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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

Sodium bicarbonate (SB) inhibited the growth of bacteria and yeasts in agar media model systems under certain conditions. Escherichia coli, Lactobacillus plantarum, Staphylococcus aureus, and Pseudomonas aeruginosa aerobic plate counts were reduced 10,000-fold by 0.12M (1% w/v) SB. Saccharomyces cerevisiae and Hansenula wingei were more sensitive; counts were reduced 100,000-fold by 0.06M SB. Potassium bicarbonate was equally inhibitory, but equimolar sodium chloride had no effect, ruling out osmotic- and sodium-mediated mechanisms of inhibition. The bicarbonate ion was identified as the probable cause of SB-mediated inhibition although, in some cases, pH elevation played a significant role.

Antimicrobial Activity of Sodium Bicarbonate

This paper examines the synergistic action of carbon dioxide and nisin on Listeria monocytogenes Scott A wild-type and nisin-resistant (Nisr) cells grown in broth at 4°C. Carbon dioxide extended the lag phase and decreased the specific growth rate of both strains, but to a greater degree in the Nisr cells. Wild-type cells grown in 100% CO2 were two to five times longer than cells grown in air. Nisin (2.5 μg/ml) did not decrease the viability of Nisr cells but for wild-type cells caused an immediate 2-log reduction of viability when they were grown in air and a 4-log reduction when they were grown in 100% CO2. There was a quantifiable synergistic action between nisin and CO2 in the wild-type strain. The MIC of nisin for the wild-type strain grown in the presence of 2.5 μg of nisin per ml increased from 3.1 to 12.5 μg/ml over 35 days, but this increase was markedly delayed for cultures in CO2. This synergism between nisin and CO2 was examined mechanistically by following the leakage of carboxyfluorescein (CF) from listerial liposomes. Carbon dioxide enhanced nisin-induced CF leakage, indicating that the synergistic action of CO2 and nisin occurs at the cytoplasmic membrane. Liposomes made from cells grown in a CO2 atmosphere were even more sensitive to nisin action. Liposomes made from cells grown at 4°C were dramatically more nisin sensitive than were liposomes derived from cells grown at 30°C. Cells grown in the presence of 100% CO2 and those grown at 4°C had a greater proportion of short-chain fatty acids. The synergistic action of nisin and CO2 is consistent with a model where membrane fluidity plays a role in the efficiency of nisin action.

Carbon dioxide and nisin act synergistically on Listeria monocytogenes.

Nisin activity is usually quantified using agar diffusion methods. The assay sensitivity (increase in zone size/increase in nisin concentration) for nisin was greatly increased by using Lactobacillus sake ATCC 15521 as the indicator organism, rather than Micrococcus luteus ATCC 10420, the previous indicator organism of choice.

Thomas J. Montville

Papers

Antimicrobial Activity of Sodium Bicarbonate

Carbon dioxide and nisin act synergistically on Listeria monocytogenes.

Improved agar diffusion assay for nisin quantification

Models and mechanisms for bacteriocin action and application.

Inhibition of Listeria monocytogenes in cold-smoked salmon by Carnobacterium piscicola CS526 isolated from frozen surimi