WWW.WA S .ORG • WORLD AQUACULTURE • SEP TEMBER 2022 35 et al. 2014). EPS affects physical characteristics of biofloc, such as floc size, volume and density (Si and Quan 2017). Physical characteristics of biofloc are affected by quorum sensing. The autoinducer AHL added to biofloc significantly enhances floc volume, particle size, floc volume index and protein content. The quorum sensing inhibitor vanillin at 0.30 mg/L caused a significant reduction in floc formation, particle size, protein content and microbial aggregation (Fatimah et al. (2019). Quorum sensing thus plays a pivotal role in the physical characteristics of biofloc, thereby affecting its functional abilities, such as nutrient uptake. Influence of Quorum Sensing on Biofilm Formation in Recirculating Aquaculture Systems Recirculating aquaculture systems use biofilters that include microbial consortia with nitrifying bacteria that control ammonia concentration. This microbial consortium grows in suspension (biofloc) or in a fixed film (biofilm) attached to biofilter media (King et al. 2008, Suhr and Pederson 2010). Both require tight interactions among the bacterial consortium to regulate the expression of those genes associated with nitrification. The expression of nitrificationrelated genes are regulated by chemical signaling molecules (Zhang and Li 2016, Ruiz et al. 2019, Fatimah et al. 2019) Quorum sensing detects the change in the cell density and thus alters the metabolic activities of microcolonies (Dickschat 2010, Zhu et al. 2016). Signalling molecules are involved in the maturation process of biofilm in Atlantic salmon RAS (Zhu et al. 2016). The quorum sensing signaling system plays a role in biofilm formation in biofilters including microbial attachment, detachment and colonization. Coordination of certain physiological attributes of microcolonies in biofilm such as stress resistance, toxin production and virulence are controlled by genes that are altered by quorum sensing signalling (Shrout et al. 2011). Therefore, quorum sensing plays a central role in controlling biofilm in recirculating aquaculture systems (Ruiz et al. 2019). QuorumQuenching as an Alternative Strategy for Disease Management in Aquaculture Two major factors of vital significance in disease outbreaks are virulence or pathogenicity of microbes and development of a biofilm; however, the expression of these phenotypes is controlled by genes that are regulated by the quorum sensing mechanism in a coordinated manner (Jayaprakashvel and Subramani 2019). One significant A synchronized population of microbes can conduct an array of energetically expensive physiological processes such as horizontal gene transfer, sporulation, symbiosis, competence, virulence and biofilm formation. These social and coordinated interactions occur in response to regulation of gene expression, modulated through the ability of bacteria to communicate from cell to cell in an orderly fashion known as quorum sensing (Novick and Geisinger 2008, Bassler 2009, Rutherford and Bassler 2012). Signaling among microbes is mediated by chemicals called autoinducers. There must be a minimal threshold concentration of these stimulatory molecules to elicit an alteration in gene expression (Miller and Bassler 2001). Examples of autoinducers that mediate quorum sensing are acylated homoserine lactone (AHL) in Gramnegative bacteria and oligo-peptides in Gram-positive bacteria (Miller and Bassler 2001). Quorum sensing regulates expression of genes that control those processes that are effective only when performed by cumulative bacterial populations (Rutherford and Bassler 2012). Various bacterial behaviors mediated by quorum sensing have implications in medicine, industries, agriculture and biotechnology and also holds significant relevance in aquaculture. Mechanism of Quorum Sensing Bacteria synthesize autoinducers during a cell reproductive cycle. Autoinducers inside the Gram-negative bacterial cell (e.g., AHL) diffuse passively through the thin cell wall. In the case of Grampositive bacteria, oligo-peptide autoinducers are actively transported through the peptidoglycan cell wall via the ATP-binding cassette transport system. Initially the concentration of autoinducers in the extracellular environment is below the threshold concentration to generate any stimulatory response. When autoinducer concentration reaches a critical threshold concentration as bacterial cell numbers increase, it becomes energetically unfavorable for autoinducers to leave the bacterial cell through diffusion or active transport, thus increasing the intracellular concentration of these chemical molecules. Autoinducers, therefore, bind to receptors inside the cell, triggering certain transcription factors that alter gene expression (Windsor 2020). Role of Quorum Sensing on the Physical Characteristics of Biofloc Quorum sensing plays a pivotal role in the secretion of extracellular polymeric substances (EPS) that are a critical aspect of microbial aggregation in a biofloc (Shrout and Nerenberg 2012, Lv Quorum Sensing in Aquaculture: A Brief Overview Rida Riyaz and Parvaiz Ahmad ( C O N T I N U E D O N P A G E 3 6 ) The ability of bacteria to communicate from cell to cell in an orderly fashion is known as quorum sensing. Quorum sensing regulates expression of genes that control those processes that are effective only when performed by cumulative bacterial populations. Various bacterial behaviors mediated by quorum sensing have implications in medicine, industries, agriculture and biotechnology and also holds significant relevance in aquaculture.
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