The diverse microbial flora found in dairy cow milk contributes to beneficial effects to human health. A group of microorganisms known as Lactic Acid Bacteria (LAB) are most commonly found and used in fermented dairy products. These bacterial strains embrace the idea of good nutrition by assisting with health maintenance, aiding in the prevention, control and treatment of many diseases. Heteropolysaccharides (HePS) produced by LAB plays an important role in the rheology, texture, body, and “mouthfeel” of fermented milk. HePS such as D-galactose, D-glucose, and L-rhamnose were tested under various conditions for comparative analysis of polysaccharide production efficiency. LAB strains were identified through biochemical tests such as gram stain, catalase, and motility tests. Even further, these strains were cultured in different temperature, pH, and incubation time. (add results).
This M.S. thesis will have an emphasis on lactic acid bacteria and their ability to produce extracellular polysaccharides under various conditions. The objectives addressed include 1) Determine LAB strains found in commercially sold dairy milk 2) Identify factors capable of impacting polysaccharide secretion. Addressing these focus points will allow further studies on these strains of bacteria capable of promoting human wellness.
1.1 Milk microbiome
Milk itself is known to contain several types of bacteria with one commonly being lactic acid bacteria (LAB). These group of bacteria are characterized as being Gram- positive, non-sporulating, anaerobic or facultative aerobic cocci or rod-shaped microorganisms. They produce lactic acid as one of the main fermentation products of the metabolism of carbohydrates. In addition to LAB, many other microorganisms are present in milk as it provides high nutrient content such as proteins, fats, carbohydrates, vitamins, minerals, and essential amino acids (10). This provides an ideal environment for the growth of many microbes. It is generally accepted that LAB is the dominant population in milk including Lactococcus, Lactobacillus, Leuconostoc, Streptococcus, and Enterococcus LAB genera (10).
Strains of non-LAB genera are also present in dairy milk including various yeasts and molds. In some cases, milk may be contaminated with microbial pathogens leading to severe illness. One prime example are bacterial strains, known as psychrotrophs, that are capable of surviving in cold storage consisting of Pseudomas and Acinetobacter spp (10). These types of bacteria can proliferate during refrigeration and produce extracellular proteins, such as lipases and proteases, negatively impacting the quality of milk resulting in spoilage (10). Another common example is Helicobacter pylori. These strains of bacteria can be found in raw sheep’s milk or in other contaminated milk products. They are responsible for cancers of the digestive tract known as gastric cancer. These microaerophilic spiral-shaped microbes deploy several mechanisms in surviving the stomach’s acidic environment including enabling their flagella to colonize human gastrointestinal tract, hydrolyzing urea and releasing ammonia with a urease enzyme to neutralize gastric acid, and adhering the gastric epithelium through receptor-mediated adhesion (20).
Several processing techniques such as thermization, Low Temperature Long Time (LTLT) pasteurization, High Temperature Short Time (HTST) pasteurization, sterilization, ultra-high temperature treatment, ultraviolet treatment, microwave treatment, membrane processing and microfiltration are used to treat raw milk for safe human consumption (13).
In contrast, dairy milk microorganisms can provide beneficial contributions to human health aiding in digestion or by reducing allergies (10). They are often defined as probiotics due to their assistance in health maintenance through treatment of diseases. For example, studies have shown that dietary supplementation of probiotic Lactobacillus reuteri to both aged humans and mice have shown to assist in younger appearance compared to their untreated counterparts (17). In addition, consumption of L. reuteri have shown to accelerate healing of skin wounds by up-regulating pituitary neuropeptide hormone oxytocin (17). Moreover, bacteria such as Propionibacterium freudenreichii derived from complex dairy products were shown to induce apoptosis among human colon cancer cell lines. While co-cultured with these cancer cells, P. freudenreichii secretes active compounds to trigger intrinsic mitochondrial apoptotic pathway in the human colorectal cancer cells (18). The release of these anti-carcinogenic metabolites were identified to be short chain fatty acids (SCFAs) and other unknown compounds (18). These studies have led to further investigations on probiotics and their ability to cease cancer development. Milk from cows, sheep, goats and humans all are source of microorganism that play a number of roles in the health and food industry.
1.2 Lactic acid bacteria
As mentioned, LAB is the predominate population in dairy milk. They can be naturally present in milk, cheese, meat, beverages, vegetables and could be isolated from soil, lakes, intestinal tract of animals and humans (6). In the food industry, LAB is highly utilized as a major application in food fermentation. They are grouped as Homofermenters or Heterofermenters in which the Homofermenters produce lactic acid as their main product of fermentation of glucose and Heterofermenters produce lactic acid, carbon dioxide, acetic acid, and ethanol from the fermentation of glucose (6). Hence, these group of bacteria are recognized for their fermentative ability to enhance food safety and supplying health benefits. These microorganisms through metabolic activities, including lipolysis and proteolysis, can produce organoleptic properties of products such as aroma and flavor compounds contributing to overall texture of fermented food (14).
The probiotic properties of LAB play an important role in maintaining undesirable pathogens and harmful bacteria. They have antibacterial aptness against Gram-negative and positive bacteria such as Esherichia coli, Pseudomas aeruginosa, and Staphlococcus aureus (12). In addition, LAB is resistant to lysozyme, gastric acid, gastrointestinal juice, and bile salts (12). Today, these bacteria are gaining attention medically and environmentally as potential tools for pathogenic treatment. LAB also stimulate a wide range of activities of the immune system of the host including the prevention of diarrhea caused by antibiotic treatment or viral infections, vitamin production, and reduction of cholesterol levels in the blood (14).
Recent observations of metagenomic data supported that LAB are a part of the microbiomes of humans and other animals. LAB are classified as gram-positive, non-spore forming bacteria that are microaerophilic or anaerobic. They generally have a low GC content (