Regulatory function of sigma factors RpoS/RpoN in adaptation and spoilage potential of Shewanella baltica

Shewanella baltica is a typical specific spoilage organism causing the deterioration of seafood, but the exact regulation of its adaptive and competitive dominance in diverse environments remains undefined. In this study, the regulatory function of two sigma factors, RpoS and RpoN, in environmental adaptation and spoilage potential were evaluated in S. baltica SB02. Two in-frame deletion mutants, Delta rpoS and Delta rpoN, were constructed to explore the roles in their motility, biofilm formation, stress response and spoilage potential, as well as antibiotics by comparing the phenotypes and transcription with those of wild type (WT) strain. Compared with WT strain, the Delta rpoN showed the slower growth and weaker motility due to loss of flagella, while swimming of the Delta rpoS was increased. Deletion of rpoN significantly decreased biofilm biomass, and production of exopolysaccharide and pellicle, resulting in a thinner biofilm structure, while Delta rpoS formed the looser aggregation in biofilm. Resistance of S. baltica to NaCl, heat, ethanol and three oxidizing disinfectants apparently declined in the two mutants compared to WT strain. The Delta rpoN mutant decreased sensory score, accumulation of trimethylamine, putrescine and TVB-N and protease activity, while a weaker effect was observed in Delta rpoS. The two mutants had significantly higher susceptibility to antibiotics than WT strain, especially Delta rpoN. Deficiency of rpoN and rpoS significantly repressed the activities of two diketopiperazines related to quorum sensing (QS). Furthermore, transcriptome analyses revealed that RpoN was involved in the regulation of the expression of 143 genes, mostly including flagellar assembly, nitrogen and amino acid metabolism, ABC transporters. Transcript changes of seven differentially expressed coding sequences were in agreement with the phenotypes observed in the two mutants. Our findings reveal that RpoN, as a central regulator, controls the fitness and bacterial spoilage in S. baltica, while RpoS is a key regulatory factor of stress response. Characterization of these two sigma regulons in Shewanella has expanded current understanding of a possible co-regulatory mechanism with QS for adaptation and spoilage potential.

Automated summary

This study evaluated the regulatory function of sigma factors RpoS and RpoN in environmental adaptation and spoilage potential in Shewanella baltica SB02. Deletion of RpoN resulted in slower growth and weaker motility, while deletion of RpoS increased swimming ability. Both mutants showed significant effects on biofilm formation, stress response, and susceptibility to antibiotics. RpoN was found to be a central regulator controlling fitness and bacterial spoilage, while RpoS played a key role in stress response.