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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

The natural preservative nisin can be effectively used to control the growth of Listeria monocytogenes, a foodborne pathogen. The psychrotrophic pathogen is capable of multiplying at refrigeration temperatures, altering its membrane fluidity by decreasing the length and degree of saturation of the membrane fatty acids (8). The presence of L. monocytogenes in hot dogs, smoked salmon, cheese, and ready-to-eat chicken and turkey, for example, has resulted in recalls of these products owing to the zero-tolerance standards for the organism in ready-to-eat foods (U.S. Food and Drug Administration, 2001 [www.fda.gov/oc/po/firmrecalls/archive.html]; U.S. Department of Agriculture Food Safety Inspection Service, 2001 [www.fsis.usda.gov/OA/recalls/recdb/rec2000.htm]).

The mode of action of nisin has been extensively researched. As a Class Ia bacteriocin, its bactericidal action results from the formation of transient pores in the target cell membranes (1, 21). Lipid II, a potential docking molecule for nisin, appears to enhance the activity of the 34-amino-acid peptide (4).

Nisin resistance is well documented, but the exact mechanism of nisin resistance remains to be elucidated. The frequency of nisin resistance ranges from 10−2 to 10−7 but can be decreased by exposing cells to an acidic environment low in salt at 10°C (10). Several authors have correlated nisin resistance with a change in membrane composition. Others have noted the up or down regulation of proteins with functions that do not have a clear relationship to nisin resistance (11). Davies et al. (7) concluded that mutations causing nisin resistance were the result of “mechanism(s) other than an alteration of membrane composition,” citing cell wall modifications as a probable cause. However, Crandall and Montville (6) observed changes in the composition of the nisin-resistant cell membrane when cells were grown in the presence of nisin. Lipid analysis of membranes from nisin-resistant cells shows that there is an increase in the proportion of straight-chain and saturated fatty acids and that the ratio of C15:C17 is decreased, resulting in a more rigid membrane (14).

Our laboratory has found that a spontaneous nisin-resistant L. monocytogenes strain had increased sensitivity to acid (15). After finding that the changes in the membrane of the nisin-resistant strain did not alter the permeability to acid, we hypothesized that differences in acid sensitivity were due to differences in the energy status of the cells. Here we show that although there is no difference in acidification of the cytoplasm, the ATPase activity of the mutant is greater than that of the wild type under acidic conditions. Upon exposure to acid (either organic or inorganic), intracellular ATP levels rapidly drop. This suggests that when the nisin-resistant strain is treated with acid, the ATPase depletes cellular ATP levels and the cells are unable to survive.

Increased ATPase Activity Is Responsible for Acid Sensitivity of Nisin-Resistant Listeria monocytogenes ATCC 700302

The emergence of Listeria monocytogenes as a foodborne pathogen in the USA during the 1980s prompted rapid, but often overlapping and conflicting, responses by the US Food and Drug Administration, the US Department of Agriculture and the food industry. Nonetheless, because of these responses, there have been no outbreaks of foodborne listeriosis reported in the USA during the past nine years. This article briefly discusses the history and etiology of listeriosis, and examines the nature of and reasons for the varied regulatory and industrial responses aimed at preventing listeriosis and L. monocytogenes contamination in ready-to-eat foods. Based on the experience gained from the response to L. monocytogenes , a paradigm for dealing with novel foodborne pathogens in the future is proposed.

The regulatory and industrial responses to listeriosis in the USA: A paradigm for dealing with emerging foodborne pathogens

Bicarbonate inhibition of Saccharomyces cerevisiae and Hansenula wingei growth in apple juice.

Sodium Bicarbonate Inhibition of Aflatoxigenesis in Corn.

We modeled nisin's anticlostridial activity and assessed the antagonistic or potentiating influences of food ingredients. The model systems contained yeast extract, proteose peptone, and glucose; were supplemented with protein (0.075, 0.75, 7.5% w/v), phospholipid (0.075, 0.75, 7.5% w/v), or soluble starch (5, 17.5, 30% w/v); and were adjusted to pH 5.5, 6.0, or 6.5. Samples inoculated with 104/mL spores were incubated at 15, 25, or 35°C. Statistical analysis developed an equation (r2= 0.76) that modeled the response and identified temperature as the most significant (α 0.001) variable. Nisin lost effectiveness with increasing temperature. Nisin concentration had significant positive and phospholipid negative, linear effects. Many interactive effects were significant (α 0.20). Nisin inhibited C. botulinum until its residual level dropped below a threshold, which decreased from 154 IU/mL at 35°C to 12 IU/mL at 15°C.

Thomas J. Montville

Papers

Increased ATPase Activity Is Responsible for Acid Sensitivity of Nisin-Resistant Listeria monocytogenes ATCC 700302

The regulatory and industrial responses to listeriosis in the USA: A paradigm for dealing with emerging foodborne pathogens

Bicarbonate inhibition of Saccharomyces cerevisiae and Hansenula wingei growth in apple juice.

Sodium Bicarbonate Inhibition of Aflatoxigenesis in Corn.

Quantification of Factors Which Influence Nisin's Inhibition of CIostridium botulinum 56A in a Model Food System