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Essential oils: their antibacterial properties and potential applications in foods--a review.

Antibiotics have always been considered one of the wonder discoveries of the 20th century. This is true, but the real wonder is the rise of antibiotic resistance in hospitals, communities, and the environment concomitant with their use. The extraordinary genetic capacities of microbes have benefitted from man's overuse of antibiotics to exploit every source of resistance genes and every means of horizontal gene transmission to develop multiple mechanisms of resistance for each and every antibiotic introduced into practice clinically, agriculturally, or otherwise. This review presents the salient aspects of antibiotic resistance development over the past half-century, with the oft-restated conclusion that it is time to act. To achieve complete restitution of therapeutic applications of antibiotics, there is a need for more information on the role of environmental microbiomes in the rise of antibiotic resistance. In particular, creative approaches to the discovery of novel antibiotics and their expedited and controlled introduction to therapy are obligatory.

Origins and Evolution of Antibiotic Resistance

Bacteriocins of gram-positive bacteria.

There is an urgent need in aquaculture to develop microbial control strategies, since disease outbreaks are recognized as important constraints to aquaculture production and trade and since the development of antibiotic resistance has become a matter of growing concern. One of the alternatives to antimicrobials in disease control could be the use of probiotic bacteria as microbial control agents. This review describes the state of the art of probiotic research in the culture of fish, crustaceans, mollusks, and live food, with an evaluation of the results obtained so far. A new definition of probiotics, also applicable to aquatic environments, is proposed, and a detailed description is given of their possible modes of action, i.e., production of compounds that are inhibitory toward pathogens, competition with harmful microorganisms for nutrients and energy, competition with deleterious species for adhesion sites, enhancement of the immune response of the animal, improvement of water quality, and interaction with phytoplankton. A rationale is proposed for the multistep and multidisciplinary process required for the development of effective and safe probiotics for commercial application in aquaculture. Finally, directions for further research are discussed.

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Probiotic Bacteria as Biological Control Agents in Aquaculture

Bacteriocins are bacterially produced antimicrobial peptides with narrow or broad host ranges. Many bacteriocins are produced by food-grade lactic acid bacteria, a phenomenon which offers food scientists the possibility of directing or preventing the development of specific bacterial species in food. This can be particularly useful in preservation or food safety applications, but also has implications for the development of desirable flora in fermented food. In this sense, bacteriocins can be used to confer a rudimentary form of innate immunity to foodstuffs, helping processors extend their control over the food flora long after manufacture.

Bacteriocins: developing innate immunity for food

This chapter provides an overview of the biologically based preservation technologies termed “biopreservation.” Acid production by lactic acid bacteria (LAB) in temperature-abused foods (controlled acidification) is covered in the chapter. Some LAB produce antimicrobial proteins, called bacteriocins, which inhibit spoilage and pathogenic bacteria without changing (e.g., through acidification, protein denaturation, and other processes) the physicochemical nature of the food. While organic acids are usually added to foods, LAB can produce lactic acid in situ. The controlled production of acid in situ is an important form of biopreservation. There are many different ways to use bacteriocins in foods. The first is to add bacteriocins directly to the food for the purpose of inhibiting spoilage or pathogenic bacteria. The second way to use bacteriocins is to add bacteriocinogenic cultures to the food or use them as starter cultures that produce the bacteriocin in situ. A third way to use bacteriocins is to facilitate the use of defined starter cultures in fermented foods. Emulsifiers such as Tween 80 or the entrapment of the pediocin in multilamellar vesicles increases pediocin effectiveness in fatty foods. Nisin is the only bacteriocin approved internationally for use in foods. The biological methods of food preservation covered here mark only the beginning of the biopreservation era in the food industry.

Biopreservation of Foods

Treatment with solutions of alkali metal bicarbonates extends the shelf life of seafood by inhibiting the growth of bacteria that cause spoilage and odors, and improve the texture and moisture retention of the seafood.

Process for inhibiting the growth of bacteria on seafood

1,6-Diphenyl-1,3,5-hexatrine as a reporter of inner spore membrane fluidity in Bacillus subtilis and Alicyclobacillus acidoterrestris.

Treatment of Fusarium tricinctum NRRL 13426 cultures with dilute sodium bicarbonate resulted in dramatic reductions in the production of trichothecene mycotoxins, geraniol, and carotenoids. All of these compounds are biosynthesized from mevalonic acid (I). In addition, three apparent mevalonic acid metabolites were found in the bicarbonate-treated cultures at concentrations of several parts per million each. Two of these compounds were tentatively identified as 3-methyl-4-oxo-2-pentenoic acid (IV) and its rearrangement product, 4,5-dimethyl-5-hydroxy-2(5H)-furanone (VI). These results suggest a pH-related inhibition of mevalonate kinase or a pH-related activation of an enzyme that is responsible, at least in part, for the conversion of mevalonic acid to the pentenoic acid or its precursor

Mechanism for sodium bicarbonate inhibition of trichothecene biosynthesis in Fusarium tricinctum

The influence of oxygen on the metabolic regulation of glucose catabolism by Lactobacillus plantarum 8014 was examined in a chemostat (D=0.2 h−1, pH 5.5). A steadystate anaerobic culture was shifted to an aerobic condition by imposing an oxygen transfer rate of 1.84 mmol L−1 min−1. Enzyme activities, end product concentrations, and ATP levels were determined during the resultant transition and under steady-state aerobic conditions. During the transition, oxygen was consumed by the aerobic culture and the intracellular ATP concentration increased until hydrogen peroxide, which also increased, became inhibitory. Under the aerobic condition, the steady-state specific rate of substrate utilization and acetate production increased. The lactate dehydrogenase, NAD-dependent lactate dehydrogenase, acetate kinase, pyruvate oxidase, and NADH oxidase specific activities all increased under aerobic conditions. Changes in the intracellular lactate and acetate concentrations had a temporal correspondence with the changes in the aforementioned catabolic activities. Subsequent experiments using chloramphenicol to inhibit de novo protein synthesis and kinetic analysis of crude enzyme extracts suggested that these changes were due to both changes in synthesis and activity levels.

Thomas J. Montville

Papers

Biopreservation of Foods

Process for inhibiting the growth of bacteria on seafood

1,6-Diphenyl-1,3,5-hexatrine as a reporter of inner spore membrane fluidity in Bacillus subtilis and Alicyclobacillus acidoterrestris.

Mechanism for sodium bicarbonate inhibition of trichothecene biosynthesis in Fusarium tricinctum

Enzymatic regulation of glucose catabolism by Lactobacillus plantarum in an aerobic chemostat