Role of RpoS in stress resistance, quorum sensing and spoilage potential of Pseudomonas fluorescens

Microorganism activities are considered the main cause of most food spoilage, leading to great economic losses. Pseudomonas fluorescent is a Gram-negative bacterium that is widely found in food and has high spoilage activity. RpoS is considered an important global regulator involved in stress survival and virulence in many pathogens. Thus, it is very possible that RpoS plays an important role in spoilage regulation in P. fluorescent. In this study an in-frame deletion mutation of rpoS was constructed to explore its function in P. fluorescent. The results showed that RpoS positively regulated the resistance of P. fluorescent to H2O2, heat, ethanol and crystal violet, negatively regulated the resistance to acetic acid, and had no effect on the resistance to NaCl. Further studies indicated that acylated homoserine lactone (AHL) production and the transcription levels of five AHL-related genes were significantly decreased in the rpoS mutant compared with those in the wild-type strain. Interestingly, the two homologous genes coding for AHL synthases contained RpoS-dependent -10 elements, suggesting that AHL quorum sensing is directly regulated by RpoS. RpoS also contributed to the spoilage activities of P. fluorescent by regulating extracellular protease and total volatile basic nitrogen (TVB-N) production in sterilized salmon juice. Our results reveal that RpoS was a key regulatory factor involved in stress resistance, the AHL quorum sensing system, and spoilage potential of P. fluorescent. Our study may benefit food safety control and food preservation.