Chemical preservatives are still commonly used to prevent food spoilage and control infectious agents and food poisoning. Among consumers they are undesirable because of the potentially negative effects, including the dangers to human health of chemical residues in food products and the acquisition of resistance of microorganisms to the chemical substances used. This has increased the need to find effective natural antimicrobials. In the thesis we investigated the antimicrobial efficacy of extracts of nettle (Urtica dioica) and two dead nettle species (Lamium orvala and Lamium galeobdolon) on the food strains of Listeria monocytogenes, Staphylococcus aureus, Escherichia coli and Pseudomonas fragi. First, ethanol extracts were prepared from the dried plants and their effect was tested by microdilution method, which was evaluated as minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The extracts showed antimicrobial activity against all bacteria tested, while the extract of stinging nettle (U. dioica) was slightly more effective than the extract of dead nettle (L. orvala and L. galeobdolon). Surprisingly, the results showed that U. dioica extract had a comparable or better effect on gram-negative bacteria than on gram-positive ones, as it best inhibited the growth of gram-negative bacteria P. fragi. The antimicrobial effect of the dead nettle extracts of L. orvala and L. galeobdolon was comparable, with the difference only in P. fragi, which was better inhibited by the L. galeobdolon extract.
|
