Frequency Of Resistant Phenotypes Research Articles

Frequency of Alternaria Genotypes Resistant to SDHI Fungicides in California Pistachio Orchards Determined by Real-Time PCR.

Real-time PCR methods were developed to quantify the frequency of SDHC-H134R and SDHB-H277Y mutants associated with succinate dehydrogenase inhibitors (SDHI) resistance in Alternaria populations from pistachio. The linearity of the standard curves demonstrated the applicability in the quantification of the assays. The accuracy and reliability of the qPCR protocols to determine the frequency of mutants in real samples were corroborated. Orchards visibly affected by Alternaria late blight were sampled. The frequency of mutants was determined using the qPCR assays, while the frequency of resistant phenotypes was determined using a single discriminatory dose. The statistical analysis showed that the frequencies of the mutation SDHC-H134R determined with the qPCR assay were highly correlated with those estimated with the conventional method. The survey also evidenced that resistance to boscalid is still widespread in California. Results also indicated the possible contribution of other mutations to SDHI resistance. Our results confirmed the prevalence of SDHC-H134R mutants and the occurrence of mutation SDHB-H277Y at low frequencies. The real-time PCR methods developed in this study were able to detect differences in the frequencies of resistant mutants caused by the use of chemical fungicides. Finally, the effects of two fungicide programs on the frequency of mutants resistant to SDHI and quinone outside inhibitors fungicides were studied using qPCR assays. The experiments demonstrated that the use of anilinopyrimidine and demethylation inhibitors fungicides in the same program reduced the frequency of these mutations in Alternaria populations. The qPCR methods developed and used in this study can be used to track resistance levels in the pistachio orchards on a large scale.

Bacterial and fungal pathogens isolated from patients with bloodstream infection: frequency of occurrence and antimicrobial susceptibility patterns from the SENTRY Antimicrobial Surveillance Program (2012–2017)

The SENTRY Antimicrobial Surveillance Program has monitored bloodstream infections (BSIs) from patients in medical centers worldwide since 1997. In this report, we examine the frequency of occurrence and antimicrobial susceptibility profiles of 6741 bacterial and 222 fungal pathogens causing BSI in 16 medical centers from 2012 to 2017. These results were stratified according to patient age, intensive care unit (ICU) location, and hospital onset (HO) versus community onset (CO) of infection. The leading pathogen isolated from patients in all age groups (range, 20.3–32.5%), except for those >64 years old (19.9%), was Staphylococcus aureus. Escherichia coli was the most common agent in patients over 64 years of age (26.7%). S. aureus was frequently recovered from patients with HO or CO BSI (20.9–24.1%). However, E. coli was the most commonly isolated species (24.5%) from CO infections. BSIs caused by vancomycin-resistant enterococci, penicillin-nonsusceptible S. pneumoniae, extended-spectrum β-lactamase–producing Klebsiella spp., carbapenem-resistant Enterobacteriaceae, and multidrug-resistant Pseudomonas aeruginosa were more common among patients in ICUs compared to patients hospitalized in a non-ICU setting. The frequency of methicillin-resistant S. aureus (MRSA) was slightly higher in the non-ICU population (37.5%) compared with the ICU group (34.1%). A trend toward a decrease in BSIs due to Gram-positive cocci and an increase in infections with Gram-negative bacilli were observed. Overall, the frequency of resistant phenotypes was high for S. aureus (MRSA; 37.0%), enterococci (vancomycin-resistant enterococci; 24.6%), Klebsiella spp. (extended-spectrum β-lactamase phenotype; 21.5%), and P. aeruginosa (multidrug-resistant; 15.4%) and generally declined from 2012 to 2017, whereas the frequency of penicillin-nonsusceptible Streptococcus pneumoniae (3.4%) and carbapenem-resistant Enterobacteriaceae (1.5%) was low overall and both resistant phenotypes declined over time. Fluconazole-resistant Candida spp. isolates were only detected in years 2013–2015.

Predicting Antibiotic Resistance in Urinary Tract Infection Patients with Prior Urine Cultures.

To improve antibiotic prescribing, we sought to establish the probability of a resistant organism in urine culture given a previous resistant culture in a setting endemic for multidrug-resistant (MDR) organisms. We performed a retrospective analysis of inpatients with paired positive urine cultures. We focused on ciprofloxacin-resistant (cipro(r)) Gram-negative bacteria, extended-spectrum-beta-lactamase (ESBL)-producing Enterobacteriaceae, carbapenem-resistant Enterobacteriaceae (CRE), and carbapenem-resistant nonfermenters (CRNF). Comparisons were made between the frequency of each resistance phenotype following a previous culture with the same phenotype and the overall frequency of that phenotype, and odds ratios (ORs) were calculated. We performed a regression to assess the effects of other variables on the likelihood of a repeat resistant culture. A total of 4,409 patients (52.5% women; median age, 70 years) with 19,546 paired positive urine cultures were analyzed. The frequencies of cipro(r) bacteria, ESBL-producing Enterobacteriaceae, CRE, and CRNF among all cultures were 47.7%, 30.6%, 1.7%, and 2.6%, respectively. ORs for repeated resistance phenotypes were 1.87, 3.19, 48.25, and 19.02 for cipro(r) bacteria, ESBL-producing Enterobacteriaceae, CRE, and CRNF, respectively (P < 0.001 for all). At 1 month, the frequencies of repeated resistance phenotypes were 77.4%, 66.4%, 57.1%, and 33.3% for cipro(r) bacteria, ESBL-producing Enterobacteriaceae, CRE, and CRNF, respectively. Increasing time between cultures and the presence of an intervening nonresistant culture significantly reduced the chances of a repeat resistant culture. Associations were statistically significant over the duration of follow-up (60 months) for CRE and for up to 6 months for all other pathogens. Knowledge of microbiology results in the six preceding months may assist with antibiotic stewardship and improve the appropriateness of empirical treatment for urinary tract infections (UTIs).

Prevalence and potential fitness cost of weak phosphine resistance in Tribolium castaneum (Herbst) in eastern Australia

The prevalence of resistance to phosphine in the rust-red flour beetle, Tribolium castaneum, from eastern Australia was investigated, as well as the potential fitness cost of this type of resistance. Discriminating dose tests on 115 population samples collected from farms from 2006 to 2010 showed that populations containing insects with the weakly resistant phenotype are common in eastern Australia (65.2% of samples), although the frequency of resistant phenotypes within samples was typically low (median of 2.3%). The population cage approach was used to investigate the possibility that carrying the alleles for weak resistance incurs a fitness cost. Hybridized populations were initiated using a resistant strain and either of two different susceptible strains. There was no evidence of a fitness cost based on the frequency of susceptible phenotypes in hybridized populations that were reared for seven generations without exposure to phosphine. This suggests that resistant alleles will tend to persist in field populations that have undergone selection even if selection pressure is removed. The prevalence of resistance is a warning that this species has been subject to considerable selection pressure and that effective resistance management practices are needed to address this problem. The resistance prevalence data also provide a basis against which to measure management success.

Diversity in the erg27 Gene of Botrytis cinerea Field Isolates from Strawberry Defines Different Levels of Resistance to the Hydroxyanilide Fenhexamid.

The hydroxyanilide (Hyd) fenhexamid has played a major role in gray mold management in Florida strawberry fields since 2000. Recent monitoring of the sensitivity of Botrytis cinerea to fenhexamid indicated that resistance has emerged. In this study, mutations in the target gene erg27 encoding the 3-keto reductase enzyme were investigated and the shift in fenhexamid sensitivity over time was evaluated in 630 isolates collected between 2005 and 2013 from locations sprayed with different spray programs. Overall, 227, 155, 48, and 200 isolates were sensitive (HydS), had reduced sensitivity (HydR2), or were moderately (HydR3-) and highly (HydR3+) resistant, respectively. Analysis of complete sequences of the erg27 gene from 70 isolates revealed seven and five mutations and one deletion in the HydR3- and HydR3+ isolates, respectively, at eight and five different codons, respectively. In addition to the three mutations (F412S, -I, and -V) reported previously at codon 412, two new mutations from glycine to arginine at codon 170 (G170R, two isolates) and from alanine to glycine at codon 210 (A210G, eight isolates) were detected for the first time in HydR3+ isolates from Florida. These isolates were not controlled on detached fruit sprayed with the recommended field rate of fenhexamid, whereas all HydS, HydR2, and HydR3- isolates were controlled on detached fruit. Overall, there was no clear correlation between the spray frequency of fenhexamid and the frequency of resistant phenotypes. This study provides an overview of the current distribution of erg27 genotypes in Florida and will serve as a baseline for future studies on shifts in population diversity and resistance. The frequency of fenhexamid-resistant populations has increased progressively in different strawberry fields but has not reached a plateau yet, indicating that the effective life of fenhexamid could be extended if appropriate rotation and management strategies are implemented. Sensitivity to other sterol biosynthesis inhibitors is discussed.

Phosphine resistance in Sitophilus oryzae (L.) from eastern Australia: Inheritance, fitness and prevalence

The inheritance and fitness of phosphine resistance was investigated in an Australian strain of the rice weevil, Sitophilus oryzae (L.), as well as its prevalence in eastern Australia. This type of knowledge may provide insights in to the development of phosphine resistance in this species with the potential for better management. This strain was 12.2 × resistant at the LC50 level based on results for adults exposed for 20 h. Data from the testing of F1 adults from the reciprocal crosses (R♀ × S♂ and S♀ × R♂) showed that resistance was autosomal and inherited as an incompletely recessive trait with a degree of dominance of −0.88. The dose–response data for the F1 × S and F1 × R test crosses, and the F2 progeny were compared with predicted dose–response assuming monogenic recessive inheritance, and the results were consistent with resistance being conferred by one major gene. There was no evidence of fitness cost based on the frequency of susceptible phenotypes in hybridized populations that were reared for seven generations without exposure to phosphine. Lack of fitness cost suggests that resistant alleles will tend to persist in field populations that have undergone selection even if selection pressure is removed. Discriminating dose tests on 107 population samples collected from farms from 2006 to 2010 show that populations containing insects with the weak resistant phenotype are common in eastern Australia, although the frequency of resistant phenotypes within samples was typically low. The prevalence of resistance is a warning that this species has been subject to considerable selection pressure and that effective resistance management practices are needed to address this problem.

Determinants of fenhexamid effectiveness against grey mould on grapevine: Respective role of spray timing, fungicide resistance and plant defences

Botrytis bunch rot of grapes is mainly controlled by applying fungicides at three crop growth stages: the end of flowering (BBCH 68), bunch closure (BBCH 77) and the beginning of veraison (BBCH 81). The hydroxyanilide derivative fenhexamid is among the most effective fungicides registered to control Botrytis cinerea. Its effectiveness was examined in relation to spray timing, fungicide resistance and defense responses of grapevine. Overall, the earlier fenhexamid was applied, the more effective it was at controlling B. cinerea. Frequencies of B. cinerea strains which were resistant to fungicides were evaluated at harvest. The frequencies of resistant phenotypes were similar among treatments and years with the exception of a class of multidrug resistant strain (MDR 2) whose frequency appeared to increase after fenhexamid applications. If current spray programs including fenhexamid appear to control bunch rot at the current MDR frequency, a propagation of MDR 2 strains might lead to a decline in disease control. Finally, defense responses were studied in grapevine flowers/berries following fenhexamid application. None of the defense processes tested was induced in flowers/berries at stages 68 and 77. Only an increase in chitinase activity was observed in treated-berries at stage 81, suggesting that fenhexamid effectiveness was not related to a stimulation of defense responses.

Influence of genetic background and heterozygosity on meiotic recombination in Arabidopsis thaliana.

Plant breeding relies on genetic variability generated by meiotic recombination. Control of recombination frequencies is not yet possible, but would significantly extend the options for plant-breeding strategies. A prerequisite would be variability of recombination frequencies. In this study, 15 transgenic kanamycin (KR) and hygromycin (HR) resistance gene insertions mapping to the five Arabidopsis thaliana chromosomes were used as genetic markers. Recombination frequencies were determined from the frequencies of resistance phenotypes within populations segregating for linked KR and HR markers. Recombination frequencies of marker pairs were compared among these four ecotypes, among F1s in both reciprocal forms derived from these ecotypes, and between F1s and their parent lines. On average, the recombination frequencies in F1 crosses were substantially higher (up to 2-fold) than in the homozygous parental ecotypes. A strong negative correlation between genetic similarities of ecotypes and recombination frequencies was detected for two adjacent marker pairs located on the long arm of chromosome 3, but not for marker pairs in other genomic regions. Our results suggest that heterozygosity influences recombination in plant breeding, and cannot be ignored in genetic mapping of genomes.

Population biology of the saw-toothed grain beetle, Oryzaephilus surinamensis (Coleoptera: Silvanidae), in an experimental model of a fabric treatment

AbstractThe response of populations of the saw-toothed grain beetle, Oryzaephilus surinamensis (Linnaeus) (7022/1 strain), to insecticide treatments of different extent and standard concentration was studied over 16 weeks (no more than two generations) in experimental bins containing a wheat bulk and 16 kibbled wheat-filled refuges. Populations comprised initially low frequencies of resistant phenotypes, as would occur in natural populations where resistant alleles had newly arisen. The degree of control achieved was related to the extent of treatment. Distribution of adult beetles was greatly influenced by the effects of insecticide treatment superimposed on intrinsic microhabitat preferences. Live beetles were recovered from all bins, although numbers sampled from wheat bulks of treated bins were 1–6% of that of the untreated bin. More live beetles were sampled from wheat bulks than were recovered from all respective refuges, except in one treated bin. Live beetles were more numerous in refuges in corners and in those furthest from refuges on treated walls. Beetles were more likely to die in refuges in coiners and lower down in the bins or in refuges on treated walls adjacent to untreated walls. Although treatments produced heavy mortality, the frequencies of resistant individuals did not increase significantly. This could either be because treatments were equally successful in eliminating homozygous susceptible and heterozygous individuals, or because selection in favour of resistant individuals was too weak to be detectable, especially if there were negative correlations between viability and fecundity. Significant genetic structuring of refuge subpopulations with respect to the resistance phenotypes was noted and appeared to be related to behavioural preferences for microhabitat.

Field Measurements of Selection for European Red Mite (Acari: Tetranychidae) Resistance to Dicofol in Apple Orchards

Mortality of Panonychus ulmi (Koch) susceptible and resistant to dicofol was measured in apple trees to estimate the selective effect of dicofol applications on mite populations. Mortality of P. ulmi i from a susceptible (0% survivors in discriminating dose bioassay) and a resistant (70% survivors in discriminating dose bioassay) population were compared in a field bioassay. With conventional application procedures used in apple orchards, mites from the resistant population had 50% mortality; those from the susceptible population had 95% mortality. Frequencies of resistant phenotypes (RP) of P. ulmi were measured in five commercial orchards before and after treatment with dicofol, and one year after treatment. Frequencies before treatment averaged 7 and 3%, respectively, in dicofoltreated and untreated control plots. Two weeks after treatment, RP frequencies averaged 32% (dicofol) and 8% (control); 1 yr after treatment, RP frequencies were 41%(dicofol) and 14% (control). The measurements of selection resulting from single dicofol applications are discussed within the context of establishment of appropriate acaricide rotations for control of spider mites in New York apple orchards.

Expression of Resistance to Pyrethroid Insecticides in Adults and Larvae of Tobacco Budworm (Lepidoptera: Noctuidae): Implications for Resistance Monitoring

Resistance to pyrethroid insecticides was diagnosed in Heliothis virescens (F.) from cotton fields in Texas in 1985 and in Mississippi in 1986. Susceptible third instars on cotton terminals sprayed with recommended rates of pyrethroids suffered 90-95% mortality; however, only 20-71% of the field-collected, resistant larvae died. We confirmed the presence of pyrethroid resistance in Texas in 1985. Topical application bioassays in the laboratory failed to show more than sixfold resistance levels in larvae of some of these strains from Texas and Mississippi, indicating that standard topical bioassays do not always accurately diagnose resistance in this species. However, the frequency of resistant phenotypes in the cotton terminal assays and results with an adult bioassay technique were highly correlated.

Selection for metalaxyl resistance in potato crops infected with Phytophthora infestans: effects of fungicides and initial frequency of resistant sporangia

Potato crops (cv. Alpha) were grown in walk‐in plastic tunnels (33 m2) in winter 1987 and spring 1988. Crops were left untreated or treated (four sprays at intervals of 2 weeks) with either metalaxyl, Ridomil‐MZ (metalaxyl+ mancozeb) or Ridocym (metalaxyl +mancozeb+cymoxanil). Crops were inoculated with sporangial suspensions of Phytophthora infestans containing 01, 1 or 10% metalaxyl‐resistant sporangia. Disease progress and percentage resistant sporangia in tunnels were estimated for a period of 50 days. In the treated crops cumulative disease, rate of disease progress and increase in frequency of resistant phenotypes (selection coefficient) were higher in metalaxyl‐treated crops than in Ridomil‐MZ or Ridocym‐treated crops. Cumulative disease, rate of disease progress and increase in frequency of resistant phenotypes were unaffected by the frequency of resistant sporangia in the original inoculum. Metalaxyl, Ridomil‐MZ and Ridocym speeded up the selection for resistance by 92, 29 and 17%, respectively, relative to untreated crops. The time interval required for the resistant subpopulation to predominate in treated crops was inversely related to initial resistance. It was longest in crops inoculated with 0.1 % resistant sporangia and treated with Ridocym. Results indicated that metalaxyl mixtures were advantageous over metalaxyl alone, both in delaying the buildup of resistance and in improving disease control, but did not prevent resistant subpopulations increasing from 01 to 100% in a single season.

Laboratory and Field Investigations of Spider Mite (Acari: Tetranychidae) Resistance to the Selective Acaricide Propargite

Evaluations of spider mite, Tetranychus spp., susceptibility to propargite, initiated in response to grower concerns about potential resistance in the field, revealed 0- to 42-fold resistance in Tetranychus urticae Koch in a whole-plant residual bioassay. Surveys of spider mites collected from throughout the San Joaquin Valley in June and August 1984 indicated that propargite-resistant phenotypes exist throughout the San Joaquin Valley; 22 of 32 sites sampled in August contained individuals that survived 1,000 ppm in residual bioassays. However, resistant phenotypes were much less common in June than August; only 3 of 23 sites sampled in June had survivors of 1,000 ppm. The shift in susceptibility of populations during the season was shown to result from intraspecific increases in frequency of resistant phenotypes or interspecific shifts in spider mite species abundance, or both. Although resistant phenotypes of T. urticae and Tetranychus pacificus McGregor were discovered, no propargite-resistant Tetranychus turkestani Ugarov and Nikolski were found. Widely differing susceptibilities to propargite exist both within and between T. urticae and T. pacificus populations. A dicofol-resistant laboratory culture of T. urticae originating from the San Joaquin Valley and maintained under pressure for 2 yr possessed a 26-fold resistance to propargite. Although resistance to propargite was discovered in the laboratory, we did not observe diminished performance of propargite in field trials at a location possessing spider mites with up to 45% survivorship of the discriminating concentration of 1,000 ppm propargite.