Quorum Sensing Inhibition by Sponge-Associated

Background/Objectives: The growing threat of antimicrobial resistance has intensified the search for alternative therapeutic approaches. Quorum sensing (QS) inhibition, which disrupts bacterial communication and virulence, represents a promising approach to mitigating infection. Given the complexity of the sponge holobiont, sponge-associated microorganisms may demonstrate QS inhibitory properties and serve as potential sources of novel anti-virulence agents. This study aimed to investigate the QS inhibitory potential of sponge-associated Bacillus species against Pseudomonas aeruginosa, a multidrug-resistant pathogen that relies on QS for virulence regulation. Methods: Ninety-eight bacterial isolates were obtained from seven intertidal South African sponges. Biosensor-based sandwich assays using Chromobacterium violaceum identified 15 isolates with putative QS inhibition (QSI) activity, including five classified as Bacillus species via 16S rRNA gene sequencing. Crude extracts from these isolates, cultivated in medium Mannitol (Mann) and medium 5294, were screened for their ability to inhibit QS-regulated virulence factors in P. aeruginosa. Results: Extracts, particularly from medium 5294, exhibited significant QSI activity without cytotoxic effects. The five most potent extracts, i.e., Bacillus mobilis SP2-AB7 (5294), Bacillus wiedmannii SP5-AB7 (Mann), B. mobilis SP2-AB7 (Mann), and Bacillus cereus SP1-AB4 (Mann and 5294), inhibited both Las- and Rhl-regulated virulence factors, including pyocyanin, pyoverdine, elastase, protease, rhamnolipid production, motility, and initial adhesion, achieving inhibition rates of up to 93% (p < 0.05). Molecular analysis confirmed the presence of the aiiA lactonase gene in key isolates, while GC-MS and FTIR profiling revealed medium-specific differences in metabolite production. Conclusions: Sponge-associated Bacillus species from KwaZulu-Natal exhibit robust QSI activity against P. aeruginosa, highlighting their potential as sources of alternative anti-virulence agents. Further characterization and in vivo validation are needed to assess their therapeutic application in combatting resistant infections.