Faecalibacterium duncaniae A2-165 growth is strongly promoted by yeast extract and vitamin B5 in cGMP medium.

Several gut microbial species within the Faecalibacterium genus have emerged as promising next-generation probiotics (NGP) due to their multifunctional protective effects against gastrointestinal and systemic disorders. To enable clinical studies and further applications, improved methods for cultivating Faecalibacterium must be developed in compliance with current Good Manufacturing Practice regulations, which is complicated by its oxygen sensitivity and complex nutritional requirements. Different yeast-based nutrients (YBNs), including yeast extracts (YEs) and yeast peptones (YPs), are ubiquitously used when cultivating microbes to supply a broad range of macro- and micronutrients. In this study, we evaluated six experimental YBNs, namely three YEs, two YPs and a yeast cell wall product (YCW), and eight B-vitamins in the cultivation of Faecalibacterium duncaniae A2-165, former Faecalibacterium prausnitzii, using growth assays in microtitre plates, dose-effect studies in Hungate tube fermentations and fully controlled bioreactor experiments. We demonstrated that YEs promote F. duncaniae A2-165 growth in a nutritionally limited medium, while YPs and YCW lacked essential growth factors for enabling cell propagation. High cell density was obtained in controlled bioreactors using a medium containing 2-4% of a selected YE and 1% casein peptone (3.4 ± 1.7 × 109 -5.1 ± 1.3 × 109 cells mL-1 ). Among all tested B-vitamins, we identified B5 as a strong growth promoter. Replacing casein peptone with YP and supplementing with vitamin B5 further increased biomass by approximately 50% (6.8 ± 1.7 × 109 cells mL-1 ). Hence, empirical selection of YE, YP and B5 allowed formulation of a high-yielding animal allergen-free nutritive medium to produce F. duncaniae A2-165. Selecting nutritionally suitable YBNs and combining these with other key nutrients are important steps for optimizing production of NGP with high yields and lower cost.

Universal microfluidic platform for bioassays in anchored droplets.

In spite of the large number of droplet-based microfluidic tools that have appeared in recent years, their penetration into non-specialist labs remains limited to a small number of applications. This is partly due to the lack of a generic platform that integrates all of the necessary operations for end-users, and partly to the increasing complexity that emerges as several operations are combined together. Here we report the development of a platform that provides the capabilities of multiwell plates in a two-dimensional array of nanoliter droplets: encapsulation, time-resolved monitoring and variation of well contents, as well as the ability to selectively extract the contents of any of the wells. We demonstrate these capabilities by encapsulating thousands of individual bacterial cells in droplets that are stored on a two-dimensional array of surface-energy anchors. Bacterial culture can be performed either in liquid or hydrogel droplets, both of which allow precise quantification using either standard measurements or digital enumeration. Using hydrogels allows the removal of the external oil that surrounds the aqueous drops, for instance in order to apply a gradient of antibiotics across the droplet population. This defines a protocol to obtain an antibiogram in a single experiment. Finally, the liquid to gel transition provides a robust way to selectively extract any droplet from the array, by melting it with a focused laser. When combined with further off-chip culture or genotyping, this platform provides a unique culturing environment to relate phenotype and genotype measurements on monoclonal colonies.

Plant root transcriptome profiling reveals a strain-dependent response during Azospirillum-rice cooperation.

Cooperation involving Plant Growth-Promoting Rhizobacteria results in improvements of plant growth and health. While pathogenic and symbiotic interactions are known to induce transcriptional changes for genes related to plant defense and development, little is known about the impact of phytostimulating rhizobacteria on plant gene expression. This study aims at identifying genes significantly regulated in rice roots upon Azospirillum inoculation, considering possible favored interaction between a strain and its original host cultivar. Genome-wide analyzes of Oryza sativa japonica cultivars Cigalon and Nipponbare were performed, by using microarrays, seven days post-inoculation with Azospirillum lipoferum 4B (isolated from Cigalon) or Azospirillum sp. B510 (isolated from Nipponbare) and compared to the respective non-inoculated condition. A total of 7384 genes were significantly regulated, which represent about 16% of total rice genes. A set of 34 genes is regulated by both Azospirillum strains in both cultivars, including a gene orthologous to PR10 of Brachypodium, and these could represent plant markers of Azospirillum-rice interactions. The results highlight a strain-dependent response of rice, with 83% of the differentially expressed genes being classified as combination-specific. Whatever the combination, most of the differentially expressed genes are involved in primary metabolism, transport, regulation of transcription and protein fate. When considering genes involved in response to stress and plant defense, it appears that strain B510, a strain displaying endophytic properties, leads to the repression of a wider set of genes than strain 4B. Individual genotypic variations could be the most important driving force of rice roots gene expression upon Azospirillum inoculation. Strain-dependent transcriptional changes observed for genes related to auxin and ethylene signaling highlight the complexity of hormone signaling networks in the Azospirillum-rice cooperation.

Genome wide profiling of Azospirillum lipoferum 4B gene expression during interaction with rice roots.

Azospirillum-plant cooperation has been mainly studied from an agronomic point of view leading to a wide description of mechanisms implicated in plant growth-promoting effects. However, little is known about genetic determinants implicated in bacterial adaptation to the host plant during the transition from free-living to root-associated lifestyles. This study aims at characterizing global gene expression of Azospirillum lipoferum 4B following a 7-day-old interaction with two cultivars of Oryza sativa L. japonica (cv. Cigalon from which it was originally isolated, and cv. Nipponbare). The analysis was done on a whole genome expression array with RNA samples obtained from planktonic cells, sessile cells, and root-adhering cells. Root-associated Azospirillum cells grow in an active sessile-like state and gene expression is tightly adjusted to the host plant. Adaptation to rice seems to involve genes related to reactive oxygen species (ROS) detoxification and multidrug efflux, as well as complex regulatory networks. As revealed by the induction of genes encoding transposases, interaction with root may drive bacterial genome rearrangements. Several genes related to ABC transporters and ROS detoxification display cultivar-specific expression profiles, suggesting host specific adaptation and raising the question of A. lipoferum 4B/rice cv. Cigalon co-adaptation.

Engineering adherent bacteria by creating a single synthetic curli operon.

The method described here consists in redesigning E. coli adherence properties by assembling the minimum number of curli genes under the control of a strong and metal-overinducible promoter, and in visualizing and quantifying the resulting gain of bacterial adherence. This method applies appropriate engineering principles of abstraction and standardization of synthetic biology, and results in the BBa_K540000 Biobrick (Best new Biobrick device, engineered, iGEM 2011). The first step consists in the design of the synthetic operon devoted to curli overproduction in response to metal, and therefore in increasing the adherence abilities of the wild type strain. The original curli operon was modified in silico in order to optimize transcriptional and translational signals and escape the "natural" regulation of curli. This approach allowed to test with success our current understanding of curli production. Moreover, simplifying the curli regulation by switching the endogenous complex promoter (more than 10 transcriptional regulators identified) to a simple metal-regulated promoter makes adherence much easier to control. The second step includes qualitative and quantitative assessment of adherence abilities by implementation of simple methods. These methods are applicable to a large range of adherent bacteria regardless of biological structures involved in biofilm formation. Adherence test in 24-well polystyrene plates provides a quick preliminary visualization of the bacterial biofilm after crystal violet staining. This qualitative test can be sharpened by the quantification of the percentage of adherence. Such a method is very simple but more accurate than only crystal violet staining as described previously with both a good repeatability and reproducibility. Visualization of GFP-tagged bacteria on glass slides by fluorescence or laser confocal microscopy allows to strengthen the results obtained with the 24-well plate test by direct observation of the phenomenon.

Which specificity in cooperation between phytostimulating rhizobacteria and plants?

Plant growth-promoting rhizobacteria (PGPR) are found in association with a large range of host plants. Although the subject of plant host specificity has been well studied in parasitic and mutualistic interactions, the question of whether phytostimulating rhizobacteria efficiently interact only with a specific host remains poorly discussed. This review presents elements suggesting the existence of specificity in three-step establishment of associative symbiosis between phytostimulating rhizobacteria and plants: bacterial attraction by the host plant, bacterial colonization of roots, and functioning of associative symbiosis.

Genome Sequence of Azospirillum brasilense CBG497 and Comparative Analyses of Azospirillum Core and Accessory Genomes provide Insight into Niche Adaptation.

Bacteria of the genus Azospirillum colonize roots of important cereals and grasses, and promote plant growth by several mechanisms, notably phytohormone synthesis. The genomes of several Azospirillum strains belonging to different species, isolated from various host plants and locations, were recently sequenced and published. In this study, an additional genome of an A. brasilense strain, isolated from maize grown on an alkaline soil in the northeast of Mexico, strain CBG497, was obtained. Comparative genomic analyses were performed on this new genome and three other genomes (A. brasilense Sp245, A. lipoferum 4B and Azospirillum sp. B510). The Azospirillum core genome was established and consists of 2,328 proteins, representing between 30% to 38% of the total encoded proteins within a genome. It is mainly chromosomally-encoded and contains 74% of genes of ancestral origin shared with some aquatic relatives. The non-ancestral part of the core genome is enriched in genes involved in signal transduction, in transport and in metabolism of carbohydrates and amino-acids, and in surface properties features linked to adaptation in fluctuating environments, such as soil and rhizosphere. Many genes involved in colonization of plant roots, plant-growth promotion (such as those involved in phytohormone biosynthesis), and properties involved in rhizosphere adaptation (such as catabolism of phenolic compounds, uptake of iron) are restricted to a particular strain and/or species, strongly suggesting niche-specific adaptation.