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Lactic Acid Bacteria (page 4)

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Bacteriocins and LAB

Class I bacteriocins or lantibiotics are small peptides containing the unusual dehydroamino acids and thioether amino acids lanthionin and 3-methyllanthionine, which are synthesized by Gram-positive bacteria during posttranslational modifications. These peptides are thought to attach to the membrane of target cells and, by an as yet unknown conformational rearrangement, lead to increased permeability and disruption of the membrane potential. There are two types of lantibiotics, types A and B. The lantibiotics produced by LAB all belong to type A, which are elongated screw-shaped peptides, whereas type B lantibiotics are mainly globular. Nisin produced by Lactococcus lactis ssp. lactis has been studied extensively. It has a broad spectrum of activity against Gram-positive bacteria. The primary target is believed to be the cell membrane. Unlike some other antimicrobial peptides, nisin does not need a receptor for its interaction with the cell membrane; however, the presence of a membrane potential is required.

Starter cultures

Starter cultures consisting of lactic acid bacteria are added at the beginning of the cheesemaking process. Lactic acid bacteria are essential for manufacture of cheese, yogurt, sour cream, cultured butter and most fermented milk products.

Starter cultures play an essential part in the manufacture of fermented dairy products. They produce the lactic acid that coagulates milk and they contribute to texture, moisture content, freedom from pathogenic microorganism, and taste of the product. The rate of acid production is critical in the manufacture of certain products, e.g. Cheddar cheese. Depending on the product, especially in mechanized cheese production units, starters may also be required to produce acid at a consistently fast rate through the manufacturing period each day and every day. The negative redox potential created by starter growth in cheese also aids in preservation and the development of flavor in Cheddar and similar cheeses. Additionally antibiotic substances, now referred to as bacteriocins, produced by starters, e.g. nisin, may also have a role in preservation.

Ecology of starter bacteria
Most starters in use to today have their origins as lactic acid bacteria naturally present as part of the contaminating microflora of milk. These bacteria probably came from vegetation in the case of lactococci or the intestinal tract in the case of bifidobacteria, enterococci and Lactobacillus acidophilus.

Modern starter cultures developed from the practice of retaining small quantities of whey or cream from the successful manufacture of a fermented product on a previous day and using this as the inoculum or starter for the following day’s production. In the foods industry his practice has been referred to as "back-slopping".

Classification of starter  cultures
While the microbes used in the manufacture of fermented dairy products are generally lactic acid bacteria, Propionibacterium shermanii and Bifidobacterium spp. which are not lactic acid bacteria, are also used. In addition, other bacteria including Brevibacterium linens, responsible for the flavor of Limburger cheese; and molds (Penicillium species) are used in the manufacture of Camembert, Roquefort and blue cheeses.

Probiotics and Lactic Acid Bacteria

Probiotics are products designed to deliver potentially beneficial bacterial cells to the microbiotic ecosystem of humans and other animals. Strains of lactic acid bacteria are the most common microbes employed as probiotics, especially Lactobacillus and Bifidobacterium species, but lactococci, some enterococci and some streptococci are also included as probiotics.

Lactic acid-producing Bacteria Used as Probiotics

Lactobacillus species are facultative anaerobes. They grow in the presence of O2, however, and may convert it to H2O or H2O2.
Lactobacilli normally predominate in the small intestine, and they are known for their beneficial effects which may antagonize potential pathogens. Of the more than 100 Lactobacillus species, the following are commonly used probiotics:
L. acidophilus 

L. fermentum
L. paracasei
L. brevis
L. gasseri
L. plantarum
L. bulgaricus
L. helveticus
L. reuteri
L. casei
L. jensenii
L. rhamnosus
L. crispatus
L. johnsonii
L. salivarius

Bifidobacterium is not included in the traditional Lactic Acid Bacteria due to its genetic unrelatedness, but the bacterium has a habitat that overlaps with LAB, and it has a metabolism that produces lactic acid as a primary end-product
of fermentation.  Bifidobacteria are strictly anaerobic and normally vie for predominance in the large intestine. Among 30 species, those recognized as probiotics include:
B. adolescentis
B. breve
B. longum
B. animalis
B. infantis
B. thermophilum
B. bifidum
B. lactis

Bifidobacterium longum. Bifidobacteria are an obligately anaerobic bacteria, not classified with the lactic acid bacteria, but which occupy similar habitats and produce lactic acid as a sole end-product. They are a prominant Gram-positve bacterium in the large intestine (colon). Bifidobacterium infantum is the predominant bacterium in the intestine of breast-fed infants because mother's milk contains a specific growth factor that enriches for the growth of the bacterium.

Streptococcus species are not typically associated with health benefits and some are highly pathogenic. However, one facultative anaerobic species, Streptococcus thermophilus, is known to promote health. It is one of the two primary species found in yogurt cultures, the other being L. bulgaricus.

Found in a number of probiotic products, the facultative anaerobe Enterococcus faecium is invariably a component of the normal intestinal microbiota and is considered a beneficial microbe. However, E. faecium has evolved from a relatively nonpathogenic commensal bacterium to the third most common cause of hospital-acquired infections and now accounts for over 10% of enterococcal clinical isolates. Furthermore, it has developed extensive resistance to antibiotics, which it is capable of transferring to other bacteria.

The human body, primarily the gastrointestinal tract, is home to a large number of different species of bacteria, and it is likely we could not survive without their presence. Two of the most common bacteria that comprise the intestinal microbiota ("normal flora") are Lactobacillus acidophilus and Bifidobacterium bifidum. Hence, they are a main component of probiotics.

The indigenous bacteria of humans serve a wide range of functions, which include manufacture of some B vitamins and vitamin K, synthesis of some digestive enzymes (e.g. lactase), competition with pathogens for colonization sites, production of antibacterial and antifungal substances that protect against harmful organisms, production of chemicals that have been shown to be anti-carcinogenic, and stimulation of the development and activity of the immune system.

The natural balance of the body’s bacteria can be upset by several factors such as certain medicines, antibiotics and steroids, increased acidity in the digestive system caused by stress, lack of sleep and poor diet, constipation or diarrhea, yeast overgrowth, fatigue, IBS and other intestinal conditions.

It has been been suggested, in a few cases proven, that one way to combat these conditions is by supplementation of the diet with probiotic bacteria in natural foods or artificial supplements.

Probiotics have been recommended or suggested for patients receiving radiation treatment, individuals who have recurrent thrush, vaginal yeast infections, or urinary tract infections, persons suffering from irritable bowel syndrome (IBS) or other bowel problems, for travelers abroad to protect against food poisoning and during any period where antibiotics may be taken.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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