**1. Introduction**

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

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Many species of lactic acid bacteria (LAB), *Bacillus*, and fungi such as *Saccharomyces* and *Aspergillus* have been used over the years in the food industry. A few have gained the probiotic status – defined as live microorganisms, which when administered in adequate amounts confer a health benefit on the host (Joint FAO/WHO, 2002) – and most of this belong to *Lactobacillus* (e.g., *L. bulgaricus*, *L. acidophilus*, *L. rhamnosus*, *L. casei*, *L. johnsonii*, *L. reuteri,* etc*.*), *Streptococcus* (e.g., *S. thermophilus,* etc.), and *Bifidobacterium* (e.g., *B. bifidum*, *B. longum*, *B. breve*, *B. infantis*) genera. Bifidobacteria is the predominant species of bacteria in the normal intestinal flora of healthy breast-fed newborns where they constitute more than 95% of the total population (Yildirim & Johnson, 1998). Numerous *Bifidobacterium* strains have gained recognition as probiotics because of their various therapeutic health benefits, including resistance to enteric pathogens (*Clostridium spp., Salmonella spp., Candida spp., Escherichia coli spp. and Listeria monocytogenes*), aid in lactose digestion and/or help to regulate digestion, anti–colon cancer effect, the immune system modulation, anti-allergy, and hepatic encephalopathy (Jia *et al*., 2010), and also for having a protective effect against acute diarrhoea (Liepke *et al*., 2002). The food industry recognized the market potential of the numerous strain-specific positive health benefits of the bifidobacteria cultures, namely in beverages. Bifidobacteria can also be administered as capsules or tablets or incorporated into food as dietary adjuncts and into baby foods (Lourens-Hattingh & Viljoen, 2001; Patrignani *et al.,* 2006). In addition, bifidobacteria lower inositol phosphate content during bread making (Palacios *et al*., 2008).

Several investigators have speculated that the survival of most bifidobacteria is not exceptionally high in most dairy products due to low pH and/or exposure to oxygen.

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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 intestine. (Figure 1).

*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.

**Low O2**

 **ORAL CAVITY** *Streptococcus Lactobacillus* 

**High O2**

 **SALIVA** *Bifidobacterium* *Actinomyces*   **TOOTH SURFACES** *Prevotella* *Fusobacterium* 

 **CREVICES** *Veillonella*

bacterial genera found in human distal oesophagus are given in Figure 1.

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

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

**1.2. The oesophagus** 

**1.3. The stomach** 

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