Diversity and Activity of Bacteria Cultured from a Cup—The Sponge Calyx nicaeensis

Marine sponges are well-known for hosting rich microbial communities. Sponges are the most prolific source of marine bioactive compounds, which are frequently synthesized by their associated microbiota. <i>Calyx nicaeensis</i> is an endemic Mediterranean sponge with scarce information regarding its (bioactive) secondary metabolites. East Mediterranean specimens of mesophotic <i>C. nicaeensis</i> have never been studied. Moreover, no research has inspected its associated bacteria. Thus, we studied the sponge's bacterial diversity and examined bacterial interspecific interactions in search of a promising antibacterial candidate. Such novel antimicrobial agents are needed since extensive antibiotic use leads to bacterial drug resistance. Bacteria cultivation yielded 90 operational taxonomic units (OTUs). A competition assay enabled the testing of interspecific interactions between the cultured OTUs. The highest-ranked antagonistic bacterium, identified as <i>Paenisporosarcina indica</i> (previously never found in marine or cold habitats), was mass cultured, extracted, and separated using size exclusion and reversed-phase chromatographic methods, guided by antibacterial activity. A pure compound was isolated and identified as 3-oxy-anteiso-C₁₅-fatty acid-lichenysin. Five additional active compounds await final cleaning; however, they are lichenysins and surfactins. These are the first antibacterial compounds identified from either the <i>C. nicaeensis</i> sponge or <i>P. indica</i> bacterium. It also revealed that the genus <i>Bacillus</i> is not an exclusive producer of lichenysin and surfactin.