Nitrogen regulation of the <i>xyl</i> genes of <i>Pseudomonas putida</i> mt-2 propagates into a significant effect of nitrate on <i>m</i>-xylene mineralization in soil

The nitrogen species available in the growth medium are key factors determining expression of xyl genes for biodegradation of aromatic compounds by Pseudomonas putida. Nitrogen compounds are frequently amended to promote degradation at polluted sites, but it remains unknown how regulation observed in the test tube is propagated into actual catabolism of, e.g. m‐xylene in soil, the natural habitat of this bacterium. To address this issue, we have developed a test‐tube‐to‐soil model system that exposes the end‐effects of remediation practices influencing gene expression of P. putida mt‐2. We found that NO (3) (−) compared with NH (4) (+) had a stimulating effect on xyl gene expression in pure culture as well as in soil, and that this stimulation was translated into increased m‐xylene mineralization in soil. Furthermore, expression analysis of the nitrogen‐regulated genes amtB and gdhA allowed us to monitor nitrogen sensing status in both experimental systems. Hence, for nitrogen sources, regulatory patterns that emerge in soil reflect those observed in liquid cultures. The current study shows how distinct regulatory traits can lead to discrete environmental consequences; and it underpins that attempts to improve bioremediation by nitrogen amendment should integrate knowledge on their effects on growth and on catabolic gene regulation under natural conditions

<i>Cupriavidus pinatubonensis</i> AEO106 deals with copper-induced oxidative stress before engaging in biodegradation of the herbicide 4-chloro-2-methylphenoxyacetic acid

Abstract Background Microbial degradation of phenoxy acid (PA) herbicides in agricultural soils is important to minimize herbicide leaching to groundwater reservoirs. Degradation may, however, be hampered by exposure of the degrader bacteria to toxic metals as copper (Cu) in the soil environment. Exposure to Cu leads to accumulation of intracellular reactive oxygen species (ROS) in some bacteria, but it is not known how Cu-derived ROS and an ensuing oxidative stress affect the degradation of PA herbicides. Based on the previously proposed paradigm that bacteria deal with environmental stress before they engage in biodegradation, we studied how the degradation of the PA herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) by the model PA degrader Cupriavidus pinatubonensis AEO106 was affected by Cu exposure. Results Exposure of C. pinatubonensis in batch culture to sublethal concentrations of Cu increased accumulation of ROS measured by the oxidant sensing probe 2,7-dichlorodihydrofluorescein diacetate and flow cytometry, and resulted in upregulation of a gene encoding a protein belong to the Ohr/OsmC protein family. The ohr/osmC gene was also highly induced by H2O2 exposure suggesting that it is involved in the oxidative stress response in C. pinatubonensis. The increased ROS accumulation and increased expression of the oxidative stress defense coincided with a delay in the catabolic performance, since both expression of the catabolic tfdA gene and MCPA mineralization were delayed compared to unexposed control cells. Conclusions The current study suggests that Cu-induced ROS accumulation in C. pinatubonensis activates a stress response involving the product of the ohr/osmC gene. Further, the stress response is launched before induction of the catabolic tfdA gene and mineralization occurs

Suppression of the activity of arbuscular mycorrhizal fungi by the soil microbiota

Arbuscular mycorrhizal fungi (AMF) colonise roots of most plants; their extra-radical mycelium (ERM) extends into the soil and acquires nutrients for the plant. The ERM coexists with soil microbial communities and it is unresolved whether these communities stimulate or suppress the ERM activity. This work studied the prevalence of suppressed ERM activity and identified main components behind the suppression. ERM activity was determined by quantifying ERM-mediated P uptake from radioisotope-labelled unsterile soil into plants, and compared to soil physicochemical characteristics and soil microbiome composition. ERM activity varied considerably and was greatly suppressed in 4 of 21 soils. Suppression was mitigated by soil pasteurisation and had a dominating biotic component. AMF-suppressive soils had high abundances of Acidobacteria, and other bacterial taxa being putative fungal antagonists. Suppression was also associated with low soil pH, but this effect was likely indirect, as the relative abundance of, e.g., Acidobacteria decreased after liming. Suppression could not be transferred by adding small amounts of suppressive soil to conducive soil, and thus appeared to involve the common action of several taxa. The presence of AMF antagonists resembles the phenomenon of disease-suppressive soils and implies that ecosystem services of AMF will depend strongly on the specific soil microbiome.publishedVersio

Food Safety versus the Green transition

New microbial and chemical risks come with the greater incorporation of plant-based raw materials into human diets and the introduction of reusable packagin

Lipopeptide biosurfactant viscosin enhances dispersal of <i>Pseudomonas fluorescens</i> SBW25 biofilms

Pseudomonads produce several lipopeptide biosurfactants that have antimicrobial properties, but also facilitate surface motility and influence biofilm formation. Detailed studies addressing the significance of lipopeptides for biofilm formation and architecture are rare. Hence the current study sets out to determine the specific role of the lipopeptide viscosin for Pseudomonas fluorescens SBW25 biofilm formation, architecture and dispersal, and to relate viscA gene expression with viscosin production and effect. Initially, we compared biofilm formation of SBW25 and the viscosin-deficient mutant strain SBW25ΔviscA in static microtiter assays. These experiments demonstrated that viscosin had little influence on the amount of biofilm formed by SBW25 during the early stages of biofilm development. Later, however, SBW25 formed significantly less biofilm than SBW25ΔviscA. The indication that viscosin is involved in biofilm dispersal was confirmed by chemical complementation of the mutant biofilm. Further, a fluorescent bioreporter showed that viscA expression was induced in biofilms 4 hours prior to dispersal. Subsequent detailed studies of biofilms formed in flow-cells for up to 5 days revealed that SBW25 and SBW25ΔviscA developed comparable biofilms dominated by well-defined mushroom-shaped structures. Carbon-starvation was required to obtain biofilm dispersal in this system. Dispersal of SBW25 biofilms was significantly larger than of SBW25ΔviscA biofilms after 3 hours, and importantly, carbon-starvation strongly induced viscA expression, in particular for cells that were apparently leaving the biofilm. Hence the current study points towards a role for viscosin-facilitated motility in dispersal of SBW25 biofilms