The amount of natural resources in the Earth’s environment is in flux, which can trigger catastrophic collapses of ecosystems. How populations survive under nutrient-poor conditions is a central question in ecology. Curiously, some bacteria persist for a long time in nutrient-poor environments. Although this survival may be accomplished through cell death and the recycling of dead cells, the importance of these processes and the mechanisms underlying the survival of the populations have not been quantitated. Here, we use microbial laboratory experiments and mathematical models to demonstrate that death and recycling are essential activities for the maintenance of cell survival. We also show that the behavior of the survivors is governed by population density feedback, wherein growth is limited not only by the available resources but also by the population density. The numerical simulations suggest that population density-dependent recycling could be an advantageous behavior under starvation conditions.
Microbial cooperation drives ecological and epidemiological processes and is affected by the ecology and demography of populations. Population density influences the selection for cooperation, with spatial structure and the type of social dilemma, namely public-goods production or self-restraint, shaping the outcome. While existing theories predict that in spatially structured environments increasing population density can select either for or against cooperation, experimental studies with both public-goods production and self-restraint systems have only ever shown that increasing population density favours cheats. We suggest that the disparity between theory and empirical studies results from experimental procedures not capturing environmental conditions predicted by existing theories to influence the outcome. Our study resolves this issue and provides the first experimental evidence that high population density can favour cooperation in spatially structured environments for both self-restraint and public-goods production systems. Moreover, using a multi-trait mathematical model supported by laboratory experiments we extend this result to systems where the self-restraint and public-goods social dilemmas interact. We thus provide a systematic understanding of how the strength of interaction between the two social dilemmas and the degree of spatial structure within an environment affect selection for cooperation. These findings help to close the current gap between theory and experiments.
Existing theory, empirical, clinical and field research all predict that reducing the virulence of individuals within a pathogen population will reduce the overall virulence, rendering disease less severe. Here, we show that this seemingly successful disease management strategy can fail with devastating consequences for infected hosts. We deploy cooperation theory and a novel synthetic system involving the rice blast fungus Magnaporthe oryzae. In vivo infections of rice demonstrate that M. oryzae virulence is enhanced, quite paradoxically, when a public good mutant is present in a population of high-virulence pathogens. We reason that during infection, the fungus engages in multiple cooperative acts to exploit host resources. We establish a multi-trait cooperation model which suggests that the observed failure of the virulence reduction strategy is caused by the interference between different social traits. Multi-trait cooperative interactions are widespread, so we caution against the indiscriminant application of anti-virulence therapy as a disease-management strategy.DOI: http://dx.doi.org/10.7554/eLife.18678.001
5Microbes commonly deploy a risky strategy to acquire nutrients from their environment, involving the 6 production of costly public goods that can be exploited by neighbouring individuals. Why engage in 7 such a strategy when an exploitation-free alternative is readily available whereby public goods are 8 kept private? We address this by examining metabolism of Saccharomyces cerevisiae both in its 9 native form and by creating a novel three-strain synthetic community deploying different strategies of 10 sucrose metabolism. Public-metabolisers digests resources externally, private-metabolisers 11 internalise resources before digestion, and cheats avoid the metabolic costs of digestion but exploit 12 external products generated by competitors. A combination of mathematical modelling and ecological 13 experiments reveal that private-metabolisers invade and take over an otherwise stable community of 14 public-metabolisers and cheats. However, owing to the reduced growth rate of private-metabolisers 15 and population bottlenecks that are frequently associated with microbial communities, privatising 16 public goods can become unsustainable, leading to population decline. 17 have been described as 'private goods' 17 . However, such 'private goods' do not overcome the 51 evolutionary incentive for cheating 43 , as is the case with our definition. 52We assess the benefits of public goods metabolism using a combination of mathematical modelling 53 and synthetic ecology. We focus on different strategies that Saccharomyces cerevisiae deploys to 54 metabolise disaccharides as an ideal exemplar. S. cerevisiae can metabolise the disaccharides 55 maltose and sucrose, which have the same molecular formula (C12H22O11) but differ in their 56 constituent monosaccharides: maltose consists of two glucose molecules whereas sucrose is one 57 glucose and one fructose molecule. Disaccharides are broken down into their constituent 58 monosaccharides, before being catabolised by the glycolytic pathway. Curiously, considering the 59 similarities, S. cerevisiae deploys different strategies for their consumption. Maltose is internalised 60 through maltose transporters before being hydrolysed by maltase, whereas sucrose is hydrolysed 61 externally by an extracellular invertase, and the resulting monosaccharides are subsequently 62 internalised 44 . 63By comparing features of wild type public sucrose metabolism and private maltose metabolism when 64 grown alone (axenically), we demonstrate how public and private metabolism by S. cerevisiae can 65 provide different environment-dependent selective benefits. A number of important differences 66 emerge. Growth on maltose was less hampered by diminishing resource concentrations than growth 67 on sucrose. However, sucrose metabolism facilitated a higher growth rate than maltose metabolism 68 when population density and resource concentrations were sufficiently high. 69To understand how ecological interactions between such contrasting metabolic strategies influence 70 their evolutionary potential, we d...
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