Facultative Control of Matrix Production Optimizes Competitive Fitness in Pseudomonas aeruginosa PA14 Biofilm Models

As biofilms grow, resident cells inevitably face the challenge of resource limitation. In the opportunistic pathogen Pseudomonas aeruginosa PA14, electron acceptor availability affects matrix production and, as a result, biofilm morphogenesis. The secreted matrix polysaccharide Pel is required for pellicle formation and for colony wrinkling, two activities that promote access to O₂. We examined the exploitability and evolvability of Pel production at the air-liquid interface (during pellicle formation) and on solid surfaces (during colony formation). Although Pel contributes to the developmental response to electron acceptor limitation in both biofilm formation regimes, we found variation in the exploitability of its production and necessity for competitive fitness between the two systems. The wild type showed a competitive advantage against a non-Pel-producing mutant in pellicles but no advantage in colonies. Adaptation to the pellicle environment selected for mutants with a competitive advantage against the wild type in pellicles but also caused a severe disadvantage in colonies, even in wrinkled colony centers. Evolution in the colony center produced divergent phenotypes, while adaptation to the colony edge produced mutants with clear competitive advantages against the wild type in this O₂-replete niche. In general, the structurally heterogeneous colony environment promoted more diversification than the more homogeneous pellicle. These results suggest that the role of Pel in community structure formation in response to electron acceptor limitation is unique to specific biofilm models and that the facultative control of Pel production is required for PA14 to maintain optimum benefit in different types of communities.     

Anomalous invasion dynamics due to dispersal polymorphism and dispersal–reproduction trade-offs

Dispersal polymorphism and mutation play significant roles during biological invasions, potentially leading to evolution and complex behaviour such as accelerating or decelerating invasion fronts. However, life-history theory predicts that reproductive fitness—another key determinant of invasion dynamics—may be lower for more dispersive strains. Here, we use a mathematical model to show that unexpected invasion dynamics emerge from the combination of heritable dispersal polymorphism, dispersal-fitness trade-offs, and mutation between strains. We show that the invasion dynamics are determined by the trade-off relationship between dispersal and population growth rates of the constituent strains. We find that invasion dynamics can be ‘anomalous’ (i.e. faster than any of the strains in isolation), but that the ultimate invasion speed is determined by the traits of, at most, two strains. The model is simple but generic, so we expect the predictions to apply to a wide range of ecological, evolutionary, or epidemiological invasions.     

Coeliac disease: A review of the causative agents and their possible mechanisms of action

Summary This review outlines the main theories for the aetiology of coeliac disease and presents in more detail the work carried out in an attempt to define the nature of the toxins in wheat gluten. This includes the results of work with synthetic peptides and a discussion of the various assays used.Evidence is presented for an enzyme deficiency in coeliac disease which leads to abnormally high concentrations of certain peptides in the small bowel. These peptides can bring about damage by direct toxic action and by immunological mechanisms.Investigations of activity of synthetic peptides based on the structure of Agliadin appear to be making good progress and point to certain regions of that molecule as being responsible for toxicity. Certain key sequences of amino acids appear to be of fundamental importance to these studies.     

Biological losses and the quarantine policy for mars

An international committee known as COSPAR has agreed that the probability of a single viable organism aboard any unmanned spacecraft intended for planetary landing should be kept less than 10^–3, in agreement with work by Sagan and Coleman. At present, the U.S.A. is committed to remain consistent with this standard. Using a model which includes both expected losses from failures to collect data and from contamination to analyze the quarantine problem, evidence is given which suggests that the current quarantine requirements may be too strict if their implementation forces a program delay. U.S. policy should be re-examined, keeping more fully in mind both the types and the relative sized of the losses which might be encountered.