In addition, we demonstrated that imidacloprid and pymetrozine competed for a CYP6CM1 variant more efficiently than dinotefuran, using a luminescent substrate competition assay. These results suggest that lack of metabolic activity of CYP6CM1 variants against dinotefuran caused no or low level of cross-resistance.

First report of thiamethoxam resistance selection, cross resistance to various insecticides and realized heritability in Asian citrus psyllid Diaphorina citri from Pakistan

Diaphorina citri Kuwayama (Hemiptera: Liviidae) is one of the devastating pests of citrus orchards in different regions of the world and its management depends upon the use of insecticides. The work was conducted to evaluate the toxicities and resistance levels to various insecticides in a field collected population of citrus psylla from an area of Pakistan. The field population was also kept under selection pressure with thiamethoxam for several generations to study the impact of selection on thiamethoxam resistance development. The resistant population was then studied for cross-resistance with insecticides from different groups and realized heritability of thiamethoxam resistance was determined to assess the potential of resistance development. Selection of D. citri from G1-G7 with thiamethoxam produced 229-fold resistant strain in the laboratory. Selected strain exhibited very low cross-resistance with flonicamid, buprofezin, and spirotetramat; low cross-resistance with diafenthiuron and fenpropathrin, moderate cross-resistance with nitenpyram and pyriproxyfen, and a very high cross-resistance with acephate and methomyl. The estimated realized heritability of thiamethoxam resistance was 0.36. This study provides information on the impact of selection on resistance development and its management by restoring the susceptibility of D. citri through rotation of insecticides with low level of cross-resistance.

Dissimilar Crystal Proteins Cry5Ca1 and Cry5Da1 Synergistically Act against Meloidogyne incognita and Delay Cry5Ba-Based Nematode Resistance.

Cry proteins of Bacillus thuringiensis (Bt) have been successfully used as biopesticides and in transgenic crops throughout the world. However, resources against the most serious agricultural pathogens, plant root-knot nematodes, are limited. The genomes of several highly nematicidal virulent Bt strains from our laboratory have been sequenced, facilitating the identification of novel Cry proteins and other virulence factors. We identified two novel Cry proteins, Cry5Ca1 and Cry5Da1, that exhibit high toxicity against Meloidogyne incognita Using the Caenorhabditis elegans model, the two Cry5 toxins were shown to negatively affect nematode life span, fertility, and survival. The 50% lethal concentrations (LC50s) of Cry5Ca1 and Cry5Da1 were 57.22 μg/ml and 36.69 μg/ml, respectively. Moreover, a synergistic effect (synergism factor, 1.61 to 2.04) was observed for nematicidal toxicity of Cry5Ca1 and Cry5Da1, which is accordant with the phylogenetic results suggesting that domain II of the two novel Cry5 toxins evolved into two independent clades. Through comparison of the depressed degree of toxicity in the β-methylgalactoside detoxification test, we found that the novel toxin Cry5D possesses a different galactose-binding epitope; meanwhile, the finding that Cry5D does not share a motif (GXXXE) in the corresponding loop of domain II with Cry5B could explain the different galactose binding performance. Additionally, low-level cross-resistance of C. elegans bre mutant strains was evident between Cry5B and Cry5D. These results suggest that Cry5D can be used as an alternative to delay the potential resistance of nematodes to Cry5B.IMPORTANCE Although proper gene resources for Bt crops against the most serious agricultural pathogens, plant root-knot nematodes, are limited, we have identified two novel nematicidal toxins, Cry5Ca1 and Cry5Da1, against M. incognita, which have supplied more gene candidates for Bt crops designed against nematodes. Moreover, the association of the dissimilarity between Cry5Da1 and Cry5Ba1 and their low cross-resistance can be attributed not only to a low sequence similarity of domain II but also to the structural difference of the key motif and receptor-binding epitope in the loops. This association facilitates the selection of a proper candidate for the prospective design of pyramided Bt crops that can delay potential resistance.

Resistance risk assessment to chlorpyrifos and cross-resistance to other insecticides in a field strain of Phenacoccus solenopsis Tinsley

The organophosphate insecticide chlorpyrifos is recommended for control of a number of insect pests, including cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) in Pakistan. This work assessed chlorpyrifos resistance evolution and cross-resistance to other insecticides. After 23 generations of selection, the P. solenopsis strain (Chlor-SEL) had a 26652-fold level of resistance to chlorpyrifos compared to a susceptible strain. Realized heritability (h2) of resistance to chlorpyrifos was 0.04. The Chlor-SEL strain also had a low level of cross resistance to lambda-cyhalothrin (14-fold) and a very low level cross-resistance to nitenpyram and profenofos after 23 generations of selection. The projected rate of resistance development indicated that if 50–90 percent of a P. solenopsis population were selected with chlorpyrifos, a ten-fold increase in the lethal concentration 50 (LC50) would occur in 22–10 generations (h2 = 0.04, Slope = 0.70). At a similar slope, if h2 = 0.14, then only 6-3 generations are required for a ten-fold increase in the LC50 at 50–90 percent selection intensity, respectively. Likewise, if h2 = 0.24, then the same would occur in 4–2 generations. This study showed that P. solenopsis has the ability to become resistant to chlorpyrifos but insect resistance management strategies such as rotation of different group of insecticides are needed to prolong the effectiveness of chlorpyrifos in controlling P. solenopsis.

Pyriproxyfen is metabolized by P450s associated with pyrethroid resistance in An. gambiae

Pyrethroid resistance is widespread in the malaria vector Anopheles gambiae leading to concerns about the future efficacy of bednets with pyrethroids as the sole active ingredient. The incorporation of pyriproxyfen (PPF), a juvenile hormone analogue, into pyrethroid treated bednets is being trialed in Africa. Pyrethroid resistance is commonly associated with elevated levels of P450 expression including CYPs 6M2, 6P2, 6P3, 6P4, 6P5, 6Z2 and 9J5. Having expressed these P450s in E. coli we find all are capable of metabolizing PPF. Inhibition of these P450s by permethrin, deltamethrin and PPF was also examined. Deltamethrin and permethrin were moderate inhibitors (IC50 1–10 μM) of diethoxyfluorescein (DEF) activity for all P450s apart from CYP6Z2 (IC50 > 10 μM), while PPF displayed weaker inhibition of all P450s (IC50 > 10 μM) except CYP's 6Z2 and 6P2 (IC50 1–10 μM). We found evidence of low levels of cross resistance between PPF and other insecticide classes by comparing the efficacy of PPF in inhibiting metamorphosis and inducing female sterility in an insecticide susceptible strain of An. gambiae and a multiple resistant strain from Cote d’Ivoire.

Susceptibility of Sogatella furcifera and Laodelphax striatellus (Hemiptera: Delphacidae) to Six Insecticides in China.

The whitebacked planthopper, Sogatella furcifera (Horváth), and small brown planthopper, Laodelphax striatellus (Fallén), both are important crop pests throughout China, especially in rice. Application of chemical insecticides is the major control practice. Consequently, insecticide resistance has become an urgent issue. In this study, resistance levels to six conventional insecticides were evaluated for these two species collected from major occurring areas of China. Additionally, imidacloprid- (resistance ratio [RR] = 10.4-fold) and buprofezin (RR = 15.1-fold)-resistant strains of whitebacked planthopper were obtained through laboratory selections for cross-resistance profiling and synergism assessment to understand resistance mechanisms. The results showed that all tested populations of both species exhibited low to high levels of resistance to chlorpyrifos, while remaining susceptible to thiamethoxam. Three of the 14 whitebacked planthopper populations showed low to moderate resistance to imidacloprid, while all small brown planthopper populations reminded susceptible. All small brown planthopper and whitebacked planthopper (except one) populations showed at least moderate resistance (RR = 10.1-271.1) to buprofezin. All small brown planthopper populations remained susceptible to pymetrozine and nitenpyram, and all whitebacked planthopper populations remained susceptible to isoprocarb. The imidacloprid-resistant whitebacked planthopper strain showed no significant cross-resistance to other tested insecticides. However, the buprofezin-resistant strain exhibited a low-level cross-resistance (CR = 3.1) to imidacloprid. Piperonyl butoxide, triphenyl phosphate, and diethylmaleate displayed no synergism effect on the resistant whitebacked planthopper strains.

Abamectin resistance in strains of vegetable leafminer, Liriomyza sativae (Diptera: Agromyzidae) is linked to elevated glutathione S-transferase activity.

Abamectin resistance was selected in the vegetable leafminer, Liriomyza sativae (Blanchard) (Diptera: Agromyzidae) under laboratory conditions, and cross-resistance patterns and possible resistance mechanisms in the abamectin-resistant strains (AL-R, AF-R) were investigated. Compared with the susceptible strain (SS), strain AL-R displayed 39-fold resistance to abamectin after 20 selection cycles during 25 generations, and strain AF-R exhibited 59-fold resistance to abamectin after 16 selection cycles during 22 generations. No cross-resistance to cyromazine was found in both abamectin-resistant strains. However, we failed to select for cyromazine resistance in L. sativae under laboratory conditions by conducting 17 selection cycles during 22 generations. However, moderate levels of cross-resistance to abamectin (6-9 fold) were observed in strains which received cyromazine treatments. Biochemical analysis showed that glutathione S-transferase (GST) activity in both abamectin-resistant strains (AL-R, AF-R) was significantly higher than in the susceptible strain (SS), suggesting metabolically driven resistance to abamectinin L. sativae. Recommendations of mixtures or rotation of cyromazine and abamectin should be considered carefully, as consecutive cyromazine treatments may select for low-level cross-resistance to abamectin.

Mechanism of Fipronil Resistance in Laodelphax striatellus (Hemiptera: Delphacidae)

The WX-F strain of Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae) was derived from a field-collected strain (WX strain) after 77 generations of continuous selection with fipronil. The selection resulted in 86.6-fold resistance to fipronil; WX-F showed no cross-resistance to dieldrin (1.8-fold) and low-level cross-resistance to endosulfan (3.7-fold) compared with the primary WX strain. Bioassay with synergists and analysis of detoxification enzyme activities indicated that metabolic detoxification is not significantly associated with fipronil resistance in the WX-F strain. Further molecular analysis indicated that the fipronil resistance was most likely attributed to target site insensitivity. Sequencing a cloned single cDNA fragment of the Rdl γ-aminobutyric acid (GABA) receptor gene showed a high frequency (80%) of the A2′N mutation in the fipronil-selected strain (WX-F). It carried a mutant LsRdl allele with the amino acid replacement of Ala (GCC) to Asn (AAC). This mutation was not detected in the 23 individuals in the WX strain. The high frequency of the LsRdl-Asn allele in the WX-F strain indicates that the A2′N mutation of the γ-aminobutyric acid (GABA) receptor gene may be associated with fipronil resistance in L. striatellus. In addition, a novel R305Q or R305W mutation was found in the cytoplasmic loop between M3 and M4 of the L. striatellus Rdl γ-aminobutyric acid (GABA) receptor subunit (Ls-Rdl). These were only found in cDNA clones carrying the A2′N mutation.

Inheritance of Cry1F resistance, cross-resistance and frequency of resistant alleles inSpodoptera frugiperda(Lepidoptera: Noctuidae)

Transgenic maize, Zea maize L., expressing the Cry1F protein from Bacillus thuringiensis has been registered for Spodoptera frugiperda (J. E. Smith) control since 2003. Unexpected damage to Cry1F maize was reported in 2006 in Puerto Rico and Cry1F resistance in S. frugiperda was documented. The inheritance of Cry1F resistance was characterized in a S. frugiperda resistant strain originating from Puerto Rico, which displayed >289-fold resistance to purified Cry1F. Concentration-response bioassays of reciprocal crosses of resistant and susceptible parental populations indicated that resistance is recessive and autosomal. Bioassays of the backcross of the F1 generation crossed with the resistant parental strain suggest that a single locus is responsible for resistance. In addition, cross-resistance to Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba, Cry2Aa and Vip3Aa was assessed in the Cry1F-resistant strain. There was no significant cross-resistance to Cry1Aa, Cry1Ba and Cry2Aa, although only limited effects were observed in the susceptible strain. Vip3Aa was highly effective against susceptible and resistant insects indicating no cross-resistance with Cry1F. In contrast, low levels of cross-resistance were observed for both Cry1Ab and Cry1Ac. Because the resistance is recessive and conferred by a single locus, an F1 screening assay was used to measure the frequency of Cry1F-resistant alleles from populations of Florida and Texas in 2010 and 2011. A total frequency of resistant alleles of 0.13 and 0.02 was found for Florida and Texas populations, respectively, indicating resistant alleles could be found in US populations, although there have been no reports of reduced efficacy of Cry1F-expressing plants.

Similar evolution of hepatitis B virus quasispecies in patients with incomplete adefovir response receiving tenofovir/emtricitabine combination or tenofovir monotherapy

Adefovir (ADV) resistance mutations induce low-level cross-resistance to tenofovir in vitro. Our aim was to compare viral kinetics, nucleos(t)ide analog resistance mutations, and quasispecies (QS) evolution during therapy with tenofovir disoproxil fumarate (TDF) or emtricitabine + TDF (FTC/TDF) in selected patients with incomplete ADV responses. Patients with chronic hepatitis B and incomplete response to ADV were randomized in a double-blind trial of TDF vs. FTC/TDF. Extensive analysis of QS evolution was performed in 17 patients through 48 weeks of treatment. At week 24, 48% of patients (9/17) achieved HBV DNA undetectability (<69 IU/ml) with no difference between treatment groups. ADV and/or LAM resistance mutations were detected in all 17 patients at baseline and in 5/6 analyzable patients at week 48. A total of 1224 reverse transcriptase clones were analyzed. Clonal analysis revealed no significant difference at baseline in QS complexity or diversity between treatment groups. There was a trend in both treatment groups for an increase in QS complexity at week 12, followed by a decrease in complexity and diversity by week 48. Analysis of individual patients showed no consistent selection/accumulation of specific viral resistance patterns during treatment, but at week 48, mutations at rtA181 persisted in 4 patients. TDF or FTC/TDF demonstrated strong viral suppression in patients with an incomplete response to ADV and no significant selective pressure on pre-existing ADV or LAM resistant strains. TDF monotherapy and FTC/TDF combination therapy had a comparable impact on QS evolution.

Susceptibility of Cry1Ab maize-resistant and -susceptible strains of sugarcane borer (Lepidoptera: Crambidae) to four individual Cry proteins

Sugarcane borer, Diatraea saccharalis (F.), is a major target of Bt maize in South America and many areas of the US mid-south region. Six laboratory strains of D. saccharalis were established from six single-pair F2 families possessing major resistance alleles to Cry1Ab maize hybrids. Susceptibility of the six strains was evaluated on diet treated with each of four purified trypsin-activated Bt proteins, Cry1Ab, Cry1Aa, Cry1Ac and Cry1F. Bt susceptibility of the six strains was compared with that of known Cry1Ab-susceptible and -resistant strains of D. saccharalis. At least two of the six strains demonstrated a similar level (>526-fold) of resistance to Cry1Ab as shown in the known Cry1Ab-resistant strain, while resistance levels were relatively lower for other strains (116- to 129-fold). All the six strains were highly cross-resistant to Cry1Aa (71- to 292-fold) and Cry1Ac (30- to 248-fold), but only with a low level to Cry1F (<7-fold). Larval growth of all six strains was also inhibited on Bt-treated diet, but, except for Cry1F, the growth inhibition of the six strains was considerably less than that of the Cry1Ab-susceptible larvae. The results provide clear evidence that the observed resistance to Cry1Ab maize in the six strains is a result of resistance to the Cry1Ab protein in the plants. The low level of cross-resistance between Cry1A and Cry1F suggests that pyramiding these two types of Bt proteins into a plant could be a good strategy for managing D. saccharalis.

Mutations in Multiple Domains of Gag Drive the Emergence of In Vitro Resistance to the Phosphonate-Containing HIV-1 Protease Inhibitor GS-8374

GS-8374 is a potent HIV protease inhibitor (PI) with a unique diethyl-phosphonate moiety. Due to a balanced contribution of enthalpic and entropic components to its interaction with the protease (PR) active site, the compound retains activity against HIV mutants with high-level multi-PI resistance. We report here the in vitro selection and characterization of HIV variants resistant to GS-8374. While highly resistant viruses with multiple mutations in PR were isolated in the presence of control PIs, an HIV variant displaying moderate (14-fold) resistance to GS-8374 was generated only after prolonged passaging for >300 days. The isolate showed low-level cross-resistance to darunavir, atazanavir, lopinavir, and saquinavir, but not other PIs, and contained a single R41K mutation in PR combined with multiple genotypic changes in the Gag matrix, capsid, nucleocapsid, and SP2 domains. Mutations also occurred in the transframe peptide and p6* domain of the Gag-Pol polyprotein. Analysis of recombinant HIV variants indicated that mutations in Gag, but not the R41K in PR, conferred reduced susceptibility to GS-8374. The Gag mutations acted in concert, since they did not affect susceptibility when introduced individually. Analysis of viral particles revealed that the mutations rendered Gag more susceptible to PR-mediated cleavage in the presence of GS-8374. In summary, the emergence of resistance to GS-8374 involved a combination of substrate mutations without typical resistance mutations in PR. These substrate changes were distributed throughout Gag and acted in an additive manner. Thus, they are classified as primary resistance mutations indicating a unique mechanism and pathway of resistance development for GS-8374.

Complete Deletion of the ramR Gene in an In Vitro -Selected Mutant of Klebsiella pneumoniae Overexpressing the AcrAB Efflux Pump

In Klebsiella pneumoniae, overexpression of nonspecific efflux pumps causes low-level cross-resistance to β-lactams (mainly cefoxitin) and to antibiotics belonging to other families, such as quinolones, chloramphenicol, and tetracycline ([1][1], [2][2]). Among these efflux systems, the pump AcrAB

Cyromazine resistance detected in Australian sheep blowfly

To investigate the cause of cyromazine failure to protect lambs from flystrike. Lucilia cuprina larvae from a Nimmitabel (New South Wales) population associated with failure of a cyromazine spray-on to protect lambs from flystrike were compared with larvae from a susceptible field strain and a reference susceptible laboratory strain in laboratory bioassays. Batches of neonate blowfly larvae were transferred onto homogenised bovine liver containing varying concentrations of cyromazine or dicyclanil and the numbers of larvae pupating and completing development were recorded. Based on the ability of larvae to complete development on liver homogenate containing 1 mg/kg cyromazine, the phenotypic frequency of resistance in the Nimmitabel population was estimated to be approximately 4%. Compared with a susceptible field strain, the Nimmitabel population was 3-fold more resistant to cyromazine and twice as resistant to dicyclanil at the LC95 level (lethal concentration killing 95% of larvae). In the laboratory, the Nimmitabel strain responded to sequential exposure of larvae to food containing cyromazine by becoming more resistant. Resistance to cyromazine was incompletely dominant, giving resistant larvae a survival advantage over susceptible types over a relatively narrow range of cyromazine concentrations. Cyromazine resistance was detected in a field population of L. cuprina. Low-level cross-resistance to dicyclanil was also confirmed. Until more is known about the resistance, the prudent recommendation to control flystrike by this blowfly population is topical treatment with ivermectin.

Hepatitis B virus wild‐type and rtN236T populations show similar early HBV DNA decline in adefovir refractory patients on a tenofovir‐based regimen

Hepatitis B virus (HBV) pol/RT mutations that confer clinical resistance to tenofovir disoproxil fumarate (TDF) have not been detected to date. In vitro, the rtN236T adefovir dipivoxil (ADV)-associated resistance mutation confers low-level cross-resistance to tenofovir: 3- to 13-fold changes in EC(50) from wild type. This study evaluated the clinical response of rtN236T mutant viruses by comparing their early viral load decay kinetics to wild-type viruses in chronic HBV monoinfected patients harbouring rtN236T prior to initiating TDF or emtricitabine (FTC)/TDF therapy. Baseline samples (n = 105) from adefovir refractory patients were tested for the presence of rtN236T using a highly sensitive allele-specific PCR assay with an rtN236T detection cut-off of 0.5%. The rtN236T mutation was detected at baseline in 14.3% (14/98) of analysable patient samples (0.5-93.2%, rtN236T percentage range). The median change in total HBV DNA at week 24 was comparable for patients with rtN236T detected at baseline (-3.7 log(10) copies/mL, n = 14) as compared to patients with wild-type HBV (-3.2 log(10) copies/mL, n = 90). In patients with rtN236T, wild-type and rtN236T mutant virus showed similar rates of HBV DNA decline with no statistically significant difference observed at week 4. Moreover, the proportion of rtN236T remained unchanged in patients in either arm of the study during treatment. In conclusion, the rtN236T mutant virus showed similar HBV DNA decline kinetics to wild-type virus in adefovir refractory patients who switched to TDF or FTC/TDF. Despite low levels of cross-resistance in vitro, TDF similarly suppresses wild-type and rtN236T mutant viruses in vivo.

Antimicrobials that affect the synthesis and conformation of nucleic acids

Several antimicrobials act by inhibiting the synthesis of nucleic acids (rifamycins, sulfamides, diaminopyridines), modifying their conformation (quinolones, coumarins) or causing irreversible lesions (nitroimidazoles, nitrofurans). The resistance mechanisms are: a reduction in intracytoplasmic accumulation, modification of the target or the production of a new low-affinity target and, more rarely, enzyme inactivation. Although the mechanisms affecting the targets are specific to each family and can lead to high-level resistance, the reduced permeability of the membrane and the increased efflux are non-specific and result in low-level cross-resistance between several families. The genetic mediation is usually chromosomal for rifamycins and quinolones, although plasmid-mediated resistant genes have been observed. On the other hand, for sulfamides and trimethoprim, plasmid-borne genes are frequent. Resistance to nitroimidazoles and nitrofurans is still not widely understood.

Susceptibility of the sheep body louse [Bovicola ovis (Schrank), Phthiraptera: Trichodectidae] to temephos and concentration stability during sheep dipping with a commercial temephos product

Laboratory bioassays were used to measure the susceptibility of sheep lice [Bovicola ovis (Schrank)] strains that were resistant to pyrethroid or benzoylphenyl urea insect growth regulator insecticides to the organophosphate temephos, the most commonly used wet dipping insecticide. Compared with the reference susceptible strain, only the pyrethroid-resistant ‘Rowena’ strain displayed low level (2.2×) tolerance of temephos. Lice exposed to the minimum lethal concentration of temephos began to be affected within 6–7 h and all had responded to the insecticide within 24 h. In a dipping trial with sheep, the working concentration of temephos in a 5000-L plunge dip operating according to constant replenishment principles remained close to the nominal application rate of 350 mg/L. The ease with which temephos dip concentration was maintained simplified the dipping process. If typical of similar field populations, the absence, or low level of cross-resistance to temephos reported here for insect growth regulator-resistant and pyrethroid-resistant lice, respectively, should make wool producers confident that thorough dipping with temephos according to product label directions should be effective in controlling lice on their sheep.