Environmental and genetic influence on the rate and spectrum of spontaneous mutations in Escherichia coli

Spontaneous mutations are the ultimate source of novel genetic variation on which evolution operates. Although mutation rate is often discussed as a single parameter in evolution, it comprises multiple distinct types of changes at the level of DNA. Moreover, the rates of these distinct changes can be independently influenced by genomic background and environmental conditions. Using fluctuation tests, we characterized the spectrum of spontaneous mutations in Escherichia coli grown in low and high glucose environments. These conditions are known to affect the rate of spontaneous mutation in wild-type MG1655, but not in a ΔluxS deletant strain - a gene with roles in both quorum sensing and the recycling of methylation products used in E. coli's DNA repair process. We find an increase in AT>GC transitions in the low glucose environment, suggesting that processes relating to the production or repair of this mutation could drive the response of overall mutation rate to glucose concentration. Interestingly, this increase in AT>GC transitions is maintained by the glucose non-responsive ΔluxS deletant. Instead, an elevated rate of GC>TA transversions, more common in a high glucose environment, leads to a net non-responsiveness of overall mutation rate for this strain. Our results show how relatively subtle changes, such as the concentration of a carbon substrate or loss of a regulatory gene, can substantially influence the amount and nature of genetic variation available to selection.

Eco-evolutionary dynamics of experimental Pseudomonas aeruginosa populations under oxidative stress

Within-host environments are likely to present a challenging and stressful environment for opportunistic pathogenic bacteria colonizing from the external environment. How populations of pathogenic bacteria respond to such environmental challenges and how this varies between strains is not well understood. Oxidative stress is one of the defences adopted by the human immune system to confront invading bacteria. In this study, we show that strains of the opportunistic pathogenic bacterium Pseudomonas aeruginosa vary in their eco-evolutionary responses to hydrogen peroxide stress. By quantifying their 24 h growth kinetics across hydrogen peroxide gradients we show that a transmissible epidemic strain isolated from a chronic airway infection of a cystic fibrosis patient, LESB58, is much more susceptible to hydrogen peroxide than either of the reference strains, PA14 or PAO1, with PAO1 showing the lowest susceptibility. Using a 12 day serial passaging experiment combined with a mathematical model, we then show that short-term susceptibility controls the longer-term survival of populations exposed to subinhibitory levels of hydrogen peroxide, but that phenotypic evolutionary responses can delay population extinction. Our model further suggests that hydrogen peroxide driven extinctions are more likely with higher rates of population turnover. Together, these findings suggest that hydrogen peroxide is likely to be an effective defence in host niches where there is high population turnover, which may explain the counter-intuitively high susceptibility of a strain isolated from chronic lung infection, where such ecological dynamics may be slower.

Mutators can drive the evolution of multi-resistance to antibiotics

Antibiotic combination therapies are an approach used to counter the evolution of resistance; their purported benefit is they can stop the successive emergence of independent resistance mutations in the same genome. Here, we show that bacterial populations with 'mutators', organisms with defects in DNA repair, readily evolve resistance to combination antibiotic treatment when there is a delay in reaching inhibitory concentrations of antibiotic-under conditions where purely wild-type populations cannot. In populations of Escherichia coli subjected to combination treatment, we detected a diverse array of acquired mutations, including multiple alleles in the canonical targets of resistance for the two drugs, as well as mutations in multi-drug efflux pumps and genes involved in DNA replication and repair. Unexpectedly, mutators not only allowed multi-resistance to evolve under combination treatment where it was favoured, but also under single-drug treatments. Using simulations, we show that the increase in mutation rate of the two canonical resistance targets is sufficient to permit multi-resistance evolution in both single-drug and combination treatments. Under both conditions, the mutator allele swept to fixation through hitch-hiking with single-drug resistance, enabling subsequent resistance mutations to emerge. Ultimately, our results suggest that mutators may hinder the utility of combination therapy when mutators are present. Additionally, by raising the rates of genetic mutation, selection for multi-resistance may have the unwanted side-effect of increasing the potential to evolve resistance to future antibiotic treatments.

Competition delays multi-drug resistance evolution during combination therapy

Combination therapies have shown remarkable success in preventing the evolution of resistance to multiple drugs, including HIV, tuberculosis, and cancer. Nevertheless, the rise in drug resistance still remains an important challenge. The capability to accurately predict the emergence of resistance, either to one or multiple drugs, may help to improve treatment options. Existing theoretical approaches often focus on exponential growth laws, which may not be realistic when scarce resources and competition limit growth. In this work, we study the emergence of single and double drug resistance in a model of combination therapy of two drugs. The model describes a sensitive strain, two types of single-resistant strains, and a double-resistant strain. We compare the probability that resistance emerges for three growth laws: exponential growth, logistic growth without competition between strains, and logistic growth with competition between strains. Using mathematical estimates and numerical simulations, we show that between-strain competition only affects the emergence of single resistance when resources are scarce. In contrast, the probability of double resistance is affected by between-strain competition over a wider space of resource availability. This indicates that competition between different resistant strains may be pertinent to identifying strategies for suppressing drug resistance, and that exponential models may overestimate the emergence of resistance to multiple drugs. A by-product of our work is an efficient strategy to evaluate probabilities of single and double resistance in models with multiple sequential mutations. This may be useful for a range of other problems in which the probability of resistance is of interest.

Mathematical modelling for antibiotic resistance control policy: do we know enough?

BACKGROUND

Antibiotics remain the cornerstone of modern medicine. Yet there exists an inherent dilemma in their use: we are able to prevent harm by administering antibiotic treatment as necessary to both humans and animals, but we must be mindful of limiting the spread of resistance and safeguarding the efficacy of antibiotics for current and future generations. Policies that strike the right balance must be informed by a transparent rationale that relies on a robust evidence base.

MAIN TEXT

One way to generate the evidence base needed to inform policies for managing antibiotic resistance is by using mathematical models. These models can distil the key drivers of the dynamics of resistance transmission from complex infection and evolutionary processes, as well as predict likely responses to policy change in silico. Here, we ask whether we know enough about antibiotic resistance for mathematical modelling to robustly and effectively inform policy. We consider in turn the challenges associated with capturing antibiotic resistance evolution using mathematical models, and with translating mathematical modelling evidence into policy.

CONCLUSIONS

We suggest that in spite of promising advances, we lack a complete understanding of key principles. From this we advocate for priority areas of future empirical and theoretical research.

Measuring Microbial Mutation Rates with the Fluctuation Assay

Fluctuation assays are widely used for estimating mutation rates in microbes growing in liquid environments. Many cultures are each inoculated with a few thousand cells, each sensitive to a selective marker that can be assayed phenotypically. These parallel cultures grow for many generations in the absence of the phenotypic marker. A subset of cultures is used to estimate the total number of cells at risk of mutations (i.e., the population size at the end of the growth period, or Nt). The remaining cultures are plated onto the selective agar. The distribution of observed resistant mutants among parallel cultures is then used to estimate the expected number of mutational events, m, using a mathematical model. Dividing m by Nt gives the estimate of the mutation rate per locus per generation. The assay has three critical aspects: the chosen phenotypic marker, the chosen volume of parallel cultures, and ensuring that the surface on the selective agar is completely dry before the incubation. The assay is relatively inexpensive and only needs standard laboratory equipment. It is also less laborious than alternative approaches, such as mutation accumulation and single-cell assays. The assay works on organisms that go through many generations rapidly and it depends on assumptions about the fitness effects of markers and cell death. However, recently developed tools and theoretical studies mean these issues can now be addressed analytically. The assay allows mutation rate estimation of different phenotypic markers in cells with different genotypes growing in isolation or in a community. By conducting multiple assays in parallel, assays can be used to study how an organism's environmental context affects spontaneous mutation rate, which is crucial for understanding antimicrobial resistance, carcinogenesis, aging, and evolution.

Divergent evolution peaks under intermediate population bottlenecks during bacterial experimental evolution

There is growing evidence that parallel molecular evolution is common, but its causes remain poorly understood. Demographic parameters such as population bottlenecks are predicted to be major determinants of parallelism. Here, we test the hypothesis that bottleneck intensity shapes parallel evolution by elucidating the genomic basis of adaptation to antibiotic-supplemented media in hundreds of populations of the bacterium Pseudomonas fluorescens Pf0-1. As expected, bottlenecking decreased the rate of phenotypic and molecular adaptation. Surprisingly, bottlenecking had no impact on the likelihood of parallel adaptive molecular evolution at a genome-wide scale. However, bottlenecking had a profound impact on the genes involved in antibiotic resistance. Specifically, under either intense or weak bottlenecking, resistance predominantly evolved by strongly beneficial mutations which provide high levels of antibiotic resistance. In contrast with intermediate bottlenecking regimes, resistance evolved by a greater diversity of genetic mechanisms, significantly reducing the observed levels of parallel genetic evolution. Our results demonstrate that population bottlenecking can be a major predictor of parallel evolution, but precisely how may be more complex than many simple theoretical predictions.

Spontaneous mutation rate is a plastic trait associated with population density across domains of life

Rates of random, spontaneous mutation can vary plastically, dependent upon the environment. Such plasticity affects evolutionary trajectories and may be adaptive. We recently identified an inverse plastic association between mutation rate and population density at 1 locus in 1 species of bacterium. It is unknown how widespread this association is, whether it varies among organisms, and what molecular mechanisms of mutagenesis or repair are required for this mutation-rate plasticity. Here, we address all 3 questions. We identify a strong negative association between mutation rate and population density across 70 years of published literature, comprising hundreds of mutation rates estimated using phenotypic markers of mutation (fluctuation tests) from all domains of life and viruses. We test this relationship experimentally, determining that there is indeed density-associated mutation-rate plasticity (DAMP) at multiple loci in both eukaryotes and bacteria, with up to 23-fold lower mutation rates at higher population densities. We find that the degree of plasticity varies, even among closely related organisms. Nonetheless, in each domain tested, DAMP requires proteins scavenging the mutagenic oxidised nucleotide 8-oxo-dGTP. This implies that phenotypic markers give a more precise view of mutation rate than previously believed: having accounted for other known factors affecting mutation rate, controlling for population density can reduce variation in mutation-rate estimates by 93%. Widespread DAMP, which we manipulate genetically in disparate organisms, also provides a novel trait to use in the fight against the evolution of antimicrobial resistance. Such a prevalent environmental association and conserved mechanism suggest that mutation has varied plastically with population density since the early origins of life.

Multicopy plasmids potentiate the evolution of antibiotic resistance in bacteria

Plasmids are thought to play a key role in bacterial evolution by acting as vehicles for horizontal gene transfer, but the role of plasmids as catalysts of gene evolution remains unexplored. We challenged populations of Escherichia coli carrying the bla_TEM-1 β-lactamase gene on either the chromosome or a multicopy plasmid (19 copies per cell) with increasing concentrations of ceftazidime. The plasmid accelerated resistance evolution by increasing the rate of appearance of novel TEM-1 mutations, thereby conferring resistance to ceftazidime, and then by amplifying the effect of TEM-1 mutations due to the increased gene dosage. Crucially, this dual effect was necessary and sufficient for the evolution of clinically relevant levels of resistance. Subsequent evolution occurred by mutations in a regulatory RNA that increased the plasmid copy number, resulting in marginal gains in ceftazidime resistance. These results uncover a role for multicopy plasmids as catalysts for the evolution of antibiotic resistance in bacteria.

Epistatic interactions between ancestral genotype and beneficial mutations shape evolvability in Pseudomonas aeruginosa

The idea that interactions between mutations influence adaptation by driving populations to low and high fitness peaks on adaptive landscapes is deeply ingrained in evolutionary theory. Here, we investigate the impact of epistasis on evolvability by challenging populations of two Pseudomonas aeruginosa clones bearing different initial mutations (in rpoB conferring rifampicin resistance, and the type IV pili gene network) to adaptation to a medium containing l-serine as the sole carbon source. Despite being initially indistinguishable in fitness, populations founded by the two ancestral genotypes reached different fitness following 300 generations of evolution. Genome sequencing revealed that the difference could not be explained by acquiring mutations in different targets of selection; the majority of clones from both ancestors converged on one of the following two strategies: (1) acquiring mutations in either PA2449 (gcsR, an l-serine-metabolism RpoN enhancer binding protein) or (2) protease genes. Additionally, populations from both ancestors converged on loss-of-function mutations in the type IV pili gene network, either due to ancestral or acquired mutations. No compensatory or reversion mutations were observed in RNA polymerase (RNAP) genes, in spite of the large fitness costs typically associated with mutations in rpoB. Although current theory points to sign epistasis as the dominant constraint on evolvability, these results suggest that the role of magnitude epistasis in constraining evolvability may be underappreciated. The contribution of magnitude epistasis is likely to be greatest under the biologically relevant mutation supply rates that make back mutations probabilistically unlikely.

Parasite diversity drives rapid host dynamics and evolution of resistance in a bacteria-phage system

Host–parasite evolutionary interactions are typically considered in a pairwise species framework. However, natural infections frequently involve multiple parasites. Altering parasite diversity alters ecological and evolutionary dynamics as parasites compete and hosts resist multiple infection. We investigated the effects of parasite diversity on host–parasite population dynamics and evolution using the pathogen Pseudomonas aeruginosa and five lytic bacteriophage parasites. To manipulate parasite diversity, bacterial populations were exposed for 24 hours to either phage monocultures or diverse communities containing up to five phages. Phage communities suppressed host populations more rapidly but also showed reduced phage density, likely due to interphage competition. The evolution of resistance allowed rapid bacterial recovery that was greater in magnitude with increases in phage diversity. We observed no difference in the extent of resistance with increased parasite diversity, but there was a profound impact on the specificity of resistance; specialized resistance evolved to monocultures through mutations in a diverse set of genes. In summary, we demonstrate that parasite diversity has rapid effects on host–parasite population dynamics and evolution by selecting for different resistance mutations and affecting the magnitude of bacterial suppression and recovery. Finally, we discuss the implications of phage diversity for their use as biological control agents.

Environmental variation alters the fitness effects of rifampicin resistance mutations in Pseudomonas aeruginosa

The fitness effects of antibiotic resistance mutations in antibiotic-free conditions play a key role in determining the long-term maintenance of resistance. Although resistance is usually associated with a cost, the impact of environmental variation on the cost of resistance is poorly understood. Here, we test the impact of heterogeneity in temperature and resource availability on the fitness effects of antibiotic resistance using strains of the pathogenic bacterium Pseudomonas aeruginosa carrying clinically important rifampicin resistance mutations. Although the rank order of fitness was generally maintained across environments, fitness effects relative to the wild type differed significantly. Changes in temperature had a profound impact on the fitness effects of resistance, whereas changes in carbon substrate had only a weak impact. This suggests that environmental heterogeneity may influence whether the costs of resistance are likely to be ameliorated by second-site compensatory mutations or by reversion to wild-type rpoB. Our results highlight the need to consider environmental heterogeneity and genotype-by-environment interactions for fitness in models of resistance evolution.

Evolutionary reversals of antibiotic resistance in experimental populations of Pseudomonas aeruginosa

Antibiotic resistance mutations are accompanied by a fitness cost, and two mechanisms allow bacteria to adapt to this cost once antibiotic use is halted. First, it is possible for resistance to revert; second, it is possible for bacteria to adapt to the cost of resistance by compensatory mutations. Unfortunately, reversion to antibiotic sensitivity is rare, but the underlying factors that prevent reversion remain obscure. Here, we directly study the evolutionary dynamics of reversion by experimentally mimicking reversion mutations-sensitives-in populations of rifampicin-resistant Pseudomonas aeruginosa. We show that, in our populations, most sensitives are lost due to genetic drift when they are rare. However, clonal interference from lineages carrying compensatory mutations causes a dramatic increase in the time to fixation of sensitives that escape genetic drift, and mutations surpassing the sensitives' fitness are capable of driving transiently common sensitive lineages to extinction. Crucially, we show that the constraints on reversion arising from clonal interference are determined by the potential for compensatory adaptation of the resistant population. Although the cost of resistance provides the incentive for reversion, our study demonstrates that both the cost of resistance and the intrinsic evolvability of resistant populations interact to determine the rate and likelihood of reversion.

Modelling colony population growth in the filamentous fungus Aspergillus nidulans

Filamentous fungi are ubiquitous in nature and have high societal significance, being both major (food-borne) pathogens and important industrial organisms in the production of antibiotics and enzymes. In addition, fungi are important model organisms for fundamental research, such as studies in genetics and evolutionary biology. However, mechanistic models for population growth that would help understand fungal biology and fundamental processes are almost entirely missing. Here we present such a mechanistic model for the species Aspergillus nidulans as an exemplar of models for other filamentous fungi. The model is based on physiological parameters that influence colony growth, namely mycelial growth rate and sporulation rate, to predict the number of individual nuclei present in a colony through time. Using population size data for colonies of differing ages, we find that our mechanistic model accurately predicts the number of nuclei for two growth environments, and show that fungal population size is most dependent on changes in mycelial growth rate.

Model and test in a fungus of the probability that beneficial mutations survive drift

Determining the probability of fixation of beneficial mutations is critically important for building predictive models of adaptive evolution. Despite considerable theoretical work, models of fixation probability have stood untested for nearly a century. However, recent advances in experimental and theoretical techniques permit the development of models with testable predictions. We developed a new model for the probability of surviving genetic drift, a major component of fixation probability, for novel beneficial mutations in the fungus Aspergillus nidulans, based on the life-history characteristics of its colony growth on a solid surface. We tested the model by measuring the probability of surviving drift in 11 adapted strains introduced into wild-type populations of different densities. We found that the probability of surviving drift increased with mutant invasion fitness, and decreased with wild-type density, as expected. The model accurately predicted the survival probability for the majority of mutants, yielding one of the first direct tests of the extinction probability of beneficial mutations.

The length of adaptive walks is insensitive to starting fitness in Aspergillus nidulans

Adaptation involves the successive substitution of beneficial mutations by selection, a process known as an adaptive walk. Gradualist models of adaptation, which assume that all mutations are small relative to the distance to a fitness optimum, predict that adaptive walks should be longer when the founding genotype is less well adapted. More recent work modeling adaptation as a sequence of moves in phenotype or genotype space predicts, by contrast, much shorter adaptive walks irrespective of the fitness of the founding genotype. Here, we provide what is, to the best of our knowledge, the first direct test of these alternative models, measuring the length of adaptive walks in evolving lineages of fungus that differ initially in fitness. Contrary to the gradualist view, we show that the length of adaptive walks in the fungus Aspergillus nidulans is insensitive to starting fitness and involves just two mutations on average. This arises because poorly adapted populations tend to fix mutations of larger average effect than those of better-adapted populations. Our results suggest that the length of adaptive walks may be independent of the fitness of the founding genotype and, moreover, that poorly adapted populations can quickly adapt to novel environments.

The properties of adaptive walks in evolving populations of fungus

The rarity of beneficial mutations has frustrated efforts to develop a quantitative theory of adaptation. Recent models of adaptive walks, the sequential substitution of beneficial mutations by selection, make two compelling predictions: adaptive walks should be short, and fitness increases should become exponentially smaller as successive mutations fix. We estimated the number and fitness effects of beneficial mutations in each of 118 replicate lineages of Aspergillus nidulans evolving for approximately 800 generations at two population sizes using a novel maximum likelihood framework, the results of which were confirmed experimentally using sexual crosses. We find that adaptive walks do indeed tend to be short, and fitness increases become smaller as successive mutations fix. Moreover, we show that these patterns are associated with a decreasing supply of beneficial mutations as the population adapts. We also provide empirical distributions of fitness effects among mutations fixed at each step. Our results provide a first glimpse into the properties of multiple steps in an adaptive walk in asexual populations and lend empirical support to models of adaptation involving selection towards a single optimum phenotype. In practical terms, our results suggest that the bulk of adaptation is likely to be accomplished within the first few steps.

Design and acceptability of patient-oriented computerized diabetes care reminders for use at the point of care

Despite the widespread availability of guidelines for caring for patients with diabetes and decades of research on computerized reminder systems, large gaps in quality remain in diabetes care remain and computerized reminder systems are rarely used for patients with diabetes. We set out to develop and test the feasibility of a system that would overcome many of the barriers preventing the widespread use of point-of-care computerized reminders to improve diabetes care. Five primary care physicians and 32 patients with type 2 diabetes pilot tested the system. We set out to design and measure the preliminary acceptability of patient-oriented point of care computerized diabetes care reminders. The main findings of our study were that (1) the reports were well accepted by both patients and providers and (2) survey and audiotape data suggest that they may be helpful at improving the quality of outpatient care for patients with diabetes.

Do malpractice concerns, payment mechanisms, and attitudes influence test-ordering decisions?

Greater understanding is needed of nonclinical factors that determine neurologists' decisions to order tests. The authors surveyed 595 US neurologists and utilized demographic information, attitude scales, and clinical scenarios to evaluate the influence of nonclinical factors on test-ordering decisions. Greater test reliance, higher malpractice concerns, and receiving reimbursement for testing were all associated with a higher likelihood of test ordering. These findings have implications for training needs and suggest malpractice worries may inflate health care costs.

Systematic review of clinical prediction rules for neuroimaging in the evaluation of dementia

BACKGROUND

Clinical practice guidelines for dementia do not recommend routine neuroimaging but vary in their recommended clinical prediction rules to identify patients who should undergo neuroimaging for potentially reversible causes of dementia.

METHODS

Using a MEDLINE search supplemented by other strategies, we identified studies from January 1, 1983, through December 31, 1998, that evaluated the diagnostic performance of a clinical prediction rule. We calculated the sensitivity and specificity of each rule, then evaluated their diagnostic performance in a hypothetical cohort of 1000 patients with dementia, varying the prevalence of potentially reversible dementia from 1% to 15%.

RESULTS

We identified 7 studies that evaluated at least 1 of 6 different clinical prediction rules. Only one rule consistently had high sensitivity (>85%) across all studies; none consistently had high specificity (>85%). Six of the 7 studies included less than 15 cases of potentially reversible dementia; thus the sensitivity and specificity for each rule had relatively wide confidence intervals. At a 5% prevalence of potentially reversible dementia, all rules had low positive predictive value (<15%) in our hypothetical cohort. Depending on the rule, our analysis predicts 6 to 44 of the 50 patients with potentially reversible dementia (5% prevalence in cohort of 1000 patients) would not undergo imaging.

CONCLUSIONS

There is considerable uncertainty in the evidence underlying clinical prediction rules to identify which patients with dementia should undergo neuroimaging. Application of these rules may miss patients with potentially reversible causes of dementia.

Improving adherence to dementia guidelines through education and opinion leaders. A randomized, controlled trial

BACKGROUND

Educational methods that encourage physicians to adopt practice guidelines are needed.

OBJECTIVE

To evaluate an educational strategy to increase neurologists' adherence to specialty society-endorsed practice recommendations.

DESIGN

Randomized, controlled trial.

SETTING

Six urban regions in New York State.

PARTICIPANTS

417 neurologists.

INTERVENTION

The educational strategy promoted six recommendations for evaluation and management of dementia. It included a mailed American Academy of Neurology continuing medical education course, practice-based tools, an interactive evidence-based American Academy of Neurology-sponsored seminar led by local opinion leaders, and follow-up mailings.

MEASUREMENTS

Neurologists' adherence to guidelines was measured by using detailed clinical scenarios mailed to a baseline group 3 months before the intervention and to intervention and control groups 6 months after the intervention. In one region, patients' medical records were reviewed to determine concordance between neurologists' scenario responses and their actual care.

RESULTS

Compared with neurologists in the baseline and control groups, neurologists in the intervention group were more adherent to three of the six recommendations: neuroimaging for patients with dementia only when certain criteria are present (odds ratio, 4.1 [95% CI, 1.9 to 8.9]), referral of all patients with dementia and their families to the Alzheimer's Association (odds ratio, 2.8 [CI, 1.7 to 4.8]), and encouragement of all patients and their families to enroll in the Alzheimer's Association Safe Return Program (odds ratio, 10.8 [CI, 3.5 to 33.2]). For the other three recommendations, adherence did not differ between the intervention and the nonintervention groups. Agreement between scenario responses and actual care ranged from 27% to 99% for the six recommendations and was 95% or more for three of the recommendations.

CONCLUSION

A multifaceted educational program can improve physician adoption of practice guidelines.

A randomized trial to implement practice recommendations: design and methods of the Dementia Care Study

The objective of the Dementia Care Study was to design, implement, and evaluate, in a randomized controlled trial a multi-faceted, specialty-society sponsored intervention to encourage neurologists' adoption of practice recommendations. Eligible participants were 417 neurologists in six regions in New York State (NYS) who were identified through the American Academy of Neurology (AAN) Membership Database and the NYS Physician Masterfile. An Advisory Panel of experts on dementia, neurologists who were local opinion leaders, and local representatives of the Alzheimer's Association guided the development of the intervention. The intervention included six components: (1) a mailing of six practice recommendations in a course of continuing medical education (CME) sponsored by the AAN; (2) a mailing of supplementary, practice-based tools; (3) follow-up mailings reinforcing the recommendations; (4) an invitation to an AAN-sponsored seminar; (5) endorsement by opinion leaders; and (6) specialty-society sponsorship and endorsement. The primary outcome measure was neurologists' decision-making, as assessed through a mailed survey that used detailed clinical scenarios. Intervention and control neurologists received the survey six months after the intervention, and a baseline group received it three months prior to the intervention. To evaluate the concordance of responses to scenarios with actual processes of care, we reviewed medical records in one study region. Secondary outcome measures included number of patient referrals received by the local Alzheimer's Associations and by the Association's National Safe Return Program. The specialty society, the opinion leaders, the dementia experts, local advocacy groups, and the study investigators achieved a high degree of collaboration. Specialty societies can integrate within their educational programs the capability to design and evaluate the impact of novel strategies to encourage the adoption of practice recommendations that are linked to improved quality of care.

Survival after percutaneous endoscopic gastrostomy among older residents of Quebec

CONTEXT

Percutaneous endoscopic gastrostomy has become a mainstay of nutritional support for individuals with swallowing dysfunction. There is little population-based data to guide the use of this intervention in older individuals.

OBJECTIVE

To describe the use of percutaneous endoscopic gastrostomy among older residents of Quebec and to evaluate patient characteristics associated with subsequent survival and hospital discharge.

DESIGN

A population-based cohort study.

SETTING

Quebec, Canada.

PATIENTS

175 individuals with a billing claim for percutaneous endoscopic gastrostomy performed in 1993.

MEASUREMENTS

Billing and hospitalization databases were used to collect patient characteristics, medical diagnoses, discharge destinations, and dates of death. The relationships between demographic and diagnostic variables before gastrostomy, and subsequent survival and discharge home, were evaluated using survival analysis.

RESULTS

Median survival after gastrostomy was 210 days. Mortality at 30 days was 18.3%. Decreased survival was associated with a previous diagnosis of malignancy (risk ratio (RR) = 1.71; 95% CI, 1.09-2.68); mortality did not increase with increasing age. Of 163 individuals hospitalized at the time of gastrostomy, 42 (26%) were discharged home. Individuals with a previous diagnosis of stroke (RR = 2.80; 95% CI 1.01-7.77) were more likely to be discharged home than other individuals.

CONCLUSIONS

Survival after percutaneous endoscopic gastrostomy is poor; the requirement for such a procedure appears to be a marker for severe underlying disease. The greater likelihood of return home after gastrostomy among individuals with stroke suggests that the use of this intervention as an adjunct to rehabilitation is appropriate in these individuals.

Tacrine use in nursing homes: implications for prescribing new cholinesterase inhibitors. SAGE Study Group. Systematic Assessment of Geriatric Drug Use via Epidemiology

OBJECTIVE

To describe the use of tacrine in nursing home residents using data from a clinically based resident assessment instrument used by all US nursing homes.

METHODS

Data were from the Systematic Assessment of Geriatric Drug Use via Epidemiology (SAGE) database, a population-based data set with information on 329,520 patients admitted to all Medicare/Medicaid certified nursing homes in four US states (Maine, Mississippi, New York, and South Dakota) from 1992 through 1995. The SAGE database combines information from the Minimum Data Set (MDS) and the On-Line Survey and Certification Automated Record. We identified all residents receiving tacrine and up to five control residents per case matched on state, date of tacrine use, cognitive function, and dementia diagnosis.

RESULTS

A total of 1,640 (0.5%) nursing home residents received tacrine at least once. Only 38% of these residents had a diagnosis of AD documented on the MDS; regardless of dementia diagnosis, 25% had severe cognitive impairment, 35% were severely dependent in activities of daily living (ADL), and 17% had both severe cognitive and ADL impairment. Only 8% achieved a therapeutic dose of at least 120 mg/d. After adjusting for confounding variables, wandering and being physically abusive were the strongest predictors of tacrine use.

CONCLUSIONS

A minority of nursing home residents received tacrine. Of those who did, a significant proportion were unlikely to benefit from its use because of their level of cognitive and ADL impairment, or because low doses were used. As new medications become available for dementia, MDS data can be used by nursing homes to monitor the use of these therapies.

Curbside consultation practices and attitudes among primary care physicians and medical subspecialists

CONTEXT

Informal (curbside) consultations are an integral part of medical culture and may be of great value to patients and primary care physicians. However, little is known about physicians' behavior or attitudes toward curbside consultation.

OBJECTIVE

To describe and compare curbside consultation practices and attitudes among primary care physicians and medical subspecialists.

DESIGN

Survey mailed in June 1997.

PARTICIPANTS

Of 286 primary care physicians and 252 subspecialists practicing in Rhode Island, 213 primary care physicians and 200 subspecialists responded (response rate, 76.8%).

MAIN OUTCOME MEASURES

Self-reported practices of, reasons for, and attitudes about curbside consultation.

RESULTS

Of primary care physicians, 70.4% (150/213) and 87.5% (175/200) of subspecialists reported participating in at least 1 curbside consultation during the previous week. In the previous week, primary care physicians obtained 3.2 curbside consultations, whereas subspecialists received 3.6 requests for curbside consultations. Subspecialties most frequently involved in curbside consultations were cardiology, gastroenterology, and infectious diseases; subspecialties that were requested to provide curbside consultations more often than they were formally consulted were endocrinology, infectious diseases, and rheumatology. Curbside consultations were most often used to select appropriate diagnostic tests and treatment plans and to determine the need for formal consultation. Subspecialists perceived more often than primary care physicians that information communicated in curbside consultations was insufficient (80.2% vs 49.8%; P<.001) and that important clinical detail was not described (77.6% vs 43.5%; P<.001). More subspecialists than primary care physicians felt that curbside consultations were essential for maintaining good relationships with other physicians (77.2% vs 38.6%; P<.001).

CONCLUSIONS

Curbside consultation serves important functions in the practice of medicine. Despite the widespread use of curbside consultation, disagreement exists between primary care physicians and subspecialists as to the role of curbside consultation and the quality of the information exchanged.

Practice styles of US compared to UK neurologists

OBJECTIVE

This study assessed variation between neurologists in the United States and United Kingdom in their diagnostic and treatment decisions for commonly encountered neurologic presentations, and identified explanatory factors for any observed variation.

METHODS

All 210 consultant neurologists in the United Kingdom and a nationally representative sample of 595 US neurologists received mailed surveys containing three detailed clinical scenarios depicting patients with (1) a single unprovoked seizure occurring 3 days previously, (2) early Parkinson's disease, and (3) dementia. The main study outcome measures were self-reported decisions regarding diagnostic test ordering and treatment, which were assessed after each scenario. Neurologists' practice characteristics, certainty about the diagnosis, and attitudes toward uncertainty were also measured. Survey response rates were 92% of US and 63% of UK neurologists.

RESULTS

A higher proportion of US than UK neurologists indicated they would order additional diagnostic tests for all three scenarios (all p < 0.05); 77% of UK compared with 26% of US neurologists would manage a single unprovoked seizure without antiepileptic medication (p < 0.0001), but treatment of early Parkinson's disease was not different. Nearly all US and UK neurologists would obtain a neuroimaging study in the evaluation of dementia. International differences persisted after adjustment for differences in demographic and practice characteristics and for attitudes toward test use and clinical uncertainty.

CONCLUSIONS

We identified large international variation in clinical decisions across three common neurologic conditions. Cross-country collaboration should explore these differences to develop consensus on standards of care.

Can a specialty society educate its members to think differently about clinical decisions? Results of a randomized trial

OBJECTIVE

Measure the effect of specialty society-developed continuing medical education (CME) on clinical decision making.

DESIGN

Randomized controlled trial.

SETTING

National sample of neurologists.

PARTICIPANTS

Of 492 neurologists randomly selected from an ongoing American Academy of Neurology CME program, 248 were randomized to receive a mailed CME course, and 244 did not receive it.

INTERVENTION

A mailed educational course on movement disorders, developed by the specialty society, containing information on diseases and practice recommendations with illustrative case presentations.

MEASUREMENTS AND MAIN RESULTS

We assessed adherence to 16 practice recommendations on disease detection, diagnostic test use, and treatments by mailed survey sent to all subjects 4.5 months after the intervention group received the course (73% response rate). The survey contained detailed clinical scenarios to measure self-reported clinical decision making and short open-ended questions to measure factual knowledge. More intervention participants (up to 2.6 times more) than control subjects reported clinical decision making adherent to 9 of the 16 recommendations (p < .05). For 4 of the other 7 recommendations, adherence exceeded 85% in both groups. Within the intervention group, neurologists who read the educational course were 2 to 6 times more likely to be adherent than neurologists who did not. The intervention group had better factual knowledge than control subjects in six of seven areas (p < .01).

CONCLUSIONS

This educational course improved neurologists' reported decision making. Specialty society-developed CME that utilizes a similar format may enhance the effectiveness of mailed CME information to improve physicians' approach to clinical decisions.

Diagnostic reasoning in neurology

Studies examining physicians' clinical decisions have demonstrated considerable variation in decisions and practices and identified numerous challenges to effective, efficient, and accurate decision making. Although use of the decision aids and tools described in this article may help overcome many of these challenges, greater self-awareness of the diagnostic reasoning process and the factors influencing decisions also should help improve clinical decisions and reduce variation, irrespective of the use of these tools. Continued research into the determinants and nature of the diagnostic reasoning process will provide additional insights that can be used to develop and apply improved decision aids and corrective procedures to overcome persistent problems.

The nonlinearity of offspring-parent regression for total sternopleural bristle number of Drosophila melanogaster

A test of the linearity of the offspring-midparent regression for total sternopleural bristle number showed significant departure from linearity, with a tendency for higher heritability at higher values of the character. Offspring-midparent regression coefficients for each of four subdivisions of the total range of bristle number were not significantly different. The nonlinearity of offspring-midparent regression detected in this study may be due to either chance effects, genotypic asymmetry of genes with large effect, distribution asymmetry, or a combination of these factors. Further work is required to determine the extent and causes of nonlinearity of offspring-parent regression.

Pituitary and ovarian responses of post-partum acyclic beef cows to continuous long-term GnRH and GnRH agonist treatment

Post-partum acyclic beef cows received continuous long-term treatment with GnRH (200 or 400 ng/kg body wt/h) or the GnRH agonist buserelin (5.5 or 11 ng/kg body wt/h) using s.c. osmotic minipumps which were designed to remain active for 28 days. All treatments increased circulating LH concentrations whereas FSH remained unchanged. Ovulation and corpus luteum (CL) formation as judged by progesterone concentrations greater than or equal to 1 ng/ml occurred in 0/5 control, 4/5 200 ng GnRH, 4/4 400 ng GnRH, 4/5 5.5 ng buserelin and 3/5 11 ng buserelin cows. The outstanding features of the progesterone profiles were the synchrony, both within and across groups, in values greater than or equal to 1 ng/ml around Day 6, and the fact that most CL were short-lived (4-6 days). Only 3 cows, one each from the 400 ng GnRH, 5.5 ng buserelin and 11 ng buserelin groups, showed evidence of extended CL function. Cows failed to show a second ovulation which was anticipated around Day 10 and this could have been due to insufficient FSH to stimulate early follicular development, or the absence of an endogenously driven LH surge. The highest LH concentrations for the respective groups were observed on Days 2 and 6 and by Day 10 LH was declining, although concentrations did remain higher than in controls up to Day 20.(ABSTRACT TRUNCATED AT 250 WORDS)

Immune complex acute necrotizing glomerulonephritis with progression to diffuse glomerulosclerosis. A murine model

Male BALB/c mice given daily intraperitoneal injections of 4 mg of horse-spleen apoferritin develop, in the majority of cases, a proliferative and necrotizing glomerulonephritis with leukocytic infiltration and extensive intraglomerular thrombosis within 10 to 14 days, as previously reported. We now show that if injection of the antigen is discontinued, surviving animals develop extensive glomerulosclerosis (GS). Ten of 13 mice treated as indicated above and sacrificed 4 months after the last horse-spleen apoferritin injection developed segmental GS involving over 40% of their glomeruli. Tubulointerstitial damage of proportionate severity also developed. Ultrastructurally, pronounced mesangial expansion due to matrix deposition obliterated the glomerular architecture. We offer this as a reproducible model of immune complex-mediated GS particularly suited to the study of cellular interactions involved in the pathogenesis of GS.

Gonadotrophin secretion and ovarian responses in prepubertal heifers actively immunized against androstenedione and oestradiol-17 beta

Groups of heifer calves received a primary immunization against androstenedione (Group A; N = 11) or oestradiol-17 beta (Group E; N = 10) at 3 months of age and booster injections on 5 occasions at 2- to 3-month intervals. Controls (Group C, N = 11) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group A (1126 +/- 261; reciprocal of titre) and Group E (10,357 +/- 4067) heifers. In Groups A and E there was a general decline in the respective peak antibody titres after successive booster injections. From 3 to 9 months of age mean plasma concentrations of LH were higher (P less than 0.05) in Group E heifers (0.89 +/- 0.08 ng/ml) than in Group C (0.46 +/- 0.03 ng/ml) and Group A (0.59 +/- 0.05 ng/ml) heifers which did not differ from one another. There were no differences between groups in plasma FSH concentrations. At 10 months of age the LH response to exogenous LHRH was of higher (P less than 0.05) amplitude for heifers in Group E (2.59 +/- 0.56 ng/ml) than for those in Groups C (0.61 +/- 0.07 ng/ml) and A (1.04 +/- 0.22 ng/ml). Elevated plasma progesterone concentrations at 5 months of age were shown by 2 heifers in Group C, 10 in Group A, and 6 in Group E. From 8 to 14 months of age a consistently higher proportion of Group A heifers exhibited elevated progesterone compared with Group C and Group E heifers. After ovarian synchronization and booster injection at 15 months of age a corpus luteum was present in 2 heifers in Group C, 7 in Group A and none in Group E. The ovaries of Group A heifers were different from those of Groups C and E and were characterized by greater numbers of 2-4 mm follicles. It is concluded that active immunization against gonadal steroids influences both LH secretion and ovarian function in prepubertal heifers. Early increases in ovarian activity in androstenedione-immunized heifers are maintained after puberty and may therefore confer some lifetime reproductive advantages.

Reproductive hormone secretion and testicular growth in bull calves actively immunized against testosterone and oestradiol-17 beta

Groups of bull calves received a primary immunization against testosterone (Group T; N = 7) or oestradiol-17 beta (Group E; N = 9) at 3 months of age and booster injections on four occasions at approximately 2 month intervals. Controls (Group C, N = 7) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group T (730 +/- 231; reciprocal of titre) and Group E (12,205 +/- 4366) bulls; however, peak antibody titres generally declined with successive booster injections. Mean plasma concentrations of LH, FSH and testosterone during the period from 3 to 10 months of age were higher (P less than 0.05) in Group T bulls than in Groups C and E. Group T bulls had larger testes compared with controls from 6 months of age onwards. At castration at 14 months of age, testes of Group T bulls were heavier (P less than 0.05) than those of Groups C and E (179 +/- 13, 145 +/- 8 and 147 +/- 6 g, respectively). At 10 months of age, there were no differences among treatment groups in LH responses to LHRH, but the testosterone responses were greater (P less than 0.05) in bulls in Group T (26.2 +/- 4.9 ng/ml) and Group E (16.6 +/- 1.8 ng/ml) compared with those in Group C (6.9 +/- 0.6 ng/ml). Testosterone responses to hCG determined at 13 months of age were also greater (P less than 0.05) in Groups T and E relative to controls. At 14 months of age daily sperm production rates per bull (X 10(-9)) were higher (P less than 0.10) in Group T bulls (2.2 +/- 0.1) than those in Groups C (1.6 +/- 0.2) and E (1.6 +/- 0.1). These results indicate that early immunity against testosterone is associated with increased gonadotrophin secretion and accelerated growth of the testes in prepubertal bulls. Also, chronic immunity against testosterone or oestradiol-17 beta enhances the steroidogenic response of bull testes to gonadotrophic stimulation. If the above responses observed in young bulls are shown to be sustained, then immunity against gonadal steroids early in life may confer some reproductive advantage in mature animals.

Effect of melatonin on postpartum anestrus in beef cows

The effect of melatonin treatment on intervals from calving to first postpartum estrus and ovulation was determined in Shorthorn cows which calved May 8 to June 14. Melatonin (500 mg in beef tallow) was injected subcutaneously (s.c.) into 20 cows on June 15 (4 to 38 d postpartum). Ovulation was determined from progesterone concentrations in jugular venous blood collected weekly from June to August. Mean intervals to first estrus and first ovulation were significantly longer in primiparous than in multiparous cows (85 +/- 4 vs 55 +/- 3 d and 83 +/- 4 vs 52 +/-3 d). Melatonin treatment caused a significant increase in the intervals to first postpartum estrus (68 +/- 4 vs 58 +/- 5d) and ovulation (68 +/- 4 vs 55 +/- 5 d). Mean plasma melatonin concentrations during the daytime were significantly higher in treated than in control cows one and two weeks after melatonin injection and were within the lower range of nighttime values reported previously for cows. Thus melatonin treatment raised daytime plasma concentrations of melatonin and delayed the onset of estrus and ovulation. These results support the possibility of a role of photoperiod through melatonin secretion in the onset of postpartum ovarian activity in cattle.

Evidence of gonadal steroid-independent changes in activity of the central LH-releasing hormone pulse generator in developing bull calves

To ascertain whether temporal changes in activity of the hypothalamo-pituitary axis in prepubertal bulls may occur independently of shifts in sensitivity to steroid feedback, the acute post-castration rise in serum gonadotrophins was monitored in bull calves castrated at monthly intervals from 4 to 9 months of age. Since a major feature of the gonadotrophin profiles of developing bulls is a change in LH pulse frequency early in life, pulsatile LH secretion after castration was used as an index of activity of the central LH-releasing hormone (LHRH) pulse generator. Relative to the day of castration (day 0) bull calves (n = 4) were bled at 20-min intervals for 8 h on day -3 and at 10-min intervals for 4 h on days 3, 5 and 7. During the first week after castration, 4-month-old bulls showed a higher (P less than 0.05) frequency of LH pulses compared with bulls at 8 and 9 months (1.13, 0.88 and 0.75 pulses/h respectively; pooled S.E.M. = 0.13). Mean LH levels before castration were higher (P less than 0.05) in 4-month-old bulls than in bulls at 7, 8 and 9 months (0.92, 0.37, 0.31, 0.38 micrograms/l respectively; pooled S.E.M. = 0.12). After castration mean LH levels did not differ with age. Mean FSH levels did not differ among age groups either before or after castration. Increased serum LH levels in 4-month-old bulls confirmed the transient rise in LH secretion that occurs at this time in developing bull calves.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth and carcase characteristics of Simmental × Friesian and Friesian × Friesian cattle in a Mediterranean environment

The calving performance of 152 Friesian cows, located on two properties in the southern Adelaide Hills region of South Australia and mated to Simmental and Friesian sires, is reported. The growth of their steer and heifer progeny and the carcase characteristics of the steer progeny are also reported. Differences due to breed of sire in the incidence of both assisted calvings and calf deaths at birth were not significant on either property. The number of assisted calvings ranged from 1 in 36 calvings for Friesian x Friesian to 3 in 31 calvings for the Simmental x Friesian, both recorded on the same property. The Simmental x Friesian progeny were always significantly heavier at birth than the Friesian progeny. On one property, the Simmental x Friesian and Friesian x Friesian steers grew at a similar rate from weaning at approximately 11 weeks of age until slaughter at approximately 32 months of age, at which stage their carcases did not differ significantly in weight, length, fat cover and eye muscle area. On the other property, the Simmental x Friesian steers grew significantly faster from weaning to slaughter than did the Friesian x Friesian steers. When slaughtered at approximately 34 months of age, the Simmental x Friesian carcases were heavier (P < 0.001 ) and had a larger eye muscle area (P < 0.001 ) than the Friesian x Friesian steers, but there were no significant differences in length or fat cover. The carcases of both breed types were leaner than required by the local Adelaide market and steers would need to be slaughtered at heavier liveweights than those used in this study for acceptable levels of carcase fatness to be achieved.

Observations on the vegetation of northeastern Mato Grosso II. Forests and soils of the Rio Suiá--Missu area

The vegetation of the well drained soils along the Suiá--Missu road in the Serra do Roncador region of NE Mato Grosso is Evergreen Seasonal forest of Amazonian type. The area lies close to the meeting place of the Amazonian forest (the hylaea) and the cerrado (savanna) formation of Central Brazil. The structure of the forest is simple: the canopy is at about 18--23 m, and is exceeded by a few scattered emergents; no recognizable strata can be distinguished among the understorey trees and the shrub and herb layers are sparse. Table 1 lists the most important species and gives information on stratification and general distribution. Most of the species appear to have a hylaean centre of distribution but extend into other vegetation types. The forest differs from related communities which lie closer to the cerrado/forest boundary in its greater height and luxuriance, the presence of additional tall tree species, and the great reduction in abundance of a cerrado floristic element. A survey on the Xavantina--São Felix road allowed us to extend previous observations on the distance to which the cerrado tree Pterodon pubescens extends into the forest. The results obtained indicate a considerable extension of forest into cerrado during the life of an individual tree. A characteristic low forest occurs in the flood plain of the Rio Suiá--Missu while Swampy Gallery forests occur on permanently waterlogged soils around the headwaters of streams. The well drained soils of the Suiá--Missu forest are very uniform, deep latosols (oxisols) of very dystrophic nature with pH (in water) between 4.0 and 5.0 (see table 2, p. 203).