194 ◾ Fundamental Food Microbiology
but is currently defined as a product containing living microorganisms, which on ingestion in
certain numbers exert health benefits beyond inherent general nutrition. 1
This trend will continue not only in the developed countries but also in many developing
countries, particularly where fermented foods have been consumed for a long period of time.
However, it will be extremely important to resolve the present controversies on the health benefits
of probiotics, which can only be possible by understanding the current problems, conducting welldesigned experiments, and analyzing the results scientifically. In this chapter, the current status
of our knowledge on intestinal microbial ecology, the beneficial microorganisms of the GI tract,
the beneficial effects of fermented milk products, and the probable causes of some controversies
are briefly presented.
Microbiology of the Human Gi tract 2
The GI tract of humans contains more than 10 14 microorganisms, many more than the total
number of our body cells, and they coexist as mutualists (Figure 16.1). They are metabolically
diverse and active; thus it is quite likely that they have a great influence on our well being. The
human GI tract harbors three dominant phyla, Bacteroidetes, Firmicutes, and Actinobacteria, and
approximately 1000 microbial species, but only 30–40 species constitute 95% of the population. 1,2
Normally microbial cells in the stomach are <10 2 cfu/g, in the small intestine, duodenum (10 1–3
cfu/g), jejunum (10 3–4 cfu/g) and ilium (10 7–9 cfu/g), and in the large intestine (cecum, colon,
rectum) approximately 10 10–12 /g of the content. Aerobic microbes are present in the upper part
of the GI tract while anaerobic bacteria are present in the lower part of the GI tract. The most
predominant types in the small intestine are several species of Lactobacillus and Enterococcus,
and in the large intestine, they are several genera of Enterobacteriaceae, different species of
Bacteroides, Fusobacterium, Clostridium, Eubacterium, Enterococcus, Peptostreptococcus, Ruminococcus,
Propionibacterium, 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
Aerobes
Liver
Stomach (ph: 1–2)
<10 2 cfu/g
Small intestine
Duodenum (pH: 6−7): 10 1−3 cfu/g
Anaerobes
Rectum
Colon
Cecum
Large intestine
(pH: 5−7): 10 10−12 cfu/g
Jejunum (pH: 6−7): 10 3−4 cfu/g
Ileum (pH: 6−7): 10 7−9 cfu/g
Figure 16.1 Microbial counts in different parts of the gastrointestinal tract.
but is currently defined as a product containing living microorganisms, which on ingestion in
certain numbers exert health benefits beyond inherent general nutrition. 1
This trend will continue not only in the developed countries but also in many developing
countries, particularly where fermented foods have been consumed for a long period of time.
However, it will be extremely important to resolve the present controversies on the health benefits
of probiotics, which can only be possible by understanding the current problems, conducting welldesigned experiments, and analyzing the results scientifically. In this chapter, the current status
of our knowledge on intestinal microbial ecology, the beneficial microorganisms of the GI tract,
the beneficial effects of fermented milk products, and the probable causes of some controversies
are briefly presented.
Microbiology of the Human Gi tract 2
The GI tract of humans contains more than 10 14 microorganisms, many more than the total
number of our body cells, and they coexist as mutualists (Figure 16.1). They are metabolically
diverse and active; thus it is quite likely that they have a great influence on our well being. The
human GI tract harbors three dominant phyla, Bacteroidetes, Firmicutes, and Actinobacteria, and
approximately 1000 microbial species, but only 30–40 species constitute 95% of the population. 1,2
Normally microbial cells in the stomach are <10 2 cfu/g, in the small intestine, duodenum (10 1–3
cfu/g), jejunum (10 3–4 cfu/g) and ilium (10 7–9 cfu/g), and in the large intestine (cecum, colon,
rectum) approximately 10 10–12 /g of the content. Aerobic microbes are present in the upper part
of the GI tract while anaerobic bacteria are present in the lower part of the GI tract. The most
predominant types in the small intestine are several species of Lactobacillus and Enterococcus,
and in the large intestine, they are several genera of Enterobacteriaceae, different species of
Bacteroides, Fusobacterium, Clostridium, Eubacterium, Enterococcus, Peptostreptococcus, Ruminococcus,
Propionibacterium, 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
Aerobes
Liver
Stomach (ph: 1–2)
<10 2 cfu/g
Small intestine
Duodenum (pH: 6−7): 10 1−3 cfu/g
Anaerobes
Rectum
Colon
Cecum
Large intestine
(pH: 5−7): 10 10−12 cfu/g
Jejunum (pH: 6−7): 10 3−4 cfu/g
Ileum (pH: 6−7): 10 7−9 cfu/g
Figure 16.1 Microbial counts in different parts of the gastrointestinal tract.