Aims: Evaluation of the antibacterial activity of cultivable bacteria associated with the marine sponges Hymeniacidon perlevis and Halichondria panicea against multi-drug-resistant Staphylococcus aureus. Methods and Results: One hundred and fourteen bacterial isolates were recovered from H. perlevis and H. panicea. Antibacterial action was demonstrated by 70% of the isolates against reference strain Staphylococcus aureus ATCC 29213 and by 31Á6% against Pseudomonas aeruginosa ATCC 27853 in agar overlay assays. Antibacterial potential was further analysed against 36 multi-drug-resistant hospital Staphylococcus aureus strains with diverse resistance profiles. Among the 80 isolates positive against S. aureus ATCC 29213, 76Á3% were active against at least one clinical S. aureus pathogen and 73Á6% inhibited one or more methicillin-resistant (MRSA) and vancomycin non-susceptible S. aureus strains. In addition, 41Á3% inhibited all vancomycin nonsusceptible MRSA strains. Conclusions: Culturable bacteria associated to H. perlevis and H. panicea are promising sources of antibacterial compounds of great pharmaceutical interest. Significance and Impact of the Study: This study was the first to explore the antibacterial potential of culturable bacteria associated with the marine sponges H. perlevis and H. panicea against MDR bacteria. This is the first report of antibacterial activity by Aquimarina, Denitrobaculum, Maribacter and Vagococcus isolates against MDR S. aureus strains, including vancomycin nonsusceptible and methicillin-resistant ones, against which new antibiotics are urgently needed.

Microbial communities play a key role in water self-purification. They are primary drivers of biogenic element cycles and ecosystem processes. However, these communities remain largely uncharacterized. In order to understand the diversity-heterotrophic activity relationship facing sole carbon sources, we assembled a synthetic community composed of 20 'typical' freshwater bacterial species mainly isolated from the Zenne River (Belgium). The carbon source utilization profiles of each individual strain and of the mixed community were measured in Biolog Phenotype MicroArrays PM1 and PM2A microplates that allowed testing 190 different carbon sources. Our results strongly suggest interactions occurring between our planktonic strains as our synthetic community showed metabolic properties that were not displayed by its single components. Finally, the catabolic performances of the synthetic community and a natural community from the same sampling site were compared. The synthetic community behaved like the natural one and was therefore representative of the latter in regard to carbon source consumption.

Background Vagococcus fluvialis is a species of lactic acid bacteria found both free-living in river and seawater and associated to hosts, such as marine sponges. This species has been greatly understudied, with no complete genome assembly available to date, which is essential for the characterisation of the mobilome. Results We sequenced and assembled de novo the complete genome sequences of five V. fluvialis isolates recovered from marine sponges. Pangenome analysis of the V. fluvialis species (total of 17 genomes) showed a high intraspecific diversity, with 45.5% of orthologous genes found to be strain specific. Despite this diversity, analyses of gene functions clustered all V. fluvialis species together and separated them from other sequenced Vagococcus species. V. fluvialis strains from different habitats were highly similar in terms of functional diversity but the sponge-isolated strains were enriched in several functions related to the marine environment. Furthermore, sponge-isolated strains carried a significantly higher number of mobile genetic elements (MGEs) compared to previously sequenced V. fluvialis strains from other environments. Sponge-isolated strains carried up to 4 circular plasmids each, including a 48-kb conjugative plasmid. Three of the five strains carried an additional circular extrachromosomal sequence, assumed to be an excised prophage as it contained mainly viral genes and lacked plasmid replication genes. Insertion sequences (ISs) were up to five times more abundant in the genomes of sponge-isolated strains compared to the others, including several IS families found exclusively in these genomes. Conclusions Our findings highlight the dynamics and plasticity of the V. fluvialis genome. The abundance of mobile genetic elements in the genomes of sponge-isolated V. fluvialis strains suggests that the mobilome might be key to understanding the genomic signatures of symbiosis in bacteria.

The dynamic of a community of 20 bacterial strains isolated from river water was followed in R2 broth and in autoclaved river water medium for 27 days in batch experiments. At an early stage of incubation, a fast-growing specialist strain, Acinetobater sp., dominated the community in both media. Later on, the community composition in both media diverged but was highly reproducible across replicates. In R2, several strains previously reported to degrade multiple simple carbon sources prevailed. In autoclaved river water, the community was more even and became dominated by several strains growing faster or exclusively in that medium. Those strains have been reported in the literature to degrade complex compounds. Their growth rate in the community was 1.5-to 7-fold greater than that observed in monoculture. Furthermore, those strains developed simultaneously in the community. Together, our results suggest the existence of cooperative interactions within the community incubated in autoclaved river water.

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