Antibacterial actions of algal extract against food spoilage bacteria Pseudomonas fluorescens and Shewanella putrefaciens

Modes of antibacterial action of the extract from edible brown algae Ascophyllum nodosum against food spoilage bacteria Pseudomonas fluorescens and Shewanella putrefaciens were investigated. The colony forming units in P. fluorescens and S. putrefaciens were 0.70 and 1.03 log cfu/mL less than the negative control in the time-kill assay, indicating the inhibitory effect of the extract on the two bacteria at high cell density. The extract also inhibited the extracellular protease activity of the two bacteria, where the exopolysaccharide (EPS) content was reduced by 44.8% in P. fluorescens and 64.7% in S. putrefaciens after treating them with the extract of the minimum inhibition concentration (MIC). Reduction in live bacteria cells in the biofilm formed by the two bacteria strains exposed to 1×MIC and 2×MIC levels was also observed. Significant increase in 260-nm absorbing material, protein content, and electrical conductivity of the culture media was observed after 6-h incubation with the extract in both bacteria strains, suggesting a leakage of nucleic material, protein, and other intracellular constitutes. It was concluded that the extract of A. nodosum could exert antibacterial activity against P. fluorescens and S. putrefaciens by reducing their extracellular enzymatic activity, inhibiting EPS production and biofilm formation, and increasing cell permeability.