**1. Introduction**

50 New Advances in the Basic and Clinical Gastroenterology

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Researches concerning probiotics were initiated by a Russian scientist named Elie Metchnikoff. He emphasized the importance of *Lactobacillus* species and fermented milk products present in the gastrointestinal tract for a healthy and long life. The term "probiotics" was first introduced in 1953 by Werner Kollath, and he defined probiotics as microbially derived factors that stimulate the growth of other microorganisms. Afterwards, the term "probiotics" was defined by Roy Fuller in 1989 as a live microbial feed supplement which beneficially affects the host by improving its intestinal microbial balance. Fuller's definition emphasizes the requirement of viability for probiotics and introduces the aspect of a beneficial effect on the host. Although, this definition has been widely used by the entire scientific world, according to the currently adopted definition by FAO/WHO, probiotics are: "Live microorganisms confer a health benefit on the host when administered in adequate amounts*"* (FAO/WHO, 2001).

The most frequently used probiotic microorganisms are Lactobacillus and Bifidobacterium species. However, there are also much more bacteria and some yeast species used as probiotic. Bacteria and yeast species, which are used commonly as probiotic, are listed by Heyman & Menard (2002) as below. *Lactobacillus* species: *L. acidophilus, L. rhamnosus, L. gasseri, L. reuteri, L. bulgaricus, L plantarum, L. johnsonii, L. paracasei, L. casei, L. salivarius, L. lactis. Bifidobacterium* species: *B. bifidum, B. longum, B. breve, B. infantis, B. lactis, B. adolescentis.*  Other species: *Streptococcus thermophilus, Escherichia coli, Bacillus cereus, Clostridium butyricum, Enterococcus faecalis, Enterococcus faecium.* Yeast: *Saccharomyces boulardii, Saccharomyces cerevisiae*, VSL#3 (four strains of lactobacilli, three strains of bifidobacteria, one strain of *Streptococcus salivarius* sp. *thermophilus*).

There is a mutual interaction between intestinal cells and microorganisms present in the gastrointestinal tract. Commensal microorganisms in the gastrointestinal tract have multidirectional effects on digestion, absorption and barrier function, secretory functions, or postnatal maturation of intestinal mucosa. Furthermore, it has been known that they change gene expression in intestinal cells (Hooper et al., 2001). The changes in the function of intestinal tract (e.g. the increase of motility, or disruption of carbohydrate digestion) also affect bacteria population and colonization.

The Impact of Probiotics on the Gastrointestinal Physiology 53

weeks of application. Villus height was longer in low dose group compared to those of controls, but moderate and high doses did not affect it after two weeks. However after 4 weeks, villus height increased in all groups supplemented with probiotic. Similarly villus height increase has been reported in studies conducted on birds as an animal model (Samli

Effects of the probiotic on villus height may change depending on the species of microorganism or probiotic. For example, villus height in duodenum and ileum increased but did not changed in jejunum of broiler chicks supplemented with *Pediococcus acidlactici* as probiotic (Taheri et al., 2010). On the other hand Günal et al. (2006) reported that villus height of jejunum and ileum increased in broiler chicks applied multi-microbe probiotic product. Segmental differences were also found in comparative studies conducted on germ free and conventional animals. Shirkey et al. (2006) have reported that villus height was the longest in jejunum of pigs supplemented with *Lactobacillus fermentum*. On the other hand Shurson et al. (1990) reported that germ-free pigs had longer ileal and duodenal villi, but shorter jejunal villi compared with their conventional counterparts. *Saccharomyces boulardii* is one of the yeast that has been using as probiotic. It has been determined that there was no significance change in villus height or crypt depth of intestinal biopsy samples obtained from volunteers who were supplemented with *Saccharomyces boulardii* for 14 days (Buts et al., 2002). However, another yeast species (*Saccharomyces cerevisiae*) was reported to increased villus height of ileum in birds (Zhang et al., 2005). Meslin and Sacquet (1984) who investigated microvilli on the surface of enterocytes reported that the microvilli were significantly shorter in all small intestinal regions when the micro flora was present. The decrease in microvillus length (due to the presence of micro flora) in germ free rat, was 5% in the duodenum, 9% in the jejunum and 18% in the ileum. Because increased villus height leads to increased surface area at the same time, digestion and absorption of disaccharides and dipeptides are promoted. In addition, it was indicated that longer villi are correlated

The data related to effects of probiotics on crypt depth are inconsistent. Probably there are variations depending on the species, the dose or the application of used probiotic. Yang et al. (2009) have reported that crypt depth decreased in mice supplemented with moderate and high doses of probiotic (*Bifidobacterium adolescentis* BBMN23) for 2 weeks, but on the contrary it was increased in low dose probiotic supplemented group compared to controls. However after 4 weeks of application, increased crypt depth has been reported in moderate and high doses groups. Willing & Van Kessel (2007) have reported that crypt depth was increased in piglets inoculated with *Lactobacillus fermentum* (monoassociated with *Lactobacillus fermentum*) compared to conventional animals. Similarly, increased crypt depth in duodenum, jejunum and ileum of chicks supplemented with *Bacillus subtilis* has been

Scharek et al. (2005) have reported that there was no significant change in the crypt depth in proximal jejunum of pigs supplemented with *Enterococcus faecium* 68. Similarly, it has been stated that crypt depth didn't change in duodenum, but decreased in ileum of broiler chicks supplemented with *Lactobacillus sp* (Awad et al., 2009). In addition, it has been reported that crypt depth was not changed in broiler chicks supplemented with *Saccharomyces cerevisiae*

et al., 2007; Awad et al., 2010).

**2.2 Crypt depth** 

found (Pelicano et al., 2005).

with activation of cell mitosis (Samanya & Yamauchy, 2002).

Probiotics are widely used for the promotion and improvement of health in humans and in animal species. Probiotics have been used as a biologically active substance in a large extend of pathologic conditions ranging from antibiotic-associated or travelers' diarrhea, irritable bowel syndrome (IBS), and lactose intolerance to dental caries, ulcers due to *Helicobacter pylori,* hepatic encephalopathy, intestinal motility disorders and neonatal necrotizing enterocolitis (Deshpande et al., 2011). It has been used as a growth, or production performance promoter in poultry species or farm animals. There are also numerous scientific reports about the interaction between probiotics and immune system. On the other hand, the effects of probiotics on digestive physiology and intestinal tract morphology have not been documented sufficiently. Therefore, the objective of this chapter is to assess the effects of probiotics on gastrointestinal physiology and morphology in human and animal models. The effects of probiotics on digestive and absorptive function of the intestine, expression of brush border enzymes and nutrient transport systems have been investigated in this chapter. The relationship between probiotics and gut motility or transit time of gastrointestinal content has also been highlighted. The effects of probiotics on morphological characteristics and the proliferation capacity of crypt and villus epithelium have been focused and in addition, the effects of probiotic on enteric nervous system have been evaluated. Finally, impact of the probiotics on the physical and functional barrier of gastrointestinal tract has been evaluated in this chapter.
