The impact of inoculated plant growth-promoting rhizobacteria (PGPR) on its host physiology and nutrition depends on inoculum level. Whether the impact of the inoculated PGPR on the indigenous rhizosphere microbiota also varies with the PGPR inoculum level is unclear. Here, we tested this issue using the PGPR Azospirillum lipoferum CRT1—maize model system, where the initial seed inoculation is known to enhance maize growth and germination, and impacts the maize rhizomicrobiota, including microbial functional groups modulating plant growth. A. lipoferum CRT1 was added to the seeds at standard (10^5–6 cells.seed⁻¹) or reduced (10^4–5 cells.seed⁻¹) inoculation levels, in three fields. The effect of the two PGPR formulations was assessed on maize growth and on the nifH (nitrogen fixation), acdS (ACC deaminase activity) and phlD (2,4-diacetylphloroglucinol production) microbial functional groups. The size of the three functional groups was monitored by qPCR at the six-leaf stage and the flowering stage, and the diversity of the nifH and acdS functional groups (as well as the bacterial community) were estimated by MiSeq metabarcoding at the six-leaf stage. The results showed that the benefits of the reduced inoculant formulation were significant in two out of three fields, but different (often lower) than those of the standard formulation. The effects of formulations on the size of the three functional groups differed, and depended on field site and functional group. The reduced formulation had an impact on the diversity of nifH and acdS groups at one site, whereas the standard formulation had an impact at the two other sites. Inoculation significantly impacted the total bacterial community in the three fields, but only with the reduced formulation. In conclusion, the reduced inoculant formulation impacted the indigenous rhizosphere microbiota differently, but not less efficiently, than the standard formulation.

The Stenotrophomonas maltophilia species complex (Smc) currently comprises 19 genomospecies, known for their ecological versatility and role as opportunistic human pathogens. However, significant genomic variability among strains has complicated their classification, and only a few species within the Smc have been validly described. Despite successive taxonomic revisions, there remains no consensus on the naming of many genomospecies, largely due to the phenotypic heterogeneity across the complex. In this study, we performed a comprehensive genomic and phylogenetic re-assessment to clarify the boundaries of the Stenotrophomonas genus and its species complex. Using whole-genome phylogeny and Overall Genome-Related Indices (OGRIs) such as digital DNA-DNA hybridization (dDDH), Average Nucleotide Identity (ANIb and ANIm), and Average Amino Acid Identity (AAI), we established precise cutoffs for defining the genus and the Smc. Our analysis led to the formal revision of the Stenotrophomonas genus, resulting in the delineation of four novel bacterial genera: Allostenotrophomonas gen. Nov., Pedostenotrophomonas gen. Nov., Parastenotrophomonas gen. Nov., and Pseudostenotrophomonas gen. Nov. Additionally, we standardized the nomenclature within the Smc by identifying reference strains for each genomospecies. Among these, we describe a novel species, Stenotrophomonas thermophila sp. nov. (type strain BurA1^T = LMG 33562^T = EML 1882^T). This study provides a robust framework for future taxonomic and ecological studies within the Stenotrophomonas genus.

The Asian tiger mosquito Aedes albopictus is well adapted to urban environments and takes advantage of the artificial containers that proliferate in anthropized landscapes. Little is known about the physicochemical, pollutant, and microbiota compositions of Ae. albopictus-colonized aquatic habitats and whether these properties differ with noncolonized habitats. We specifically addressed this question in French community gardens by investigating whether pollution gradients (characterized either by water physicochemical properties combined with pollution variables or by the presence of organic molecules in water) influence water microbial composition and then the presence/absence of Ae. albopictus mosquitoes. Interestingly, we showed that the physicochemical and microbial compositions of noncolonized and colonized waters did not significantly differ, with the exception of N₂O and CH₄ concentrations, which were higher in noncolonized water samples. Moreover, the microbial composition of larval habitats covaried differentially along the pollution gradients according to colonization status. This study opens new avenues on the impact of pollution on mosquito habitats in urban areas and raises questions on the influence of biotic and abiotic interactions on adult life-history traits and their ability to transmit pathogens to humans.

Omics are the most promising approaches to investigate microbes for which no genetic tools exist such as the nitrogen-fixing symbiotic Frankia. A proteogenomic analysis of symbiotic Frankia alni was done by comparing those proteins more and less abundant in Alnus glutinosa nodules relative to N-replete pure cultures with propionate as the carbon source and ammonium as the nitrogen-source. There were 250 proteins that were significantly overabundant in nodules at a fold change (FC) ≥ 2 threshold, and 1429 with the same characteristics in in vitro nitrogen-replete pure culture. Nitrogenase, SuF (Fe–Su biogenesis) and hopanoid lipids synthesis determinants were the most overabundant proteins in symbiosis. Nitrogenase was found to constitute 3% of all Frankia proteins in nodules. Sod (superoxide dismutase) was overabundant, indicating a continued oxidative stress, while Kats (catalase) were not. Several transporters were overabundant including one for dicarboxylates and one for branched amino acids. The present results confirm the centrality of nitrogenase in the actinorhizal symbiosis.