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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

Alicyclobacillus acidoterrestris is an acidophilic, spore-forming spoilage organism of concern for the fruit juice industry The occurrence of bacterial spore-formers in low pH foods was once thought to be insignificant However, in recent years, spoilage of acidic juices by Alicyclobacillus has been recognized as a serious problem A acidoterrestris has been associated with commercially pasteurized fruit juices as well as other low pH, shelf-stable products such as bottled teas and isotonic drinks It has been isolated from garden and forest soils and may be introduced into the manufacturing process through unwashed or poorly washed fruit If spores are not destroyed by processing, they can germinate, grow and spoil product The commonly used “hot pack” process for high-acid foods has proven inadequate for the control of A acidoterrestris spores, creating the need for alternate processing technologies

PRACTICAL APPLICATIONS
A better understanding of the characteristics of Alicyclobacillus acidoterrestris, its sources and signs of spoilage in high-acid beverages, and possible interventions will assist processors in developing appropriate strategies for its control in juices, teas and isotonic waters

Alicyclobacillus Acidoterrestris: The Organism, the Challenge, Potential Interventions

This chapter addresses three issues: (i) the ability of bacteria to use different biochemical pathways to generate the energy required to grow under adverse conditions in foods; (ii) the interaction of bacteria and foods in ecosystems in which the cells may exist in a variety of physical and physiological states and in which the roles of intrinsic and extrinsic factors and (iii) the kinetics of microbial growth. It talks about microbial physiology and metabolism, and glycolytic pathways such as Embden-Meyerhof-Parnas Pathway and Entner-Doudoroff Pathway. It also discusses the heterofermentative catabolism, homofermentative catabolism, tricarboxylic acid cycle, aerobes, anaerobes, the regeneration of NAD, and respiration, bioenergetics. The food ecosystem is composed of intrinsic factors, which are inherent to the food (i.e., pH, water activity [aw], and nutrients) and extrinsic factors, which are external to it (i.e., temperature, gaseous environment, and the presence of other bacteria). In addition, the chapter focuses on the physiological and genetic responses that bacteria utilize in osmoregulation. Bacteria are classified as psychrophiles, psychrotrophs, mesophiles, and thermophiles according to how temperature influences their growth. Microbial growth in foods is a complex process governed by genetic, biochemical, and environmental factors.

Physiology, Growth, and Inhibition of Microbes in Foods

Food microbiologists must understand microbiology and food systems and be able to integrate them to solve problems in complex food ecosystems. This chapter addresses this in three parts by (i) examining foods as ecosystems and discussing intrinsic and extrinsic environmental factors that control bacterial growth, (ii) explaining first-order or pseudo-first-order kinetics which govern the log phase of microbial growth and many types of lethality, and (iii) focusing on physiology and metabolism of foodborne microbes. Growth of Clostridium botulinum in foods such as potatoes and sauteed onions exposed to air has caused botulism outbreaks. Bacteria are classified as psychrophiles, psychrotrophs, mesophiles, and thermophiles according to the way in which temperature influences their growth. Additional barriers to microbial growth should be incorporated into refrigerated foods containing no other inhibitors. Food microbiology is concerned with all four phases of microbial growth. Growth curves showing the lag, exponential logarithmic or log, stationary, and death phases of a culture are normally plotted as the number of cells on a logarithmic scale or log10 cell number versus time. These plots represent the states of microbial populations rather than individual microbes. Thus, both the lag phase and stationary phase of growth represent periods when the growth rate equals the death rate to produce no net change in cell numbers. Food microbiologists frequently use doubling times (td) to describe growth rates of foodborne microbes. Developments in molecular biology and microbial ecology will change or deepen the perspective about the growth of microbes in foods.

Growth, Survival, and Death of Microbes in Foods

Listeria monocytogenes inhibition by bacteriocinogenic Carnobacterium piscicola DX or non-bacteriocinogenic C. piscicola 2818 was examined in vacuum-packed minimally processed chicken cubes with gravy at 4, 8 and 15C. C. piscicola DX and C. piscicola 2818 at 10 4 CFU/g were coinoculated individually with a pool of three Listeria monocytogenes strains at 10 2 CFU/g. At 4 and 8C, C. piscicola DX inhibited growth of L. monocytogenes by 3 logs, significantly (P<0.05) more than did C. piscicola 2818. At 15C both C. piscicola strains inhibited L. monocytogenes by one log cycle. The pH ofall inoculated systems decreased from an initial pH of 6.14 to final values ranging from 6.06 to 5. 65 depending on the inocula used. Bacteriocin was detected in the systems coinoculated with C. piscicola DX. These studies demonstrate that lower temperatures and bacteriocin production enhanced L. monocytogenes inhibition by C. piscicola.

Inhibition of listeria monocytogenes by carnobacterium piscicola in vacuum‐packaged cooked chicken at refrigeration temperatures

Oxidation-reduction potentials (Eh) of canned foods ranged from -18 to -438 mV. Foods packed in glass had higher redox potentials than those packed in cans. Only 4 out of 26 products tested reached positive redox values after exposure to air for 24 hr at 4°C. Inoculated containers of mushrooms, whole corn, cream corn, asparagus, beef gravy, kidney beans, green beans, cream of mushroom soup, cheddar cheese soup, and lima beans supported toxin production by C. botulinum; potatoes and beets did not.

Thomas J. Montville

Papers

Alicyclobacillus Acidoterrestris: The Organism, the Challenge, Potential Interventions

Physiology, Growth, and Inhibition of Microbes in Foods

Growth, Survival, and Death of Microbes in Foods

Inhibition of listeria monocytogenes by carnobacterium piscicola in vacuum‐packaged cooked chicken at refrigeration temperatures

Oxidation‐Reduction Potentials of Canned Foods and Their Ability to Support Clostridium Botulinum Toxigenesis