Antimicrobial effect of bacteriocin from Lactobacillus fermentum isolated from goat milk on perishable foods. San Luis. Argentina
Abstract
Strain selected for this study was isolated and named as sl36, from samples of goat milk collected from stainless steel drums in a dairy farm (San Luis, Argentina). The LAB strain was biochemically typified as Lactobacillus fermentum and designated as L. fermentum sl36. This identification was confirmed by 16S rRNA full sequences. The selected strain showed antimicrobial activity against food-borne pathogens, Listeria monocytogenes, Listeria innocua, Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa and Escherichia coli. The inhibitory activity was lost after treatment with trypsin, which indicates that this activity is due to a protein nature substance compatible with bacteriocins produced by Gram positive bacteria.The inhibitory substance was stable at different pH and temperatures. Perishable food samples (semi-hard cheese, cream, cooked pork shoulder) were treated with cell free supernatant from studied strain and then with indicator S. aureus and E. faecalis bacteria suspensions. Lactobacillus fermentum sl36 caused the inhibition of the growth of E. faecalis and S. aureus in the treated foods. Our work shows that it is possible to increase the safety of food perishable directly using the bacteriocins produced by the LAB strains, difference from the more frequent practice of using the bacteria themselves as probiotics.
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Introduction
There is an increasing demand on preservatives from natural sources due to potential toxicity of food chemical preservatives. In the last years, numerous studies have been focused on the reduction of the use of chemical preservatives; additionally, new preservation techniques by physical methods or by using natural antimicrobials have been developed. [1]
Lactic acid bacteria (LAB) have been used for centuries in food processing and are very important for their contribution to the value of products. Due to several metabolic properties, LAB play an important role in the food industry, for their contribution to flavour, smell, texture, sensory characteristics, therapeutic properties and nutritional value of food products. The use of LAB and its metabolites in food preservation is accepted as natural by consumers [2] and integrate an important group of generally recognized as safe (GRAS) for the American Food and Drug Administration (FDA). [3]
These microorganisms are found in very diverse environments, including resident microbiote of humans, as well as in cereals, fruits and vegetables, milk and meat, playing an essential role in the fermentation of these substrates and for the manufacture of many fermented foods and beverages[4].
The protective role of LAB lies in their capability to decrease pH and synthesis of bacteriostatic and bactericidal substances. These substances include hydrogen peroxide, lactic acid, carbon dioxide and bacteriocins which are defined as peptides produced by bacteria that inhibit or kill other related and unrelated microorganisms. In these years, a lot of studies have focused on the inhibition, by using bacteriocins, of food spoilage and of human pathogens associated with vegetable foods and beverages, and bacteriocin application has appeared as an interesting alternative to chemical compounds and antibiotics. [5], [6]
The bacteriocins produced by LAB exhibit properties that make them suitable for food preservation: recognized as GRAS, nontoxic and have no activity against eukaryotic cells, inactivated by digestive proteases, having little influence on the gut microbiota, are stable in wide ranges of pH and temperature, have a relatively broad antimicrobial spectrum against many food-borne pathogenic and spoilage bacteria, generally have a bactericidal mechanism and no cross resistance with antibiotics; and their genetic determinants are usually plasmid encoded, facilitating genetic manipulation, including the transfer of the gene clusters involved in their production to other food grade bacteria.[7]
Conclusion
The strain of Lactobacillus fermentum sl36 isolated in this region from raw goat milk showed antimicrobial activity that remain stable at different pH values and at different heat treatment. Our work demonstrates that it is possible to increase the safety of perishable foods by utilization of bacteriocins produced by LAB strains, unlike the using the bacteria as probiotics which is most frequent practice. The development of biotechnological alternatives that use bacteriocins, such as smart packaging and aerosols for application on surfaces, would improve the conditions of storage and preservation of food by controlling microorganisms. The discovery of new bacteriocins represents an opportunity in the production of substances with preservative qualities.