Edible Seaweeds and Spirulina Extracts for Food Application: In Vitro and In Situ Evaluation of Antimicrobial Activity towards Foodborne Pathogenic Bacteria

Abstract

Research is more and more focused on studying and selecting food preservatives of natural origin. In this scenario, algae are an excellent source of bioactive compounds, among which are antimicrobials, whose presence is variable depending on the algal species and environmental conditions. The aim of the present study was to obtain, by a food grade approach, antimicrobial extracts from five species already approved as foods and to test their efficacy in vitro (agar well diffusion assay) and in situ (microbial challenge test) towards foodborne pathogenic bacteria. Moreover, the total phenolic compounds of the extracts were determined in order to evaluate possible correlations with the antimicrobial activity. Strains belonging to Salmonella spp., Listeria monocytogenes, Escherichia coli, Staphylococcus aureus, and Bacillus cereus were considered. Overall, the extracts showed a good antimicrobial activity in vitro towards all the tested microorganisms, especially L. monocytogenes (15 mm of inhibition diameter). The effect of inhibition was monitored during 24, 48 and 120 h showing a good persistence in time. Arthrospira platensis exerted the highest efficacy, further revealed towards L. monocytogenes on salmon tartare as bacteriostatic using 0.45% of the extract and bactericidal using 0.90%. The presence of phenolic compounds could be related to the antimicrobial activity but was not revealed as the main component of this activity. The extract with the highest phenolic content (18.79 ± 1.90 mg GAE/g) was obtained from Himanthalia elongata. The efficacy, confirmed also in a food matrix, might open perspectives for their application as food preservative.

Keywords: Listeria monocytogenes; algae extract; food safety; foodborne pathogenic bacteria; microbiological challenge test; natural antimicrobial; phenolic content.

Figure 1

Antimicrobial activity of algae extracts towards Salmonella spp., L. monocytogenes, E. coli, S. aureus and B. cereus represented by heatmap. A scale ranging from a minimum of 0 mm (green) to a maximum of 15 mm (red) was used to represent size of inhibition diameter calculated, as average values of triplicates, after 24 h (A), 48 h (B), and 120 h (C) of incubation at 37 °C. I and II represent two different extracts referred to the same algae species.

Figure 2

Antimicrobial activity of algae extracts along time: 24 h (Green line), 48 h (light blue line) and 120 h (Blue line). (A) Arthrospira platensis extracts; (B) Himanthalia elongata extracts. (C) Laminaria spp. extracts. (D) Palmaria palmata extracts. (E) Undaria pinnatifida extracts. The means with different letters are significantly different (p < 0.05).

Figure 3

Behavior of Listeria monocytogenes during four days’ shelf life at refrigeration temperatures in salmon tartare without extract addition (C) (―), with 0.45% (v/w) (---) and 0.9% (v/w) (∙∙∙∙∙) of A. platensis extract.