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.

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.

Aims In this study, we evaluate the antimicrobial activity of bacteria isolated from marine sponges Hymeniacidon perlevis and Halichondria panicea against clinically relevant carbapenem-resistant Acinetobacter baumannii (CRAb) isolates. Methods and results We observe the inhibitory activity of 18 (out of 114) sponge-isolated bacterial strains using medium throughput solid agar overlay assays. These strains belong to the genera Lactococcus, Pseudomonas and Vagococcus. In addition, this antimicrobial activity is validated through a liquid co-cultivation challenge using an inhibitory strain of each genus. Conclusions We show the inhibitory effect of the sponge-isolated strains against clinically relevant, modern CRAb strains and characterize this antimicrobial activity as bactericidal. Significance and impact of the study A. baumannii is one of the most problematic nosocomial ESKAPE pathogens that imposes a global threat to human health, because of its increasing resistance to last-resort antibiotics. The World Health Organization has emphasized that new alternative treatments against CRAb strains are urgently needed. Our results show promising opportunities for the isolation of potentially new bactericidal compounds from sponge-isolated bacteria of the genera Lactococcus, Pseudomonas and Vagococcus to fight modern CRAb isolates and other problematic bacterial pathogens.