AbstractIron is essential for almost all bacterial pathogens and consequently it is actively withheld by their hosts. However, the production of extracellular siderophores enables iron sequestration by pathogens, increasing their virulence. Another function of siderophores is extracellular detoxification of non-ferrous metals. Here, we experimentally link the detoxification and virulence roles of siderophores by testing whether the opportunistic pathogen Pseudomonas aeruginosa displays greater virulence after exposure to copper. To do this, we incubated P. aeruginosa under different environmentally relevant copper regimes for either two or twelve days. Subsequent growth in a copper-free environment removed phenotypic effects, before we quantified pyoverdine production (the primary siderophore produced by P. aeruginosa), and virulence using the Galleria mellonella infection model. Copper selected for increased pyoverdine production, which was positively correlated with virulence. This effect increased with time, such that populations incubated with high copper for twelve days were the most virulent. Replication of the experiment with a non-pyoverdine producing strain of P. aeruginosa demonstrated that pyoverdine production was largely responsible for the change in virulence. Therefore we here show a direct link between metal stress and bacterial virulence, highlighting another dimension of the detrimental effects of metal pollution on human health.

Disturbances can facilitate biological invasions, with the associated increase in resource availability being a proposed cause. Here, we experimentally tested the interactive effects of disturbance regime (different frequencies of biomass removal at equal intensities) and resource abundance on invasion success using a factorial design containing five disturbance frequencies and three resource levels. We invaded populations of the bacterium Pseudomonas fluorescens with two ecologically different invader morphotypes: a fast-growing "colonizer" type and a slower growing "competitor" type. As resident populations were altered by the treatments, we additionally tested their effect on invader success. Disturbance frequency and resource abundance interacted to affect the success of both invaders, but this interaction differed between the invader types. The success of the colonizer type was positively affected by disturbance under high resources but negatively under low. However, disturbance negatively affected the success of the competitor type under high resource abundance but not under low or medium. Resident population changes did not alter invader success beyond direct treatment effects. We therefore demonstrate that the same disturbance regime can either be beneficial or detrimental for an invader depending on both community resource abundance and its life history. These results may help to explain some of the inconsistencies found in the disturbance-invasion literature.

Disturbances can play a major role in biological invasions: by destroying biomass, they alter habitat and resource abundances. Previous field studies suggest that disturbance-mediated invader success is a consequence of resource influxes, but the importance of other potential covarying causes, notably the opening up of habitats, have yet to be directly tested. Using experimental populations of the bacterium Pseudomonas fluorescens, we determined the relative importance of disturbance-mediated habitat opening and resource influxes, plus any interaction between them, for invader success of two ecologically distinct morphotypes. Resource addition increased invasibility, while habitat opening had little impact and did not interact with resource addition. Both invaders behaved similarly, despite occupying different ecological niches in the microcosms. Treatment also affected the composition of the resident population, which further affected invader success. Our results provide experimental support for the observation that resource input is a key mechanism through which disturbance increases invasibility.

Successful colonisations by invasive organisms are causing catastrophic changes to communities: altering their dynamics, reducing biodiversity and impeding ecosystem services. These ecological costs are only surpassed by habitat loss, and when combined with the huge associated economic costs, makes understanding how these events occur of growing importance. This study will focus on how disturbances, which change the availability of resources and habitat, may facilitate the establishment of novel species. First, we factorially separate resource influxes and habitat opening to test the mechanism by which disturbance increases invader success; using diversified populations of Pseudomonas fluorescens. We homogenised communities to open habitat and added nutrients to increase resources. Resource influxes were key in successful establishment, habitat opening had little affect and no interaction was found. Secondly, we expanded upon this by testing if resource abundance interacts with disturbance frequency; hypothesising when more resources are available disturbance-induced influxes, and thus invader success, would be greater. To do this communities of P. fluorescens were disturbed at different frequencies in three resource concentrations and invaded multiple times. We found disturbance and resources to interact: manipulating the mortality-growth rate balance, and thus success, of the invader. Resources also interacted with evolved biodiversity to effect invasion resistance. We finish by testing disturbance and resource effects on a stably coexisting 5-species bacterial community, using a 5x5 factorial design. Disturbance and resource both manipulated the variation in fitness between species: impacting biodiversity. Interactions were only found at high-disturbance-high-resources. This highlights the suitability of this system for future disturbance-resource studies on stably coexisting systems, including future invasion work.
In conclusion, we show disturbance, through adding resources, to be a key factor in invasion success: the extent to which being strongly affected by resource abundance. We also find disturbance and resource changes are likely to impact the stability of communities.