Sensitization of Escherichia coli to nisin by maltol and ethyl maltol.

When used separately, 20 mmol l-1 maltol or 1600 AU ml-1 nisin resulted in a 0-0.6 log10 reduction in viable counts of Escherichia coli in a buffer system. However, when added in combination they yielded a 1.8-5.5-log-cycle reduction in viable counts of E. coli at pH 5.0 and 6.8 respectively. It is postulated that maltol (and ethyl maltol) destabilizes the cell outer membrane by chelation of Mg2+ and/or Ca2+, thus permeabilizing the E. coli cell to nisin.

Factors that interact with the antibacterial action of thyme essential oil and its active constituents.

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.

Response of spheroplasts and chelator-permeabilized cells of gram-negative bacteria to the action of the bacteriocins pediocin SJ-1 and nisin.

We have attempted to bypass the outer membrane (OM) barrier of Escherichia coli and Salmonella typhimurium with pediocin SJ-1 (as compared with nisin) using chelating agents as OM permeabilizers. EDTA, and to less extent EGTA, enabled nisin, but not pediocin SJ-1, to permeate the cell OM of E. coli, to have access to the cytoplasmic membrane and to cause subsequent permeability changes, indicated by an increase in ANS fluorescence intensity and a shift of its emission maximum. Such spectral changes did not occur when, prior to addition, EDTA was saturated with Ca2+ and Mg2+. ANS fluorescence data indicated that, in spite of the fact that pediocin SJ-1 did traverse the EDTA-permeabilized OM of E. coli, it did not cause perturbation of its cytoplasmic membrane and was, therefore, unable to cause cell death. Spheroplasts prepared from E. coli were lysed when treated with nisin but not with pediocin SJ-1. We suggest that the resistance of Gram-negative bacteria to pediocin SJ-1 is due not only to this material's inability to permeate the OM but also (in contrast to nisin) to its inability to interact with the cytoplasmic membrane.

Purification, partial characterization and plasmid-linkage of pediocin SJ-1, a bacteriocin produced by Pediococcus acidilactici.

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.

Effect of different isomers of dihydroxybenzoic acids (DBA) on the rate of DL-dopa oxidation by mushroom tyrosinase.

Dihydroxybenzoic acids (DBA), such as 3,4-DBA, 3,5-DBA, and 2,4-DBA--at all concentrations tested--inhibited the rate of DL-DOPA oxidation to dopachrome (lambda max = 475 nm) by mushroom tyro0sinase. 2,3-DBA and 2,5-DBA at relatively low concentration had a synergistic effect on the reaction, whereas at relatively high concentrations they inhibited the rate of DL-DOPA oxidation. The synergistic effect of 0.6-13.3 mM 2,3-DBA on the rate of DL-DOPA oxidation to dopachrome (lambda max = 475 nm) was found to be due to the ability of 2,3-DBA-o-quinone (formed by the oxidation of 2,3-DBA by mushroom tyrosinase or by sodium periodate) to oxidize DL-DOPA to dopachrome (via dopaquinone) non-enzymatically. A similar explanation is likely to be valid for the synergism exerted by 2,5-DBA on the rate of DL-DOPA oxidation by mushroom tyrosinase.

Synergism exerted by 4-methyl catechol, catechol, and their respective quinones on the rate of DL-DOPA oxidation by mushroom tyrosinase.

4-Methyl catechol and catechol, at concentrations ranging from 0.03 to 9 mM and 0.066 to 20 mM, respectively, have a synergistic effect on the rate of DL-DOPA oxidation by mushroom tyrosinase to material absorbing at 475 nm. The synergism results from the ability of 4-methyl catechol-o-quinone (4-methyl-o-benzoquinone) and of catechol-o-quinone (o-benzoquinone) to oxidize DL-DOPA non-enzymatically to dopaquinone, with the later being immediately converted to dopachrome (lambda max = 475 nm).

Antagonistic Compounds Produced by a Chicken Intestinal Strain of Lactobacillus acidophilus.

The production of antagonistic compounds was studied with a strain of Lactobacillus acidophilus isolated from chicken intestinal tract. Accumulation of lactic acid (0.15 mol/l), hydrogen peroxide (280 nmol/h/mg cell dry weight), and a bacteriocin (ca. 10,000 arbitrary activity units per ml) was observed in cultures of this strain. In conditions eliminating the effects of organic acids and hydrogen peroxide, the bacteriocin, designated LA-147, showed inhibitory activity against strains of Lactobacillus leichmannii but not against several other species of Lactobacillus , or other selected gram-positive and gram-negative species. LA-147 (MW ca. 38.5 kDa) was bactericidal against sensitive cells; it was inactivated by heating for 15 min at 100°C, and by the action of protease and alpha-chymotrysin.