Download Lb. delbrueckii

Dr. Jehan Abdul Sattar
Probiotics
I. INTRODUCTION
In Western countries, the beneficial role of fermented milk (yogurt) in
the prolongation of life was first advocated by Metchnikoff in 1907. He
suggested that the bacteria and their metabolites in yogurt neutralized the
harmful products yielded from foods in the GI tract and provided
protection to human health. This group of bacteria was thought to reduce
production of toxic compounds that adversely affect the human body,
thus enabling humans to live longer . Research done since Metchnikoff’s
period has shown that Lb. delbrueckii subsp. bulgaricus neither survives
nor establishes itself in the gastrointestinal tract. However, other species
of lactobacilli have been reported to provide some beneficial effects
through growth and action in the gastrointestinal tract. This group of
bacteria and others are now often referred to as probiotics. Although there
are other possibilities, cultures most often mentioned as probiotics for
humans include Lb. acidophilus, Lb. casei, and Bifidobacterium species.
These species along with Propionibacterium species and Lb. reuteri are
the ones most often considered for use as probiotics for livestock. All
these species can survive and grow in the intestinal tract, and thus have
the potential to provide benefits. Certain yeast cultures also are
considered as being probiotics for livestock even though the yeasts are
not expected to survive and grow in the gastrointestinal tract.
Bacteria normally used as starter cultures for some fermented milk
products, such as Lb. delbrueckii subsp. bulgaricus and Streptococcus
thermophilus used to manufacture yogurt, also may provide benefits, but
not through the ability to survive and grow in the intestinal tract. Benefits
they provide come primarily from serving as a source of enzymes needed
to improve digestion of nutrient in the gut. For example, β-galactosidase
is needed for hydrolysis of lactose in the small intestine.
Whereas some reports indicate that the nutritional value of milk can be
improved by certain fermentations . Several benefits are possible from
such microorganisms, including control of intestinal infections, control of
serum cholesterol levels, beneficial influences on the immune system,
improvement of lactose utilization in persons who are classified as being
lactose maldigestors, and anticarcinogenic action.
The term probiotics was initially used with no specific definition. In
1989, Fuller defined the term as products containing living
microorganisms, which on ingestion (by humans, animals, and birds) in
certain numbers exert health benefits beyond inherent general nutrition.
In this statement, several requirements were included, such as the
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microbial cells should be alive and consumed in high numbers (usually
109 cells/d). But it did not include which microorganisms (only intestinal
or others) should be used and consumed for how long (daily as a
preventative or for 2 or more weeks as a therapeutical. Also Probiotics
have been defined as ‘a preparation of or a product containing viable,
defined micro-organisms in sufficient numbers, which alter the
microbiota (by implantation or colonisation) in a compartment of the host
and by that exert beneficial health effects in this host’. It is currently
accepted the probiotic definition formulated in 2001 by FAO/WHO “Live
microorganisms which when administered in adequate amount confera
health benefit to the host”.
-Prebiotic is ‘a non-digestible food ingredient that beneficially affects
the host by selectively stimulating the growth and/or activity of one or a
limited number of bacteria in the colon’.
-Synbiotic is a synergistic combination of a probiotic and prebiotic.
Synbiotic supplements not only given as feed supplements or
pharmaceuticals but increasingly in suitable food specimens such as dairy
products, fruit juices and chocolates. The synbiotic concept has been
widely used by European dairy drink and yoghurt manufacturers .
Administration of synbiotics as a food supplement is safe, simple, and
convenient.
Probiotics, prebiotics, and synbiotics that may be suitable for human
consumption are listed below:
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-Postbiotics refers to the metabolic by products like enzymes, peptides,
teichoic acid, peptidoglycan, exopolysaccharides, cell surface and
secreted proteins, bacteriocins and organic aids generated by a probiotic
organism during its lifespan. Postbiotics have advantage due to their clear
chemical structure, safety dose parameters and longer shelf life which can
influence physiological function of host . Postbiotic substances contain
various metabolites and signaling molecules which display broad
antibacterial spectrum and immunomodulatory actions.
-Biogenics
The health benefit from the consumption of fermented dairy products is
attributed to the lactic acid bacteria used in fermentation as well as the
by-products of metabolism of milk nutrients by them. The latter aspect
becomes important to explain health benefits attributed to fermented
products in which nongut bacteria, such as Lactococcus lactis, Str.
thermophilus, and Lb. delbrueckii ssp. bulgaricus, are used in
fermentation. These products have recently been termed biogenics and
include components in food that are derived through the microbial
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metabolic activity of food nutrients and have health benefits. One such
group is the peptide produced by the exoproteinases of some lactic acid
bacteria, such as Lc. lactis in buttermilk and Lb. delbrueckii ssp.
bulgaricus in yogurt. Some of the peptides thus produced and present in
the fermented milk can reduce blood pressure in individuals with
hypertension.
MICROBIOLOGY OF THE HUMAN GI TRACT
The GI tract of humans contains more than 1014 microorganisms. It is
estimated that the human GI tract harbors 1000 bacterial species, but only
30 to 40 species constitute 95% of the population. Normally, the
microbial level in the small intestine (particularly in the jejunum and
ilium) is 106–7/g, and in the large intestine (colon) 109–10/g of the content.
. Bacterial numbers fall dramatically to <103 colony forming units (cfu)/
mL of gastric contents as they encounter the stomach, which provides a
highly effective barrier against invading micro-organisms, both
pathogenic and benign.
The most predominant types in the small intestine are several species of
Lactobacillus and Enterococcus, and in the large intestine are several
genera of Enterobacteriaceae, different species of Bacteroides,
Fusobacterium,
Clostridium,
Eubacterium,
Enterococcus,
Bifidobacterium, and Lactobacillus.
The intestine of a fetus in the uterus is sterile. At birth, it is inoculated
with vaginal and fecal flora from the mother. Subsequently, a large
variety of microorganisms enter in the digestive tract of infants from the
environment. From these, the normal flora of the GI tract are established.
In both breast-fed and formula-fed babies during the first couple of days,
Escherichia coli and Enterococcus appear in large numbers in the feces.
Then, in the breast-fed babies, large numbers of Bifidobacterium species
and a lower level of both E. coli and Enterococcus species appear. In
formula-fed babies, in contrast, E. coli and Enterococcus, together with
Clostridium and Bacteroides, predominate, with Bifidobacterium being
almost absent; this situation may lead to diarrhea. As breast-fed babies
are introduced to other foods, the levels of E. coli, Enterococcus,
Bacteroides, Clostridium, and others increase, but Bifidobacterium still
remains high. When breast-feeding is completely stopped, Bacteroides,
Bifidobacterium, and Lactobacillus species predominate, along with some
E. coli, Enterococcus, Clostridium, and others. By the second year of life,
the different microflora establish themselves at their specific ecological
niche in the GI tract, and the population resembles that of adult GI tracts.
The intestinal microflora are divided into indigenous and transient types.
Many indigenous species can adhere to intestinal cells, which helps
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maintain them in their specific environment. Whereas the indigenous
types are permanent inhabitants, the transient types are either passing
through or temporarily colonizing a site from where the specific
indigenous type has been removed because of some inherent or
environmental factors (such as antibiotic intake). Among the indigenous
microbial flora, several species of Lactobacillus in the jejunum and ilium,
and Bifidobacterium in the large intestine, are thought to have beneficial
effects on the health of the GI tracts of the hosts. From the intestines and
intestinal content of humans, Lactobacillus acidophilus, Lb. fermentum,
Lb. rhamonosus, Lb. reuteri, Lb. casei, Lb. lactis, Lb leichmannii, Lb.
plantarum, Bifidobacterium bifidus, Bif. longum, Bif. adolescentis, Bif.
infantis, and others have been isolated. However, age, food habits, and
health conditions greatly influence the species and their levels. There is
some belief that a portion of the intestinal Lactobacillus species is
transient. Initially, it was considered that Lb. acidophilus, Lb. reuteri, and
some Bifidobacterium are the main indigenous species. At present,
several other Lactobacillus
species, such as Lb. casei and Lb. rhamnosus, have been tested to be
beneficial. The presence of high numbers of the indigenous Lactobacillus
species in the feces (and content of the large intestine) probably results
from their constant removal from the small intestine.
The prevalence of bifidobacteria in breast-fed infants is thought to confer
protection by reducing the gut pH, which induces a concomitant
reduction in other potentially harmful species. Moreover, bifidobacteria
are able to exert directly antagonistic activities against gut pathogens.
Newborns are susceptible to intestinal infections and atopic diseases as
their immune system and GI tract develop.
Mode of delivery and diet therefore have important implications, both at
birth and later in life, as initial colonisation is involved in the
development of the GI tract, its microbiota and in maturation of the
immune system. During the first few years of life and upon weaning, the
infant microbiota normalises. This composition will remain stable
throughout most of adult life .
There is a high degree of variability between the stomach, small intestine
and colon in terms of numbers and bacterial population types. This is due
predominantly to different transit times, secretions and nutrient
availability.
Micro-organisms are also determinants as they interact with and influence
their surroundings to ensure their survival against competitors. This is
achieved through several mechanisms, such as increasing anaerobicity or
through the production of compounds such as acid or antimicrobial
substances. These compounds concurrently affect the host and can
thereby have advantageous or devastating implications .
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The rapid transit time, low pH and presence of bile associated with the
small intestine do not provide an environment that encourages growth of
bacterial populations. The duodenum also has low microbial populations
due to its short transit time and the secretion of intestinal fluids, which
create a hostile environment . But the large gut is favourable for bacterial
growth with a slow transit time, ready availability of nutrients and
favourable pH. The small intestine harbours enterococci, enterobacteria,
lactobacilli, bacteroides and clostridia. Organic acids produced from
fermentation result in a lower pH of 5.5–6.0 than the more neutral pH
found in the colon. Transit in the colon is slower and nutrient availability
is minimised, producing slower-growing populations that tend towards
more proteolytic fermentations.
Yeasts, including the opportunistic pathogen Candida albicans, are also
present in the gut microbiota, although in healthy individuals counts do
not exceed 104 cfu/ g faeces . The vast majority (>90%) of the total cells
in the body are present as
Functions of the gastrointestinal microbiota
The GI tract, along with its microbiota, is one of the most metabolically
active organs in the human body. The intestinal microbiota is involved in
the fermentation of endogenous and exogenous microbial growth
substrates. The metabolic end-products of carbohydrate fermentation are
benign, or even advantageous to human health .
Major substrates available for the colonic fermentation are starches that
for various reasons are resistant to the action of pancreatic amylases, can
be degraded by bacterial enzymes, as well as dietary fibres such as
pectins and xylans. Other carbohydrate sources available for fermentation
in lower concentrations include oligosaccharides and a variety of sugars
and non-absorbable sugar alcohols. Saccharolysis results in the
production of short-chain fatty acids (SCFA) such as butyrate, acetate,
propionate and lactate which contribute towards energy metabolism of
the large intestinal mucosa and colonic cell growth; they can also be
metabolised by host tissues such as the liver, muscle and brain. The
production of SCFA results in a lower pH that can protect against
invading micro-organisms and also reduces the transformation of primary
bile acids into secondary pro-carcinogenic bile acids . This is believed to
be one of the mechanisms utilised by the beneficial gut microbiota that
results in protection for the host. Proteins and amino acids can be
effective growth substrates for colonic bacteria, whilst bacterial
secretions, lysis products, epithelial cells and mucins may also make a
contribution. However, the diet provides, by far, the predominant source
of nutrients, witharound 70–100 g/d of dietary residues being available to
the colonic microbiota. These materials are degraded by a wide range of
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bacterial polysaccharidases, glycosidases, proteases and amino-peptidases
to smaller oligomers and their component sugars and amino acids.
The indigenous gut microbiota is better adapted to compete for nutrients
and attachment sites than the incoming micro-organism, which it may
also inhibit through the production of compounds .
Another important function of the gut microbiota is the production of
vitamins B and K. Several studies have indicated that beneficial effects of
these bacteria are produced when they are present in relatively high
numbers in the intestinal tract (106–7/g intestinal content). Diets rich in
foods from plant sources, as opposed to those rich in foods from animal
sources, seem to favor their presence in higher numbers. Many other
conditions in a host also can reduce bacterial numbers in the GI tract,
such as antibiotic intake, mental stress,starvation, improper dietary habits,
alcohol abuse, and sickness and surgery of the GI tract. This, in turn, can
allow the undesirable indigenous or transient bacteria to grow to high
levels and produce enteric disturbances,including diarrhea and infection
by enteric pathogen.
By changing the species composition of the GIT bacteria using probiotics
and prebiotics it is possible to beneficially modulate enteric immune
functions, thereby improving resistance to GIT pathogens and other
health challenge.
Microorganisms considered as probiotics
The majority of probiotic microorganisms belong to the genera
Lactobacillus and Bifidobacterium . There are also other genera of
Bacteria streptococci,enterococci, lactococci and some yeasts such as
Saccharomyces spp. widely used and reported below:
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IMPORTANT CHARACTERISTICS OF PROBIOTIC
BACTERIA
Respecting the “Guidelines on probiotics
characteristics can be
summarized as follows:
• Must not lose its properties during storage.
• Must be normally present in the human intestine.
• Must be able to survive, to overcome the gastric barrier, resisting to the
action of digestive gastric juice, intestinal enzymes and bile salts and
colonize the intestine: for this reason, the minimum effective dose, which
is very indicative because it depends on the strain and preparation used, is
107 CFU/day.
• Must be able to adhere to and to colonize the intestinal cells: the
bacterial membrane structure is involved in the mechanism of adhesion
and direct switch with the mucosa, the surface proteins and possibly also
the secreted ones. Beneficial strains differ in adherence ability and
specificity . An adherent strain should probably be favored over a
nonadherent strain. Also, strains adherent to humans should be preferred
over strains adherent to other species. The selected strains should have a
strong adherence property.
• Must exert metabolic functions at the enteric level, with beneficial
effects for human health, and antagonism against pathogenic
microorganisms by producing antimicrobial substances.
• Should not cause immune or otherwise harmful reactions and then be
considered as safe (GRAS status: generally recognized as safe).
• Resistance to antibiotics must be intrinsic or due to genetic mutations,
whereas if it is caused by a horizontal gene transfer (i.e. transposons,
genomic DNA segments that breaks off to join another, conjugative
plasmids carrying genes for resistance, virulent or temperate phages) his
choice becomes more problematic;
• Must also be administered in adequate doses and have a favourable
cost-efficacy ratio.
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