Diversity and Bioactivity of Marine Bacteria Associated with the Sponges Candidaspongia flabellata and Rhopaloeides odorabile from the Great Barrier Reef in Australia

Sponges and their associated microbial communities have sparked much interest in recent decades due on the abundant production of chemically diverse metabolites that in nature serve as functional compounds required by the marine sponge host. These compounds were found to carry therapeutic importance for medicinal applications. In the presented study, 123 bacterial isolates from the culture collection of the Australian Institute of Marine Science (AIMS) previously isolated from two different sponge species, namely Candidaspongia flabellata and Rhopaloeides odorabile, originating from different locations on the Great Barrier Reef in Queensland, Australia, were thus studied for their bioactivity. The symbiotic bacterial isolates were first identified using 16S rRNA gene analysis and they were found to belong to five different dominating classes of Domain Bacteria, namely Alphaproteobacteria, Gammaproteobacteria, Flavobacteria, Bacilli and Actinobacteria. Following their taxonomical categorization, the isolates were screened for their antimicrobial activity against human pathogenic microbial reference strains: Escherichia coli (ATCC® BAA-196™), E. coli (ATCC® 13706™), E. coli (ATCC® 25922™), Klebsiella pneumoniae (ATCC® BAA-1705™), Enterococcus faecalis (ATCC® 51575™), Bacillus subtilis (ATCC® 19659™), Staphylococcus aureus (ATCC® 29247™), Candida albicans (ATCC® 10231™) and Aspergillus niger (ATCC® 16888™). Over 50% of the isolates displayed antimicrobial activity against one or more of the reference strains tested. The subset of these bioactive bacterial isolates was further investigated to identify their biosynthetic genes such as polyketide synthase (PKS) type I and non-ribosomal peptide synthetase (NRPS) genes. This was done using polymerase chain reaction (PCR) with degenerate primers that have been previously used to amplify PKS-I and NRPS genes. These specific genes have been reported to be possibly involved in bacterial secondary metabolite production. In 47% of the bacterial isolates investigated, the PKS and NRPS genes were located. Some of the bacterial isolates were found to possess both gene types, which agrees with the previous reported biosynthetic ability of certain sponge-symbiotic bacteria such as the Actinobacteria or Gammaproteobacteria to produce secondary metabolites with antimicrobial activity. All these reported activities further confirm that sponge-symbiotic bacteria hold significant bioactivity with medicinal and biotechnological importance.