Abstract
The spoilage potential and biofilm formation of Shewanella baltica are reported to be regulated by Quorum sensing (QS) system from the phenotype point of view, but the specific mechanism is not fully understood. In the present study, the QS autoinducers were detected by UHPLC-MS/MS, cell density-dependent luxR-type genes were obtained through autoregulation experiments among a series of candidates in S. baltica OS155 (The SSO of large yellow croaker). The direct interaction between cyclo-(L-Pro-L-Phe) (PP) and LuxR01 as well as LuxR02 proteins was revealed via in vitro binding assay. Deletion of luxR-type genes (luxR01 and luxR02) impaired spoilage potential and biofilm formation of S. baltica OS155 in various degrees. Transcriptional analysis and qRT-PCR validation showed that spoilage and biofilm-related genes torS, speF, and pomA were down-regulated in luxR01 and luxR02 deletion strains. In addition, exogenous PP promoted spoilage potential and biofilm formation, which could be attenuated by luxR01 or luxR02 deletion. Our results revealed an explicit QS system employing PP as autoinducer and two orphan LuxRs as receptors which positively regulated spoilage capacity and biofilm formation via transcriptional regulation of corresponding genes in S. baltica OS155, which provides potential specific targets for seafood preservation involving QS system.
Highlights
Seafood is one of the most highly perishable food products because of the chemical effects of atmospheric oxygen and the growth of spoilage microorganisms (Özogul et al, 2004)
PL, LL, and PP were detected in the overnight culture supernatant of S. baltica OS155 in LB medium (Supplementary Figure S3A), and N-butanoylhomoserine lactone (C4-HSL), N-hexanoylhomoserine lactone (C6-HSL), PL, LL, and PP were detected in spoiled large yellow croaker (Supplementary Figure S3B)
Three DKPs (PP, PL, and LL) were detected predominant in cell-free culture fluids of S. baltica OS155 as well as spoiled large yellow croaker, we failed to detect any acylhomoserine lactones (AHLs) in pure culture of S. baltica OS155, which was largely consistent with previous studies (Gu et al, 2013; Zhu et al, 2015, 2016)
Summary
Seafood is one of the most highly perishable food products because of the chemical effects of atmospheric oxygen and the growth of spoilage microorganisms (Özogul et al, 2004). Biochemical analysis on the basis of nucleotide metabolism, production of TMA, hypoxanthine (Hx), total volatile basic nitrogen (TVB-N), and BAs have been commonly utilized to estimate fish quality. Shewanella baltica, a commonly dominant spoilage bacteria in seafood products such as iced sea salmon (Hozbor et al, 2006), gutted sea bass (Parlapani et al, 2015), chilled fresh Mediterranean swordfish (Pantazi et al, 2008) and tropical prawns (Chinivasagam et al, 1998; Zhu et al, 2015), has received increasing attention in recent years due to its significant role in seafood spoilage, and has been identified as the SSO of large yellow croaker stored at 4◦C (Gu et al, 2013; Zhu et al, 2016)
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