The viable counts of Salmonella typhimurium on nutrient agar (NA) decreased upon the addition of either the essential oil of thyme or its constituent thymol, especially under anaerobic conditions. Antagonistic effects of thymol against Staphylococcus aureus were also greater under anaerobic conditions. In contrast to the phenolic constituents of the oil, thymol and carvacrol, the chemically related terpenes p-cymene and gamma-terpinene had no antagonistic effects against Salm. typhimurium. The addition of Desferal to NA counteracted the antibacterial effects of both thyme oil and thymol. No support was obtained, however, for a possible role of iron in the oxygen-related antibacterial action of the thyme oil and thymol or for the observed effect of Desferal. In the presence of thymol, the viable counts of Salm. typhimurium obtained on a minimal medium (MM) were lower than those obtained on NA. Addition of bovine serum albumin (BSA) neutralized the antibacterial action of thymol. It is suggested that the effects of BSA or Desferal are due to their ability to bind phenolic compounds through their amino and hydroxylamine groups, respectively, thus preventing complexation reactions between the oil phenolic constituents and bacterial membrane proteins. This hypothesis is supported by the marked decrease in the viable counts of Salm. typhimurium caused by either thyme oil or thymol when the pH of the medium was changed from 6.5 to 5.5 or the concentration of Tween 80 in the medium was reduced.

Essential oils from oregano and thyme were applied for 24 h as fumigants against the mycelia and spores ofAspergillus flavus, Aspergillus niger and Aspergillus ochraceus, as well as against natural microflora of wheat grains. The minimal inhibitory concentration (MIC) of oregano oil needed to inhibit the mycelial growth of the fungi was 2.0 μl/L, while spores were eradicated following exposure to 2.0 to 2.5 μl/L. The thyme essential oil was less efficient in controlling mycelia and growth was observed even following exposure to 4.0 μl/L. However, the thyme essential oil was fungitoxic to spores (MIC = 3.0 μl/L). In another set of trials the efficacy of the oils and two of their constituents (carvacrol and thymol) in controlling natural microflora of surface-sterilized wheat grain was studied. Of the four materials investigated, only oregano essential oil exhibited fungicidal activity and, following 24 h exposure to 20 μl/L, a significant reduction in the percent of infested grain was observed even after 5 days of incubation on potato dextrose agar. A reduction in the germinability of the grains was evident following exposure to the materials tested. When the fungicidal activity of oregano essential oil was evaluated using grains with different moisture contents (MC), data revealed that the better inhibitory effect was achieved in grain with a high MC. The findings emphasize the toxicity of oregano and thyme essential oils as fumigants against fungi attacking stored grain and strengthen the possibility of using them as an alternative to chemicals for preserving stored grains.

The fumigant toxicity of 28 essential oils extracted from various spice and herb plants and some of their major constituents were assessed for adult coleopteransRhyzopertha dominica, Oryzaephilus surinamensis, Tribolium castaneum, andSitophilus oryzae. Three groups of active materials were distinguished: (1) The compounds terpinen 4-ol, 1,8-cineole, and the essential oils of three-lobed sage, sage, bay laurel, rosemary, and lavender were most active againstR. dominica; (2) The compounds linalool,α-terpineol, and carvacrol and the essential oils of oregano, basil, Syrian marjoram, and thyme were most active againstO. surinamensis; and (3) the compound 1,8-cineole and the essential oils anise and peppermint were active againstT. castaneum.

Hydrogen peroxide is responsible for certain bactericidal effects observed in biological systems, such as growth inhibition of one bacterial species by another and killing of invading microorganisms by activated phagocytic cells. H2O2 might be generated in bacteriological media by their exposure to light and/or oxygen and become an important mediator of toxic effects. H2O2 cytotoxicity is apparently due to its capacity—generally mediated by transition metal ions—to generate more reactive and cytotoxic oxygen species such as the hydroxyl radical, which is a powerful oxidant, and which can initiate oxidation of biomolecules. The conversion of H2O2 into more cytotoxic compounds may be potentiated by reducing agents and by peroxidases. Cells may protect themselves against H2O2 toxicity either by the action of catalases or, in the case of DNA damage, by repairing the damage after it has taken place. Assays for the detection and quantitation of H2O2 in cell cultures include those based on (i) catalase-dependent oxidation of formate to CO2, (ii) generation of fluorescent products due to a H2O2- mediated oxidative reaction, (iii) the loss of fluorescence upon the oxidation of scopoletin, (iv) change in absorbance upon oxidation of phenol red, or (v) formation of complexes with peroxidases. Some possible antimicrobial uses of H2O2 in the food industry are presented.

Pediococcus acidilactici SJ-1, isolated from a naturally-fermented meat product, produced an antibacterial agent active against selected strains of Lactobacillus spp., Clostridium perfringens and Listeria monocytogenes. The agent was bactericidal against sensitive indicators, and sensitive to proteolytic enzymes; it was identified as a bacteriocin, and was designated as pediocin SJ-1. It was stable over a wide pH range (3-9), and apparently most stable in the lower part of that range. At pH 3.6, pediocin SJ-1 was stable at heat-processing temperatures within the range 65-121 degrees C; its activity decreased significantly, however, when it was heated at pH 7.0. The activity of pediocin SJ-1 on sensitive indicator cells was lost in the presence of alpha-amylase, suggesting that it contains a glyco moiety, necessary for its antibacterial action. Native pediocin SJ-1 exists in the form of monomers and aggregates (with molecular weights in the range 80-150 kDa). Pediocin SJ-1 was purified 262-fold by direct application of cell-free supernatant fluids to a cation-exchange chromatography column, and was resolved by SDS-PAGE as a single peptide band with a MW of ca 4 kDa. The original pediocin SJ-1-producing strain (bac+) harbours three plasmids of 4.6, 23.5, and 45.7 MDa. Production of pediocin SJ-1, but not immunity to SJ-1, is associated with the 4.6 MDa plasmid.