196 ◾ Fundamental Food Microbiology
chronic low-grade inflammation and show a propensity for development of diabetes. 6 Long-term
feeding with a specific diet can also alter the microbial community structure. Diets consisting of
high fat and low fiber tend to increase Bacteriodes while high fiber and low fat diets enhance the
Prevetella population. 7 Scientists believe maintaining a healthy community of microbes in the
gut is essential to prevent obesity and chronic metabolic disorders, such as diabetes and microbial
diseases.
important Characteristics of Beneficial Bacteria
Some relevant characteristics of Lactobacillus acidophilus, Lactobacillus reuteri, and Bifidobacterium
species are briefly discussed here. All three are found in the GI tract of humans as well as in animals and birds. They are Gram-positive rods and grow under anaerobic conditions. Lab. acidophilus is an obligatory homolactic fermentator; Lab. reuteri is a heterolactic fermentator and produces
lactic acid, ethanol, and CO 2 ; and Bifidobacterium species produce lactic and acetic acids (in a 2:3
ratio). They are less sensitive to stomach acid than many other bacteria under a given condition and
highly resistant to bile, lysozyme, and pancreatic enzymes present in the GI tract (small intestine).
The two Lactobacillus species are present in low numbers in the jejunum but in relatively high
numbers in the ilium, especially toward the distal part, whereas Bifidobacterium species are present
in the proximal part of the colon (near the ilium). All three are able to colonize in their respective
niches, but studies with Lab. acidophilus reveal that all strains do not adhere to the GI mucosa of
the host. Other studies have shown that the ability to adhere to the intestinal epithelial cells by
Lab. acidophilus strains can be species specific; that is, a specific strain can adhere to a particular
species only, and the trait may be lost during prolonged culturing under laboratory conditions.
Under normal conditions, these three species are thought to help in maintaining the ecological
balance of GI tract microflora by controlling the growth rate of undesirable microflora. This effect
is produced through their ability to metabolize relatively large amounts of lactic and acetic acids.
In addition, they can produce specific inhibitory substances, several of which are recognized.
Many strains of Lab. acidophilus are known to produce bacteriocins although they are most effective against closely related Gram-positive bacteria. Also, because of the sensitivity of bacteriocins
to proteolytic enzymes of the GI tract, their actual role in controlling undesirable Gram-positive
bacteria in the GI tract is disputable. Some strains also produce compounds that are not well characterized but have been reported to have an antibacterial effect on both Gram-positive and Gramnegative bacteria. Some Lactobacillus reuteri strains, while growing in glycerol, produce reuterine,
which is inhibitory to both Gram-positive and Gram-negative bacteria. Antibacterial metabolite
production by a few Bifidobacterium species is also reported. Some strains of Lactobacillus acidophilus are able to deconjugate bile acids to produce compounds that are more inhibitory than
the normal bile acids. Some Lactobacillus strains can also produce H 2 O 2 but probably not under
anaerobic conditions in the GI tract. Identification of specific antibacterial compounds other than
acids (such as reuterine) in these beneficial bacteria will help in understanding their role in maintaining intestinal health.
Several studies have indicated that beneficial effects of these bacteria are produced when they
are present in relatively high numbers in the intestinal tract (≥10 6–7 /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. These, in turn, can allow the undesirable
chronic low-grade inflammation and show a propensity for development of diabetes. 6 Long-term
feeding with a specific diet can also alter the microbial community structure. Diets consisting of
high fat and low fiber tend to increase Bacteriodes while high fiber and low fat diets enhance the
Prevetella population. 7 Scientists believe maintaining a healthy community of microbes in the
gut is essential to prevent obesity and chronic metabolic disorders, such as diabetes and microbial
diseases.
important Characteristics of Beneficial Bacteria
Some relevant characteristics of Lactobacillus acidophilus, Lactobacillus reuteri, and Bifidobacterium
species are briefly discussed here. All three are found in the GI tract of humans as well as in animals and birds. They are Gram-positive rods and grow under anaerobic conditions. Lab. acidophilus is an obligatory homolactic fermentator; Lab. reuteri is a heterolactic fermentator and produces
lactic acid, ethanol, and CO 2 ; and Bifidobacterium species produce lactic and acetic acids (in a 2:3
ratio). They are less sensitive to stomach acid than many other bacteria under a given condition and
highly resistant to bile, lysozyme, and pancreatic enzymes present in the GI tract (small intestine).
The two Lactobacillus species are present in low numbers in the jejunum but in relatively high
numbers in the ilium, especially toward the distal part, whereas Bifidobacterium species are present
in the proximal part of the colon (near the ilium). All three are able to colonize in their respective
niches, but studies with Lab. acidophilus reveal that all strains do not adhere to the GI mucosa of
the host. Other studies have shown that the ability to adhere to the intestinal epithelial cells by
Lab. acidophilus strains can be species specific; that is, a specific strain can adhere to a particular
species only, and the trait may be lost during prolonged culturing under laboratory conditions.
Under normal conditions, these three species are thought to help in maintaining the ecological
balance of GI tract microflora by controlling the growth rate of undesirable microflora. This effect
is produced through their ability to metabolize relatively large amounts of lactic and acetic acids.
In addition, they can produce specific inhibitory substances, several of which are recognized.
Many strains of Lab. acidophilus are known to produce bacteriocins although they are most effective against closely related Gram-positive bacteria. Also, because of the sensitivity of bacteriocins
to proteolytic enzymes of the GI tract, their actual role in controlling undesirable Gram-positive
bacteria in the GI tract is disputable. Some strains also produce compounds that are not well characterized but have been reported to have an antibacterial effect on both Gram-positive and Gramnegative bacteria. Some Lactobacillus reuteri strains, while growing in glycerol, produce reuterine,
which is inhibitory to both Gram-positive and Gram-negative bacteria. Antibacterial metabolite
production by a few Bifidobacterium species is also reported. Some strains of Lactobacillus acidophilus are able to deconjugate bile acids to produce compounds that are more inhibitory than
the normal bile acids. Some Lactobacillus strains can also produce H 2 O 2 but probably not under
anaerobic conditions in the GI tract. Identification of specific antibacterial compounds other than
acids (such as reuterine) in these beneficial bacteria will help in understanding their role in maintaining intestinal health.
Several studies have indicated that beneficial effects of these bacteria are produced when they
are present in relatively high numbers in the intestinal tract (≥10 6–7 /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. These, in turn, can allow the undesirable