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Lactic Acid Bacteria (page 4)
(This chapter has 5 pages)
© Kenneth Todar, PhD
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.
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.
play an essential part in the manufacture of fermented dairy products.
the lactic acid that coagulates milk and they contribute to texture,
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
Ecology of starter bacteria
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
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
generally lactic acid bacteria, Propionibacterium
shermanii and Bifidobacterium
spp. which are not lactic acid bacteria, are also used.
In addition, other bacteria including Brevibacterium
responsible for the flavor of Limburger cheese; and molds
(Penicillium species) are used
in the manufacture of Camembert,
Roquefort and blue cheeses.
Lactic Acid Bacteria
Probiotics are products
designed to deliver potentially
beneficial bacterial cells to the microbiotic ecosystem of humans and
animals. Strains of lactic acid bacteria are the most common microbes
employed as probiotics, especially Lactobacillus
and Bifidobacterium species,
lactococci, some enterococci and some streptococci are also
included as probiotics.
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
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:
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
Bifidobacteria are strictly anaerobic and normally vie for predominance
in the large intestine. Among 30 species, those recognized as
Bifidobacteria are an obligately anaerobic
bacteria, not classified with the lactic acid bacteria, but which
similar habitats and produce lactic acid as a sole end-product. They
are a prominant Gram-positve bacterium in the large intestine (colon). Bifidobacterium
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.
Found in a
number of probiotic products, the facultative anaerobe Enterococcus
is invariably a component of the normal intestinal microbiota and is
considered a beneficial microbe. However, E.
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,
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
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.