Synergistic antibacterial activity and mechanism of surfactin and Ag NPs against Pseudomonas fluorescens.

Pseudomonas fluorescens (P. fluorescens) is an important specific spoilage organism in aquatic products, which has caused serious food waste due to its extreme environmental resistance and spoilage capacity. This study aimed to explore a novel composite antibacterial agent that combined surfactin and silver nanoparticles (Ag NPs) to form surfactin-Ag NPs and to investigate its synergistic antibacterial mechanism against P. fluorescens. The results showed that surfactin-Ag NPs can effectively enhance the antibacterial activity against P. fluorescens, and the minimum inhibitory concentration when synergized was only 1/8 of that when each was used alone. This might be the change of carboxyl conformation in surfactin and the presence of more lipophilic groups around surfactin-Ag NPs made it easier to enter the cell membrane. Surfactin-Ag NPs can alter outer/inner membrane permeability and cell membrane integrity, causing leakage of intracellular contents and eventually leading to bacterial death. Transcriptomic analysis indicated that surfactin-Ag NPs disrupted energy metabolism and ABC transporters in P. fluorescens, affecting nutrient acquisition. Moreover, surfactin-Ag NPs disrupted ribosome transcription and translation, and led to excessive synthesis of certain amino acids. This investigation not only revealed the synergistic antibacterial mechanism of surfactin-Ag NPs against P. fluorescens at the cellular structure level, but also conducted in-depth research at the molecular level, providing new ideas for the effective preservation of aquatic products.