Abstract
Coliform bacteria consist of both nonpathogen commensal and human opportunistic pathogen species isolated from different habitats like animals, man, vegetables, and water. Olives normally carry natural nonpathogenic epiphytic bacteria, but during growth, harvest, and processing, one of the final products, represented by virgin olive oil, can be contaminated with coliform. Present study showed that coliform bacteria can survive and reproduce in virgin olive oil containing low level of phenolic compounds. The laboratory inoculation trials demonstrated that when the bacterium Escherichia coli, isolated from the olives carposphere, was transferred in olive oil containing high polar phenols content, equal to 372 mg caffeic acid equivalent per kg, the survival was completely inhibited after 15 days of storage. On the contrary, the bacterium reproduced quickly when it was inoculated in virgin olive oil samples containing lower concentration of polar phenols. The SDS-PAGE analysis of the E. coli proteins showed different electrophoretic patterns when the bacterium was inoculated in the virgin olive oil with high phenolic compounds content, confirming the strong interaction between the olive oil phenols content and the bacterial wall proteins. The SEM ultrastructural observations confirmed the presence of a more higher number of damaged microbial cells in virgin olive oil rich of polar phenols. This finding needs further studies since, in an era of antibiotic resistance, the development of new strategies to fight unwanted food bacteria is promising way for the future.
Highlights
Olive oil is one of the basic components of the Mediterranean diet which can be found in other geographical areas where it is known for its high dietetic and nutritional value and its varied sensory characteristics
The microbiological analysis of the wash water, kneaded paste, and the extra virgin olive oil produced by three olive varieties showed the presence of yeasts and molds in all the analyzed samples, whereas coliform bacteria were found in the wash water produced, respectively, by the Lavagnina, Taggiasca, and Leccino variety, but were absent in the kneaded paste and the olive oil extracted by the same olive varieties
In detail, when the bacterium E. coli was inoculated in the olive oil characterized by a high polar phenols content, equal to 372 mg caffeic acid equivalent per kg of product, the colony form units (CFU) per mL of oil decreased drastically already during the first few days of incubation, while they were completely absent after 15 days of Survival of Escherichia coli in olive oil (Log CFU/mL)
Summary
Olive oil is one of the basic components of the Mediterranean diet which can be found in other geographical areas where it is known for its high dietetic and nutritional value and its varied sensory characteristics. It is well known that microorganisms persist for long periods in the olives’ carposphere, both during the phase of development and during the ripening of the fruit attached to the plant. This occurs both in the later stages, when the fruits are processed as table olives or when they are ready for oil extraction in the mills. Many studies demonstrate that phenolic compounds, such as those widely present in virgin olive oil, have a marked biocide effect on the human opportunistic pathogen yeast species as well as on bacteria [5, 6].
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