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Biomedical subjects

P D Roberts

Publications and source records attributed to P D Roberts.

At least 19 recordsLinked to original sources
PubMed2005

Relative importance of bacteriocin-like genes in antagonism of Xanthomonas perforans tomato race 3 to Xanthomonas euvesicatoria tomato race 1 strains.

In a previous study, tomato race 3 (T3) strains of Xanthomonas perforans became predominant in fields containing both X. euvesicatoria and X. perforans races T1 and T3, respectively. This apparent ability to take over fields led to the discovery that there are three bacteriocin-like compounds associated with T3 strains. T3 strain 91-118 produces at least three different bacteriocin-like compounds (BCN-A, BCN-B, and BCN-C) antagonistic toward T1 strains. We determined the relative importance of the bacteriocin-like compounds by constructing the following mutant forms of a wild-type (WT) T3 strain to evaluate the antagonism to WT T1 strains: Mut-A (BCN-A-), Mut-B (BCN-B-), Mut-C (BCN-C-), Mut-AB, Mut-BC, and Mut-ABC. Although all mutant and WT T3 strains reduced the T1 populations in in planta growth room experiments, Mut-B and WT T3 were significantly more effective. Mutants expressing BCN-B and either BCN-A or BCN-C reduced T1 populations less than mutants expressing only BCN-A or BCN-C. The triple-knockout mutant Mut-ABC also had a significant competitive advantage over the T1 strain. In pairwise-inoculation field experiments where plants were coinoculated with an individual mutant or WT T3 strain and the T1 strain, the mutant strains and the WT T3 strain were reisolated from more than 70% of the lesions. WT T3 and Mut-B were the most frequently reisolated strains. In field experiments where plants were group inoculated with Mut-A, Mut-B, Mut-C, Mut-ABC, and WT T1 and T3 strains, Mut-B populations dominated all three seasons. In greenhouse and field experiments, the WT and mutant T3 strains had a selective advantage over T1 strains. Bacterial strains expressing both BCN-A and BCN-C appeared to have a competitive advantage over all other mutant and WT strains. Furthermore, BCN-B appeared to be a negative factor, with mutant T3 strains lacking BCN-B having a selective advantage in the field.

Agriculture↗
PubMed2000

Systematic analysis of xanthomonads (Xanthomonas spp.) associated with pepper and tomato lesions.

The taxonomy and evolutionary relationships among members of the genus Xanthomonas associated with tomato and pepper have been a matter of considerable controversy since their original description in 1921. These bacteria, which are a major affliction of tomato and pepper crops in warm and humid regions, were originally described as a single species, but subsequent research has shown the existence of at least two genetic groups differentiated by physiological, biochemical and pathological characteristics. This work synthesizes the findings from several approaches, including pathogenicity tests, enzymic activity, restriction fragment analysis of the entire genome, DNA-DNA hybridization and RNA sequence comparisons based on a 2097 base sequence comprising the 16S rRNA gene, the intergenic spacer located between the 16S and 23S rRNA genes and a small region of the 23S rRNA gene. Within the group of xanthomonads pathogenic on pepper and tomato four distinct phenotypic groups exist, of which three form distinct genomic species. These include Xanthomonas axonopodis pv. vesicatoria (A and C group), Xanthomonas vesicatoria (B group) and Xanthomonas gardneri (D group). On the basis of phenotypic and genotypic differences between A- and C-group strains, the C strains should be considered as a subspecies within Xanthomonas axonopodis pv. vesicatoria.

Antibodies, Monoclonal↗
PubMed2000

Dynamics of temporal learning rules.

The changes of synaptic strength are analyzed on two time scales: the fast local field dynamics, and the slow synaptic modification dynamics. The fast dynamics are determined by the synaptic strengths and background noise in the system. The slow dynamics are determined by the functional form of a temporal learning rule. Temporal learning rules are defined to be functions yielding state dependent changes in synaptic strengths depending on the timing of pre- and postsynaptic states in the network. The evolution of local field dynamics that result from various learning rules are analyzed for a stochastic, discrete time neural model with no relative refractory period that receives a series of delayed adaptive inputs. A fixed point is found in the learning dynamics, and conditions for two types of instabilities are analyzed. Four universality classes of dynamics are found that are independent of the details of the temporal learning rules. Examples are given of biological systems in which these temporal learning rules have been identified, and their functional consequences are discussed.

Adaptation, Physiological↗
PubMed2000

Modeling inhibitory plasticity in the electrosensory system of mormyrid electric fish.

Mathematical analyses and computer simulations are used to study the adaptation induced by plasticity at inhibitory synapses in a cerebellum-like structure, the electrosensory lateral line lobe (ELL) of mormyrid electric fish. Single-cell model results are compared with results obtained at the system level in vivo. The model of system level adaptation uses detailed temporal learning rules of plasticity at excitatory and inhibitory synapses onto Purkinje-like neurons. Synaptic plasticity in this system depends on the time difference between pre- and postsynaptic spikes. Adaptation is measured by the ability of the system to cancel a reafferent electrosensory signal by generating a negative image of the predicted signal. The effects of plasticity are tested for the relative temporal correlation between the inhibitory input and the sensory input, the gain of the sensory signal, and the presence of shunting inhibition. The model suggests that the presence of plasticity at inhibitory synapses improves the function of the system if the inhibitory inputs are temporally correlated with a predictable electrosensory signal. The functional improvements include an increased range of adaptability and a higher rate of system level adaptation. However, the presence of shunting inhibition has little effect on the dynamics of the model. The model quantifies the rate of system level adaptation and the accuracy of the negative image. We find that adaptation proceeds at a rate comparable to results obtained from experiments in vivo if the inhibitory input is correlated with electrosensory input. The mathematical analysis and computer simulations support the hypothesis that inhibitory synapses in the molecular layer of the ELL change their efficacy in response to the timing of pre- and postsynaptic spikes. Predictions include the rate of adaptation to sensory stimuli, the range of stimulus amplitudes for which adaptation is possible, the stability of stored negative images, and the timing relations of a temporal learning rule governing the inhibitory synapses. These results may be generalized to other adaptive systems in which plasticity at inhibitory synapses obeys similar learning rules.

Action Potentials↗
PubMed1998

Classification of temporal patterns in dynamic biological networks.

A general method is presented to classify temporal patterns generated by rhythmic biological networks when synaptic connections and cellular properties are known. The method is discrete in nature and relies on algebraic properties of state transitions and graph theory. Elements of the set of rhythms generated by a network are compared using a metric that quantifies the functional differences among them. The rhythms are then classified according to their location in a metric space. Examples are given, and biological implications are discussed.

Models, Neurological↗
PubMed1998

Self-esteem and injury in competitive field hockey players.

A volunteer sample of 50 competitive field hockey players completed the Coopersmith Self-esteem Inventory at pre- and postseason and prospectively collected injury data over a 20-wk. season. Multiple regression analysis showed no relationship between scores on Self-esteem and the number of injuries, the participation time affected due to injury, and sex of players. Further multiple regression analysis showed that frequency of the more severe injuries significantly predicted scores on Self-esteem. This finding can be interpreted as evidence of the relationship between low self-esteem and injury in sport.

Adult↗
PubMed1997

Classification of rhythmic patterns in the stomatogastric ganglion.

A large class of neural pattern generators change their rhythmic output under the influence of neuromodulators. We present a method for identifying the variety of rhythmic patterns generated by small neural networks. The technique provides a tool for investigating the biological mechanisms responsible for pattern generation and pattern switching. Discrete methods based on transition graphs are applied to dynamic biological networks to generate sets of possible rhythmic behaviours. A measure is introduced onto the set of rhythms to quantify their differences and organize the set according to clusters of similar rhythms. Each cluster represents a different operational mode of the network. Examples are drawn from the stomatogastric ganglion, a well studied network that controls the muscles in the foregut of crustaceans. Classes of rhythms are found that correspond to experimentally observed patterns, and other classes of rhythms are found that have not yet been observed. Predictions are made for the rhythmic output of the stomatogastric ganglion under specific manipulations of parameters in the biological network.

Animals↗
PubMed1996

Dynamics of the sit-to-stand movement.

The strategies of the sit-to-stand movement are investigated by describing the movement in terms of the topology of an associated phase diagram. Kinematic constraints are applied to describe movement sequences, thus reducing the dimension of the phase space. This dimensional reduction allows us to apply theorems of topological dynamics for two-dimensional systems to arrive at a classification of six possible movement strategies, distinguished by the topology of their corresponding phase portrait. Since movement is treated in terms of topological structure rather than specific trajectories, individual variations are automatically included, and the approach is by nature model independent. Pathological movement is investigated, and this method clarifies how subtle abnormalities in movement lead to difficulties in achieving a stable stance upon rising from a seated position.

Humans↗
PubMed1996

The stomatogastric nervous system: a formal approach.

A discrete mathematical formalism (d-space) which is specifically designed to investigate discrete aspects of behavior is applied to the foregut of decapod crustacea. This approach differs from continuous modeling techniques in that the analysis determines a structure in which the observed behavior of the foregut is constrained. A notation for the implementation of the formalism is developed as well as a coordinate system natural to the functioning of the gastric mill. The formalism is used to organize previous observations that suggest potential courses of further experimental investigation. A detailed analysis of observed chewing modes of the gastric mill is presented, along with a discussion of the overall organization of the interrelationships between these modes. The investigation also addresses the relationship between behavioral modes of a pyloric muscle found in the shrimp Palaemon. Two alternative hypotheses are presented to describe the relationship of the behavioral components of the gastric mill: an interlaced control scheme in which the components are freely exchanged, and a top-down control system where the chewing modes are rigidly separated into packages. Flow through regions of state space in time is found to be important in determining the relations between the discrete behavioral components. The behavior of the foregut, like that of other motor control systems, is shown to fit naturally into the d-space formalism.

Animals↗
PubMed1993

The opsX locus of Xanthomonas campestris affects host range and biosynthesis of lipopolysaccharide and extracellular polysaccharide.

Xanthomonas campestris pv. citrumelo strain 3048 is the causal agent of citrus bacterial leaf spot disease and has a wide host range that includes rutaceous and leguminous plants. A spontaneous prototrophic mutant of strain 3048 (strain M28) that had lost virulence on citrus but retained virulence on bean plants was recovered. Growth studies in planta showed that M28 cells died rapidly in citrus leaves but grew normally in bean leaves. In addition to the loss of citrus-specific virulence, M28 displayed the following mutant phenotypes in culture: decreased growth rate, reduction of the amount of exopolysaccharide (to ca. 25% of the amount in 3048), loss of capsules, and significant alterations of the two 3048 lipopolysaccharide (LPS) bands visualized by silver stain on polyacrylamide gels, consistent with a defect(s) in LPS assembly. A 38-kb DNA fragment from a 3048 total DNA library that complemented the mutant phenotypes of M28 was identified. The 38-kb fragment did not hybridize to two similarly sized fragments carrying different hrp (hypersensitive response and pathogenicity) genes cloned from 3048. Subcloning, DNA sequence analyses, and gene disruption experiments were used to identify a single gene, opsX (for outer-membrane polysaccharide), responsible for the mutant phenotypes of M28. At least one other gene downstream from opsX also affected the same phenotypes and may be part of a gene cluster. We report here the DNA sequence and transcriptional start site of opsX. A search of protein sequence data bases with the predicted 31.3-kDa OpsX sequence found strong similarity to Lsi-1 of Neisseria gonorrhoeae and RfaQ of Escherichia coli (both are involved in LPS core assembly). The host-specific virulence function of opsX appears to involve biosynthesis of the extracellular polysaccharide and a complete LPS. Both may be needed in normal amounts for protection from citrus, but not bean, defense compounds.

Amino Acid Sequence↗
PubMed1989

Congenital human immunodeficiency virus (HIV) infection in the Bahamas.

In the 13-month period between June 1985 and July 1986, 27 children were found to be HIV positive in the Princess Margaret Hospital in Nassau. Nineteen of the children had clinical AIDS, four were in the prodromal phase and four were symptom free. The clinical course of these infants is presented. Of the 18 mothers 16 were screened and were all seropositive and asymptomatic. They remained healthy in spite of subsequent pregnancies in nine of them (56%) during a follow-up period of between 13 and 65 months (mean 40 months). Fifteen of the 18 mothers were Haitian but only three had other risk factors, throwing doubt on the value of selective screening in Afro-Caribbean countries.

AIDS-Related Complex↗
PubMed1981

Splenic irradiation in B-prolymphocytic leukaemia.

Twelve patients with B-cell prolymphocytic leukaemia (PLL) were treated with splenic irradiation at a weekly dose of 100 cGy to a maximum total dose of 1000 cGy. There was no morbidity associated with this treatment. SEven patients responded. Three achieved a good response and four a partial response. Two of the patients who had a partial response have subsequently died of unrelated causes. Four of the five patients who failed to respond have died as a result of their disease. When more than 25% of the prolymphocytes formed rosettes with mouse red blood cells (MRBC) the patients appeared to respond better to splenic irradiation. There was no correlation between response and the initial white cell count or the size of the spleen.

Aged↗
PubMed1981

Splenic atrophy in adult coeliac disease: is it reversible?

A study of splenic function in 28 patients with adult coeliac disease showed no significant correlation between the half life of heat-damaged red cells and either the duration of pre-treatment exposure to gluten or the length of time on a gluten free diet. A significant correlation was found between splenic size and duration of treatment; those patients who had been taking a gluten free diet for the longest time had the smallest spleens. Blood films from 11 of these 28 patients taken before treatment with a gluten free diet were compared with those taken between two and 15 years after the start of treatment. There was no tendency for the hyposplenic changes to regress. In the majority, the changes became more prominent despite strict adherence to the gluten free diet. These findings suggest that splenic atrophy in adult coeliac disease is not reversed by treatment with a gluten free diet and is unlikely to be related to the state of the jejunal mucosa or the duration of initial exposure to gluten.

Adolescent↗
PubMed1977

Erythroid hypoplasia in myelofibrosis: a feature associated with blastic transformation.

Four cases of myelofibrosis have shown a pattern which is usually found only where there is erythroid hypoplasia and in none of these was there evidence of extramedullary erythropoiesis. Three of these cases terminated in a blastic phase. The findings suggest that the occurrence of erythroid hypoplasia in myelofibrosis may be associated with blastic transformation--whether as a predisposing factor or as a secondary event is still speculative. This phenomenon is discussed in relation to the natural history of myelofibrosis.

Bone Marrow↗