CHARACTERIZATION AND ANTIMICROBIAL ACTIVITIES OF LACTIC ACID BACTERIA ISOLATED FROM SOME FERMENTED FOODS

ABSTRACT

This study was aimed at establishing the antimicrobial activity of lactic acid bacteria (LAB) on some food spoilage and pathogenic organisms. Serial dilutions of fermented maize and sorghum samples were performed and 0.1ml of the appropriate dilutions were inoculated on de Man Rogosa Sharpe medium for the isolation of lactic acid bacteria. Lactococcus and Lactobacillus species were isolated, identified and characterized. The test organisms were confirmed by biochemical tests to be Pseudomonas sp., Escherichia coli, Serratia marcescens, Bacillus sp. and Staphylococcus sp. The two LAB isolates were further confirmed molecularly to be Lactococcus specie 104.6 and Lactobacillus fermentum strain SN5. Using the agar well diffusion method, the two isolates inhibited the growth of all the test organisms except Pseudomonas aeruginosa which showed no zone of inhibition. The inhibitory activity of Lactococcus sp. 104.6 was the highest, 12.7±2.31 mm on S. marcescens, 11±1.00 mm on Staphylococcus sp. 9.6±1.52 mm on E. coli with the least inhibitory activity 8.3±1.52 mm on Bacillus sp. The highest inhibitory activity of Lactobacillus fermentum was also observed as 17.0±1.00mm on S. marcescens, followed by 14.3±1.15 mm on E. coli, 9.0±1.73 mm on Bacillus sp. with the least inhibitory activity 11±1.00mm on Staphylococcus sp. The co-culturing of lactic acid bacteria and test organisms indicated gradual reduction in the number of test organisms during the period of monitoring. The GC-MS analysis of Lactobacillus fermentum showed five volatile organic acids namely, acetic acid, propionic acid, isobutyric, butyric acid and isovaleric acid. The results of this study suggest that Lactococcus species and Lactobacillus fermentum can be used as biopreservatives in food industries rather than the synthetic chemical preservatives which is commonly used.

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CHAPTER 1

INTRODUCTION

1.1 BACKGROUND INFORMATION

In Africa, majority of the foods consumed are fermented. The fermentation of such foods is usually carried out by few groups of bacteria and yeasts. The lactic acid bacteria are involved in fermentation of many African foods and beverages (Wassie and Wassie, 2016). Lactic acid bacteria (LAB) are a group of Gram positive, non-spore forming, anaerobic or facultative aerobic cocci or rods, which produce lactic acid as one of their major fermentation products of the carbohydrates metabolism (Hayek and Ibrahim, 2013). LAB through their fermentative ability enhances the shelf life of the fermented product (Sharma et al., 2012). They produce varieties of antimicrobial metabolites during the process of fermentation such as lactic acid, acetic acid, ethanol, hydrogen peroxide, diacetyl and others. (Diop et al., 2010). These metabolites produced confer preservative ability on them as a natural competitive means to overcome other microorganisms sharing the same niche (Oliveira et al., 2008). Nowadays consumers prefer food with few or no chemical preservatives. As a result, interests in LAB have increased over period because of their safe association with fermented foods which are consumed by humans. Attention has focused on the ability of LAB to produce proteinaceous substances known as bacteriocins that inhibit the growth of pathogens such as Listeria, Clostridium, Staphylococcus, Bacillus spp. and Enterococcus spp. Intensive study of bacteriocins produced by LAB are being carried out because of their antagonistic activities against food-borne bacteria. Bacteriocin producing strains of LAB may be of great significant in competing with other organisms in intestine. They are made up of a biologically active protein moiety which have a bactericidal mode of action and are specific while attaching to cell receptors. Lactic acid bacteria thus enhance food safety, improve organoleptic attributes, enrich nutrients and increases overall health benefits (Steele et al., 2013).

Most food spoilage meant for human consumption is caused by microorganisms. Fungi and bacteria when given access to foods that are unprotected rapidly increase in population, producing distasteful chemicals and toxins in some cases (Pitt and Hocking, 2009).  Due to the ubiquitous nature of microbes, they have such enormous populations, and are often dispersed as spores in air, water, or soil thereby they colonizing unprotected foods. Humans developed two main strategies in order to prevent microbial food spoilage; reducing  their access to the susceptible foodstuffs and  inhibiting growth thereby limiting population size by creating an unfavourable environment (Rawat, 2015). Both fermented and unfermented food products are known to be susceptible to spoilage during storage. Their spoilage during storage is said to be due to the presence of the spoilage microbes and the enzymes they produce, which breakdown the food/food product into entirely different substances leading to changes in their organoleptic properties (Fadahunsi et al., 2013). Although starter cultures have been widely used in some fermentation of foods to enforce uniformity in their composition, reduced shelf life still pose as a major problem. The consumption of products contaminated with spoilage and pathogenic microorganisms could serve as a potential health threat hence the need to control microbial contamination of foods.

1.2 PROBLEM STATEMENT

These days, consumers are concerned about foods preserved with chemical substance which may leave chemical residues in their bodies after consumption and cause adverse health effects when accumulated. However, they now rely on less processed foods which may harbour microorganisms which can cause food spoilage or food-borne diseases.

1.3 JUSTIFICATION

The solution to food preservation challenges can be the use of antimicrobial metabolites produced by microorganisms with the status generally recognized as safe (GRAS) of which lactic acid bacteria is one. Most researchers have reported the antimicrobial potential of lactic acid bacteria using their cell free supernatant/bacteriocin (Adebayo and Aderiye, 2007; Sharaf and Al Habi, 2011). This work focused on the evaluation of the strains that produce antimicrobial metabolites and have wide spectrum activity on food pathogens so they can be useful in food preservation in food industries.

1.4 GENERAL OBJECTIVE

To characterize strains of Lactic acid bacteria isolated from some fermented foods consumed in our localities and to evaluate their antimicrobial activity on some food spoilage microorganisms and food-borne pathogens.

            1.4.1        Specific objectives

1.      To isolate and identify phenotypically, biochemically and molecularly, Lactic acid bacteria from fermented cereals, namely; maize and sorghum.

2.      To determine the growth characteristics of the Lactic acid bacteria isolates in various conditions.

3.      To determine the antimicrobial activities of the LAB on some food spoilage microorganisms and food-borne pathogens using agar well diffusion and co-culturing method.

4.      To identify the organic acids apart from lactic acid produced by the strains using GC-MS analysis.

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