Antimicrobial activities of virgin olive oils in vitro and on lettuce from pathogen-inoculated commercial quick salad bags

Virgin olive oil (VOO) provides an important source of healthy compounds. Its high concentrations of monounsaturated fatty acids and phenolic compounds contribute toward its nutritional, antioxidant, and antimicrobial properties. VOO is used in the preparation of many dishes worldwide and forms a staple of the Mediterranean diet; however, the antimicrobial activity of VOO in food matrices has been investigated very little to date. Considering this and the growing demand to reduce the use of synthetic antimicrobials and chemical food preservatives, especially in minimally processed foods, the aim of this work was to evaluate the antibacterial activity of VOOs obtained from 13 different Italian olive varieties. All olive varieties were harvested and processed in the same area (Oristano, Sardinia, Italy). The in vitro antimicrobial activities of the resulting VOOs were tested against: Listeria monocytogenes DSM 20600, Staphylococcus aureus DSM 20231, Escherichia coli DSM 30083, Salmonella bongori DSM13772, Lacticaseibacillus casei Shirota, Limosilactobacillus reuteri DSM 17938, Lacticaseibacillus casei RI4, and the yeast Candida albicans 3393. Orthogonal Projections to Latent Structures (OPLS) analysis showed a strong correlation between the content of certain phenolic compounds, the contact time of VOOs with the microbial cells, and the antimicrobial activity of the VOOs against the microorganisms tested. The VOOs showing the strongest antimicrobial activities (the Bosana and Sivigliana varieties) were then re-tested against L. monocytogenes DSM 20600 and S. bongori DSM 13772 in artificially contaminated commercial quick salad bags (QSB) of baby green leaf lettuces. The results show that antimicrobial activities of VOO depend on the target microbial species and the concentration of the inoculum. This study provides evidence supporting that VOOs with determinate polyphenols profiles might be used as antimicrobial ingredients in minimally processed foods to significantly reduce the presence of pathogenic bacteria.