EVALUATION OF LACTIC ACID BACTERIA AND ITS RESPONSE TO HEAT STRESS

ABSTRACT

Lactic Acid Bacteria (LAB) are the most widespread group of bacteria that is used in fermented foods. They are natural inhabitants of the human gastric intestine, and can be applied in different fermented products and probiotic foods. This study was to evaluate lactic acid bacteria response to heat stress of which two (2) samples of Ogiri were selected for the isolation and evaluation of lactic acid bacteria and its response to heat stress. From this study three (3) lactic acid bacteria were isolated and identified from the Ogiri sample using colonial morpholOgiries, Gram staining, motility and biochemical tests. These revealed the major lactic acid bacteria to be Lactobacillus plantarium, Lactobacillus helveticus, and Lactobacillus brevis. In this present study, the different temperatures at which the lactic acid bacteria species were treated for 30 minutes before inoculation and incubation at 37oC revealed that the total bacterial counts for L. planetarium had its maximum at 50oC (1.37x105) but was declined below and above 50°C temperature. This revealed that L. planetarium was able to survive heat stress at a maximum temperature of 50°C. In addition it was also revealed in this study that Lactobacillus helveticus and Lactobacillus brevis remained cultivable at 55oC (1.41x105) and 40oC (1.99x105) respectively, which demonstrates that bacteria are able to withstand such adverse environmental conditions. Bacteria activate mechanisms allowing them to adapt to new conditions, which can influence the viability and technolOgirical properties. Lactic Acid Bacteria can be used as live cells at optimum temperature since they have a Generally Recognized as Safe status during the production of bacteriocins. Further research needs to be done to find out more on the various temperatures at which lactic acid bacteria can survive.

TABLE OF CONTENTS

Title Page                                                                                                                                i

Certification                                                                                                                           iii

Dedication                                                                                                                              iv

Acknowledgements                                                                                                                v

Table of Contents                                                                                                                   vi

List of Tables                                                                                                                          vii

Abstract                                                                                                                                  ix

CHAPTER ONE: INTRODUCTION                                                                                   1

1.1       Background of Study                                                                                                  1

1.2       Aim of Study                                                                                                              4

CHAPTER TWO: LITERATURE REVIEW                                                                      5

2.1       Lactic Acid Bacteria                                                                                                   5

2.2       Classification of Lactic Acid Bacteria                                                                       6

2.2.1    Lactobacillus                                                                                                              6

2.2.2    Bifidobacteria and Propionibacteria                                                                           7

2.3       Lactic Acid Bacteria and Stress: Basic Concepts                                                       7

2.4       Lab Stress Response Mechanisms                                                                              9

2.5       Gene Regulation in the Lab Stress Response                                                             10

2.6       Principal Responses to the Most Common Stresses                                                   12

2.6.1    Heat Shock Response                                                                                                 12

2.6.2    Cold Shock Response                                                                                                 13

2.6.3    Oxidative Stress Response                                                                                         15

2.6.4    Acid Stress Response                                                                                                 16

2.6.5    Osmotic Stress Response                                                                                            18

2.6.6    High Pressure Stress Response                                                                                   19

2.6.7    Competition and Communication                                                                              20

2.7       Sensing and Signaling Stresses in Lab                                                                       21

2.7.1    Two-Component Systems                                                                                          22

2.7.2    One-Component Systems                                                                                           23

2.7.3    Thermosensors in LAB                                                                                               24

CHAPTER THREE: MATERIALS AND METHODS                                                      25

3.1       Sample Collection                                                                                                      25

3.2       Materials and Apparatus                                                                                             25

3.2.1    Sterilization of Materials                                                                                            25

3.2.2    Normal Saline Preparation                                                                                         25

3.2.3    Media Preparation for Isolation of Lactic Acid Bacteria from the Ogiri Samples             25

3.3       Isolation of Lactic Acid Bacteria                                                                               26

3.3.1    Sub-Culturing                                                                                                            26

3.4       Characterization and Identification of Lactic Acid Bacterial Isolates                        26

3.4.1    Gram Staining Techniques                                                                                         26

3.4.2    Motility test                                                                                                                27

3.5       Biochemical Test                                                                                                        27

3.5.1    Catalase test                                                                                                                27

3.5.2    Coagulase test                                                                                                             27

3.5.3    Methyl red test                                                                                                            27

3.5.4    Voges-proskaeur test                                                                                                  28

3.5.5    Indole test                                                                                                                   28

3.5.6    Citrate test                                                                                                                   28

3.5.7    Oxidase test                                                                                                                29

3.6       Determination of Lactic Acid Bacteria Response to Different Heat Stress               29

CHAPTER FOUR

4.0       Results                                                                                                                        30

CHAPTER FIVE: DISCUSSION AND CONCLUSION                                                    33

5.1       Discussion                                                                                                                   33

5.2       Conclusion                                                                                                                  35

            References

LIST OF TABLES

CHAPTER ONE

INTRODUCTION

1.1       BACKGROUND OF STUDY

Lactic Acid Bacteria (LAB) are the most widespread group of bacteria that is used in fermented foods. They are natural inhabitants of the human gastric intestine, and can be applied in different fermented products and probiotic foods (Ficco et al., 2009). They are present in products like yogurts, sourdoughs, sour vegetables, cheese, wine or meat and play a crucial role in the devel­opment of the organoleptic and hygienic quality of fer­mented products (Van-De-Gutche et al., 2002). The tech­nological benefit of Lactic Acid Bacteria depends on the ability to enhance safety, flavour, texture and nutritional value (Salminen and Von-Wright, 2004). Some LAB, due to their probiotic properties, can be used in the produc­tion of functional food and potential oral vaccines (Parente et al., 2010).

At the same time, LAB can cause spoilage of food. They can grow in improperly pasteurized beverages and juices in bottles and cans, in vacuum packed products with a deficit of oxygen. LAB can enter a given product along with the raw material, additives or with packing ma­terials (Lawlor et al., 2009). The most common species that cause spoilage of beverages are Lactobacillus paracasei and Leuconostoc mesenteroides, as well as Lactoba­cillus brevis, Lactobacillus buchneri, Lactobacillus plantarum, Lac­tobacillus perolens and Weissella confuse (Back, 2005). Many bacteria from these species are also responsible for beer spoilage. LAB mainly ferments sucrose to lactic acid. Depend­ing on the species and growth conditions, catabolism of sugars can also lead to the formation of ethanol, ace­tate, formic acid or succinate (Hammes and Hertel, 2009). Some of the bacteria can produce diacetyl that gives a bitter taste and flavour of the products. That is why LAB are undesirable in beverages and juices. It has been reported that formic acid in apple juices can indicate food spoilage (Gökmen and Acar, 2004). The L. mesenter­oides and W. confusa bacteria can synthesize compounds which cause ropiness of the final product (Back, 2005). Ropiness caused by LAB is the reason why these bacte­ria are believed to be potentially a cider spoilage indica­tor (Ibarburu et al., 2010). In alcohol beverages, LAB can influence the bitter flavor by converting glycerol to 3-hydroxypropionalde­hyde, which can transform to acrolein and bind with polyphenols creating bitter compounds (Juvonen et al., 2011).

Lactic acid bacteria (LAB) constitute a heterogeneous group of bacteria which are found in diverse environments from the human and animal body to plants. These bacteria have been used for long to produce various fermented foods from products derived from animals (milk, meat, fish, etc.) or plants (vegetables, wine, olives, etc.) (Stiles, 2006). The industrialization of food bio-transformations increased the economical importance of LAB. Although LAB are a low cost ingredient of the food transformation processes, they play a crucial role in the development of the organoleptique and hygienic quality of fermented products. Therefore, the reliability of starter cultures in terms of quality and functional properties (important for the development of aroma and texture), but also in terms of growth performance and robustness has become essential for successful fermentations.

However, in addition these strains must also resist the adverse conditions encountered in industrial processes, for example during starter handling and storage (freeze-drying, freezing or spray-drying). The development of new applications such as live vaccins and probiotic foods reinforces the need for robust LAB since they may have to survive in the digestive tract, resist the intestinal flora, eventually colonize the digestive or uro-genital mucosa and express specific functions in conditions unfavorable to growth (for example, during stationary phase or storage) (Schiffrin et al., 2001). Except probiotic strains for which high tolerance to acid and bile was used as a selection criteria, LAB have seldomly been selected for stress resistance (Dunne et al., 2009).

However, bacteria are not only submitted to potentially stressful environmental changes in industrial processes, but also in nature where the ability to quickly respond to stress is essential for survival (Stortz et al., 2000) It is now well established that LAB, like other bacteria, evolved stress-sensing systems and defenses against stress which allow them to withstand harsh conditions and sudden environmental changes. Although a microorganism could, in theory, have specific regulators tailored to each of its regulated genes and adapt their expression according to its environment, this would represent a tremendous genetic burden. Instead, regulators usually control several genes and sometimes even control other regulators (VanBogelen et al., 2009). Stress defenses are good examples of such integrated regulation systems. Bacterial stress responses rely on the coordinated expression of genes which alter different cellular processes (cell division, DNA metabolism, housekeeping, membrane composition, transport, etc.) and act in concert to improve the bacterial stress tolerance (Stortz et al., 2000).

The integration of these stress responses is accomplished by networks of regulators which allow the cell to react to various and complex environmental shifts. Identifying regulators and regulatory networks is essential if the goal is to control, predict or engineer LAB behavior (in given conditions). The knowledge of regulators and a better understanding of LAB stress responses could constitute a basis of comparison with the well known model micro-organisms, E. coli and B. subtilis. Such comparisons should reveal the specificity of LAB stress responses which may have evolved and been selected indirectly to fit the specific constraints of a given substrate and/or process (for example, milk and milk fermentation). The current knowledge on the environmental stress responses in LAB varies between species and depending on the type of stress. The best studied are acid, heat and cold stress, although for the latter most of the studies focused on a specifically on heat response.

1.2       AIM OF STUDY

The aim of this study is to evaluate lactic acid bacteria response to heat stress, while the specific objectives are;

·       To isolate and identify strains of lactic acid bacteria from known sample

·       To determine the effect of temperature on the survival of lactic acid bacteria

Buyers has the right to create dispute within seven (7) days of purchase for 100% refund request when you experience issue with the file received. 

Dispute can only be created when you receive a corrupt file, a wrong file or irregularities in the table of contents and content of the file you received. 

ProjectShelve.com shall either provide the appropriate file within 48hrs or send refund excluding your bank transaction charges. Term and Conditions are applied.

Buyers are expected to confirm that the material you are paying for is available on our website ProjectShelve.com and you have selected the right material, you have also gone through the preliminary pages and it interests you before payment. DO NOT MAKE BANK PAYMENT IF YOUR TOPIC IS NOT ON THE WEBSITE.

In case of payment for a material not available on ProjectShelve.com, the management of ProjectShelve.com has the right to keep your money until you send a topic that is available on our website within 48 hours.

You cannot change topic after receiving material of the topic you ordered and paid for.