**1.1. The oral cavity**

Ingested foodstuff first comes into contact with the oral cavity, which is composed of different niches of microbial population. In the oral cavity, bacteria are the main group of microorganisms, although viruses and yeasts can also be found. The main ecological habitants of the mouth are the mucosa of lips, cheeks and palate, the tongue, the tooth surface, the saliva, and the tonsillar area. The population of microorganisms in each section is mainly dependent on the presence of oxygen and nutrients as well as the flow rate of the saliva (see Figure 2). The major species in the oral cavity are lactic acid bacteria of the genera *Streptococcus*, *Lactobacillus* and *Bifidobacterium*. In dental plaque and oral infections, many anaerobic species have been isolated, mainly *Prevotella* and *Porphyromonas* species, as well as *Eubacterium*, *Actinomyces* and *Veillonella* (Hartemink, 1999).

The main source of nutrients and energy for oral bacteria is the ingested food, especially carbohydrates, which are rapidly metabolized to lactic and acetic acids by the predominant LAB, leading to a rapid drop in the pH of the saliva after ingestion of carbohydrates. The surplus carbohydrates can be incorporated into exopolysaccharides by a large number of bacteria and be used as energy storage compounds, or as attachment factors (Hartemink, 1999).

*Bifidobacterium* in Human GI Tract:

Screening, Isolation, Survival and Growth Kinetics in Simulated Gastrointestinal Conditions 283

**Figure 2.** Relationship between bacterial species, oxygen tension and habitat in the oral cavity.

### **1.2. The oesophagus**

282 Lactic Acid Bacteria – R & D for Food, Health and Livestock Purposes

**Figure 1.** The human gastrointestinal tract and its microbiota.

*Eubacterium*, *Actinomyces* and *Veillonella* (Hartemink, 1999).

intestine. (Figure 1).

**1.1. The oral cavity** 

1999).

Nevertheless, problems may arise as a consequence of the difficulties of isolation and cultivation of bifidobacteria. Only a few studies have been published concerning the isolation and characterization of plasmids from bifidobacteria. The human gastrointestinal (GI) tract is the largest tube, running through the body and which include mouth and/or oral cavity, oesophagus, stomach, small intestine and large

Ingested foodstuff first comes into contact with the oral cavity, which is composed of different niches of microbial population. In the oral cavity, bacteria are the main group of microorganisms, although viruses and yeasts can also be found. The main ecological habitants of the mouth are the mucosa of lips, cheeks and palate, the tongue, the tooth surface, the saliva, and the tonsillar area. The population of microorganisms in each section is mainly dependent on the presence of oxygen and nutrients as well as the flow rate of the saliva (see Figure 2). The major species in the oral cavity are lactic acid bacteria of the genera *Streptococcus*, *Lactobacillus* and *Bifidobacterium*. In dental plaque and oral infections, many anaerobic species have been isolated, mainly *Prevotella* and *Porphyromonas* species, as well as

The main source of nutrients and energy for oral bacteria is the ingested food, especially carbohydrates, which are rapidly metabolized to lactic and acetic acids by the predominant LAB, leading to a rapid drop in the pH of the saliva after ingestion of carbohydrates. The surplus carbohydrates can be incorporated into exopolysaccharides by a large number of bacteria and be used as energy storage compounds, or as attachment factors (Hartemink, In quantitative terms, the oesophagus and stomach carry the lightest microbial loads in the human GI tract. The predominant culturable bacteria are facultative anaerobes, originating in the oral cavity, such as streptococci and lactobacilli, which occur in relatively small numbers (*ca.* 102 – 103 cm−2 or ml−1 of the mucosal surface or lumenal aspirate, respectively) (Macfarlane & Dillon, 2007). The majority of oesophageal bacteria (including the largely αhaemolytic *Streptococcus* species) are cultivable and are almost 104 bacteria per mm2 mucosal surface of the distal oesophagus (Pei *et al.,* 2004). While the bacterial biota in the distal oesophagus is likely to be similar to that of the oropharynx (Kazor *et al*., 2003), many other species of *Pseudomonas tolaasii*, *Pseudomonas influorensces*, *Pseudomonas syringae*, *Pseudomonas putida*, uncultured *Duganella*, *Stenotrophomonas maltophilia*, *Janthinobacterium lividum*, *Lactobacillus paracasei*, *Propionibacterium acnes*, *Pseudomonas Antarctica / meridiana,* and *Brevundimonas bulata* exist in the oesophagus (Pei *et al.,* 2004). Other selected members of the bacterial genera found in human distal oesophagus are given in Figure 1.

### **1.3. The stomach**

In general, the human stomach has a remarkably low pH. The normal resting gastric juice's pH is below 3.0, which prevents virtually all bacterial growth, and which is bactericidal for most transient species, especially the LABs. During and shortly after a meal, the pH may increase to values around 6.0. This will allow passing bifidobacteria to survive the gastric juice prior to proceeding onto the small intestine (to battle the bile salts). The resident flora of the gut lumen is highly acidic tolerant and consists mainly of lactobacilli and streptococci.

In the stomach mucosa, the pH is much higher, and bacterial populations may be higher, as well. In addition to lactobacilli and streptococci, some other bacterial species and yeasts may be present (Hartemink, 1999). The gastric juice plays a significant role in digestion of proteins, by activating digestive enzymes, making ingested proteins unravel so that digestive enzymes can alter protein down to individual amino acids. Fermentation of ingested carbohydrates in the stomach hardly occurs.

*Bifidobacterium* in Human GI Tract:

Screening, Isolation, Survival and Growth Kinetics in Simulated Gastrointestinal Conditions 285

In the large intestine, the flow rate of the digesta decreases considerably. In addition, the bile is even more diluted, and the pH is close to neutral. Total logarithmic counts may reach up to 1011 bacteria/gram contents. Higher numbers have been reported, but it is physically impossible to achieve a number over 1012 bacteria/gram faecal dry weight, taking into account the average balance of faeces and the dimensions of an average bacterium. It is estimated that over 400 different bacterial species reside in the human large intestine. Of these, about 200 have been validly described, but often non-identifiable strains are reported. In addition to the resident bacteria, transient bacteria are often isolated. The dominant floras in the large intestine are relatively stable, and they include *Bifidobacterium*, *Bacteriodes* and anaerobic cocci. Large variations also exist in the less dominant species, especially among the facultative or aerotolerant species like *E. coli* and lactobacilli. The numbers of the dominant species are also comparable in different population. Differences in counts are more often due to the methodology used, rather than actual differences. As in individuals, the counts of less dominant species differ widely between different populations. Among the dominant bacterial groups are members of the genera *Bacteriodes*, *Bifidobacterium*, *Coprococcus*, *Peptostreptococcus*, *Eubacterium* and *Ruminococcus*. Members of the following genera are often isolated and are available in lower numbers: *Fusobacterium*, *Streptococcus*, *Lactobacillus*, *Enterococcus*, *Veillonella*, *Megasphaera*, *Propionibacterium* and *Enterobacteriaceae*.

It is indispensable to emphasize here that the principal function of the GI tract includes breakdown and absorption of food components and water. In general, degradation takes place in the upper part of the GI tract, whereas the major sites of absorption are in the lower part of small intestines and the large intestines. Degradation and absorption are enhanced by the excretion of the large number of digestive enzymes, such as glycosidases, lipases, peptidases and proteinases. The colon receives digesta from the intestinal ileum approximately 5 h after food ingestion. Thereafter, rate of motion slows progressively from the caecum towards the distal colon. Concurrent with this is an increase in water absorption; thus gut contents in the proximal colon are more or less liquid in nature but have a faecal

For most of the world's population, the standard gut transit time is 60 h, with a variety of 23 – 168 h. The colon itself has a capacity of approximately 500 ml with about 220 g contents. In general, stools weight correlates inversely with transit time. Studies with healthy volunteers have indicated that speeding up colonic transit times from 67 to 25 h resulted in an increase in stool weight from 148 to 285 g/day. Conversely, when transit time increased, stool weight decreases from 182 to 119 g/day (Hartemink, 1999). The differences are mainly due to

The structure of faeces is highly variable. Bacteria may constitute up to 55% of the total solids, whilst fibre and other non-digested, non-fermented compounds represent less than 17% of the weight of which about 24% is soluble material. Faecal water content may be as high as 70% of the total weight. Stool size is influenced by both dietary and endogenous

**1.5. The large intestine** 

like appearance distally (Hartemink, 1999).

changes in the water content of the faecal mass.
