**7. Gastric microbiota and gastroduodenal diseases**

#### **7.1 Chronic gastritis and peptic ulcer disease**

The isolation of *H. pylori* was a real breakthrough, not because it declared the stomach as a non-sterile organ but because subsequent research established it as a main etiological factor for the development of gastroduodenal diseases [2, 55, 56]. It is well known that long-term *H. pylori* infection causes various degrees of chronic inflammation of the underlying gastric mucosa. A subset of patients develops clinical symptoms, and a further subset will develop complications including peptic ulcer, gastric mucosa-associated lymphoid tissue (MALT) lymphoma, and gastric cancer. The cascade leading from chronic gastritis to neoplasm is known as Correa's cascade and involves the progression to glandular atrophy with intestinal metaplasia and dysplasia and eventually to invasive carcinoma [57]. Peptic ulcer disease is the most common complication of chronic *H. pylori* infection, with 95% of duodenal and 70% of gastric ulcers being linked to it [58]. All of this serves to prove that *H. pylori*

**59**

*Gastric Microbiota: Between Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.86926*

**7.2 Gastric cancer**

is by far the most important microbial species that can colonize the stomach, with an enormous impact on the pathogenesis and development of gastroduodenal diseases. Nevertheless, there is arising evidence that non-*H. pylori* bacteria may also play an important role in the pathogenesis of chronic gastritis and peptic ulcers. One study suggests that different gastric microbial communities, such as the overrepresentation of the *Streptococcus* genus within the *Firmicutes* phylum, can lead to gastritis as well, even in the absence of *H. pylori* [35]. Another study has found an increase of *Streptococcus* and a decrease of *Prevotella* in patients with atrophic gastritis, versus healthy subjects [59]. There are also several studies that assess the role of non-*H. pylori* species in peptic ulcer disease. The non-*H. pylori* bacteria seems to be more prevalent in patients with nonulcer dyspepsia than in those with gastric ulcer as shown by a culture-based study by Hu et al. [19]. Another study in Malaysian patients showed significant correlation between the isolation of *Streptococcus* and patients with peptic ulcers [60]. Given that most studies find that the *Streptococcus* genus is one of the most abundant non-*H. pylori* species in the stomach, a possible pathogenic relation can be a subject of further research. However, by far no other species have an established pathogenic role except *H. pylori*.

Each year approximately 990,000 people are diagnosed with gastric cancer (GC) worldwide, of whom about 738,000 die from this disease, making GC the fourth most common incident cancer and the second most common cause of cancer death. Both incidence and mortality rates are about twice as high in males as in females. Over 70% of cases occur in developing nations, concentrated in Eastern Asia, Eastern Europe, and Central and South America. Approximately 90% of gastric cancers are adenocarcinomas, with the other 10% shared between mucosaassociated lymphoid tissue (MALT) lymphomas, gastrointestinal stromal tumors (GIST), leiomyosarcomas, and other more rare types of cancer. Adenocarcinomas are histologically classified into two major types: diffuse and intestinal. These two types not only look different under the microscope but also differ in gender ratio, age at diagnosis, and other epidemiologic features. Anatomically, gastric cancers are categorized as proximal and distal. Proximal adenocarcinomas are more similar to esophageal adenocarcinomas and may be associated with the absence of *H. pylori*, while distal adenocarcinomas originate in the antrum, with approximately 90% of such cases related to *H. pylori* infection [61]. Today the correlation between *H. pylori* and the development of gastric cancer is undeniable as shown in several prospective studies [62–64]. Moreover, the eradication of *H. pylori* is proven to significantly reduce the risk of gastric cancer development, according to several international consensuses [65, 66]. *H. pylori* was recognized as a "definite carcinogen" by the World Health Organization in 1994, and this fact was reconfirmed in 2009.

However, *H. pylori* coevolved with humans for millennia, and only 1–2% of people infected with this bacterium actually develop gastric cancer or MALT lymphoma. Similar to most cancers, pathogenetic mechanisms remain unclear with a multitude of other factors to influence the final carcinogenesis [61]. Although, *H. pylori* is clearly the most relevant microbial risk factor for the development of gastric cancer, an increasing pool of evidence suggests that other microbial communities play a causative role in the pathophysiology of gastric cancer. To date, several

Studies with INS-GAS mice have revealed that male mice with intestinal microbiota

developed gastric pathology from chronic gastritis to atrophy and dysplasia independent of *H. pylori* infection. Furthermore, the presence of commensal microbiota accelerated the progression to gastric intraepithelial neoplasia, and gastric intraepithelial neoplasia became invasive in *H. pylori*-infected INS-GAS mice. Male INS-GAS mice

animal and human studies have supported this theory.

#### *Gastric Microbiota: Between Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.86926*

is by far the most important microbial species that can colonize the stomach, with an enormous impact on the pathogenesis and development of gastroduodenal diseases.

Nevertheless, there is arising evidence that non-*H. pylori* bacteria may also play an important role in the pathogenesis of chronic gastritis and peptic ulcers. One study suggests that different gastric microbial communities, such as the overrepresentation of the *Streptococcus* genus within the *Firmicutes* phylum, can lead to gastritis as well, even in the absence of *H. pylori* [35]. Another study has found an increase of *Streptococcus* and a decrease of *Prevotella* in patients with atrophic gastritis, versus healthy subjects [59]. There are also several studies that assess the role of non-*H. pylori* species in peptic ulcer disease. The non-*H. pylori* bacteria seems to be more prevalent in patients with nonulcer dyspepsia than in those with gastric ulcer as shown by a culture-based study by Hu et al. [19]. Another study in Malaysian patients showed significant correlation between the isolation of *Streptococcus* and patients with peptic ulcers [60]. Given that most studies find that the *Streptococcus* genus is one of the most abundant non-*H. pylori* species in the stomach, a possible pathogenic relation can be a subject of further research. However, by far no other species have an established pathogenic role except *H. pylori*.

#### **7.2 Gastric cancer**

*Gastrointestinal Stomas*

**6.3 Use of medications**

addressing the influence of diet on the gastric microbiota. An example is an in vivo study that compared the gastric microbiota of mice fed a non-purified diet (natural source-derived food) to mice fed a purified diet (refined food) and found higher levels of total aerobes, total anaerobes, and *Lactobacillus* in the stomach of the mice on a non-purified diet [50]. Nevertheless, it is well established how dietary factors affect the gastric microclimate, and since the microbiota is an inseparable part of this microclimate, it is not farfetched to suspect that diet affects the gastric micro-

The long-term use of proton pump inhibitors (PPIs) and H2 antagonists affects the composition of the gastric microbiota by inducing a non-*H. pylori* bacterial overgrowth [51]. This is not surprising, considering that normally the non-*H. pylori* gastric microbiota is suppressed by the significantly acidic gastric environment. Suppression of gastric acidity will alter the bacterial flora of the upper GI tract, and studies have confirmed that PPIs do alter the bacterial population in the stomach [52]. This is mainly due to oral bacteria that survive instead of being killed in the normally acidic stomach. It has also been suspected that by causing alteration and overgrowth of the microbiota, acid-suppressive treatments may increase the risk of gastric cancer [52]. It has also been shown that a previously antrum-dominant *H. pylori* infection after treatment with acid inhibitors changed to a more corpus predominant infection [51]. The less acidic corpus allows *H. pylori* to penetrate deeper in the crypts and increase the inflammation, which causes faster progression to atrophy [53]. Treatment with acid inhibitors has by culture-dependent methods been shown to affect the survival of bacteria in the stomach. However, no significant differences have been found regarding diversity and composition of the microbiota by using culture methods [10]. Antibiotics are well known to have suppressive effects on the gastrointestinal microflora. *H. pylori* eradication is dependent on combined antibiotic treatment. However, certain antibiotic treatments can have negative effects on the "healthy" gastric microbiota. Animal studies indicate that treatment with penicillins reduces *Lactobacillus* populations and promotes yeast colonization of the gastric epithelium. Furthermore, Mason et al. [54] showed that cefoperazone treatment in humans causes long-term alteration of the gastric microbiota, such as a significant reduction

bial communities. However, more research is needed.

in the number of *Lactobacillus* and overgrowth of *Enterococcus*.

The isolation of *H. pylori* was a real breakthrough, not because it declared the stomach as a non-sterile organ but because subsequent research established it as a main etiological factor for the development of gastroduodenal diseases [2, 55, 56]. It is well known that long-term *H. pylori* infection causes various degrees of chronic inflammation of the underlying gastric mucosa. A subset of patients develops clinical symptoms, and a further subset will develop complications including peptic ulcer, gastric mucosa-associated lymphoid tissue (MALT) lymphoma, and gastric cancer. The cascade leading from chronic gastritis to neoplasm is known as Correa's cascade and involves the progression to glandular atrophy with intestinal metaplasia and dysplasia and eventually to invasive carcinoma [57]. Peptic ulcer disease is the most common complication of chronic *H. pylori* infection, with 95% of duodenal and 70% of gastric ulcers being linked to it [58]. All of this serves to prove that *H. pylori*

**7. Gastric microbiota and gastroduodenal diseases**

**7.1 Chronic gastritis and peptic ulcer disease**

**58**

Each year approximately 990,000 people are diagnosed with gastric cancer (GC) worldwide, of whom about 738,000 die from this disease, making GC the fourth most common incident cancer and the second most common cause of cancer death. Both incidence and mortality rates are about twice as high in males as in females. Over 70% of cases occur in developing nations, concentrated in Eastern Asia, Eastern Europe, and Central and South America. Approximately 90% of gastric cancers are adenocarcinomas, with the other 10% shared between mucosaassociated lymphoid tissue (MALT) lymphomas, gastrointestinal stromal tumors (GIST), leiomyosarcomas, and other more rare types of cancer. Adenocarcinomas are histologically classified into two major types: diffuse and intestinal. These two types not only look different under the microscope but also differ in gender ratio, age at diagnosis, and other epidemiologic features. Anatomically, gastric cancers are categorized as proximal and distal. Proximal adenocarcinomas are more similar to esophageal adenocarcinomas and may be associated with the absence of *H. pylori*, while distal adenocarcinomas originate in the antrum, with approximately 90% of such cases related to *H. pylori* infection [61]. Today the correlation between *H. pylori* and the development of gastric cancer is undeniable as shown in several prospective studies [62–64]. Moreover, the eradication of *H. pylori* is proven to significantly reduce the risk of gastric cancer development, according to several international consensuses [65, 66]. *H. pylori* was recognized as a "definite carcinogen" by the World Health Organization in 1994, and this fact was reconfirmed in 2009.

However, *H. pylori* coevolved with humans for millennia, and only 1–2% of people infected with this bacterium actually develop gastric cancer or MALT lymphoma. Similar to most cancers, pathogenetic mechanisms remain unclear with a multitude of other factors to influence the final carcinogenesis [61]. Although, *H. pylori* is clearly the most relevant microbial risk factor for the development of gastric cancer, an increasing pool of evidence suggests that other microbial communities play a causative role in the pathophysiology of gastric cancer. To date, several animal and human studies have supported this theory.

Studies with INS-GAS mice have revealed that male mice with intestinal microbiota developed gastric pathology from chronic gastritis to atrophy and dysplasia independent of *H. pylori* infection. Furthermore, the presence of commensal microbiota accelerated the progression to gastric intraepithelial neoplasia, and gastric intraepithelial neoplasia became invasive in *H. pylori*-infected INS-GAS mice. Male INS-GAS mice with *H. pylori* infection, colonized with artificial mouse intestinal microbiota, have shown increased incidence of gastric intraepithelial neoplasia by 69% [27, 32]. On the other hand, antibiotic treatments significantly delayed the onset of gastric neoplasia in *Helicobacter*-free and specific pathogen-free INS-GAS mice [67].

A study by Wang et al. found a similar number of bacterial species in the microbiota between gastric cancer and chronic gastritis, but by using a method to explore and visualize similarities or dissimilarities of the data, a pattern suggesting the presence of a diversified microbiota in gastric cancer was found [68]. Moreover, a 16S rRNA gene sequencing analysis of gastric mucosa of patients with gastric cancer showed a prevalence of the genera *Lactobacillus*, *Streptococcus* (among which the most common species were *S. mitis* and *Streptococcus parasanguinis*), *Prevotella*, and *Veillonella* [30]. Two other studies evaluated the gastric microbiota of subjects with non-atrophic gastritis, intestinal metaplasia, and gastric cancer. The first one showed a significantly lower diversity and a higher abundance of the genus *Pseudomonas* in the microbiota of neoplastic patients compared to patients with simple gastritis. Moreover, both the progressive decrease of six taxa and the progressive increase of two taxa were observed from the gastritis group to the neoplastic group, via the metaplastic group [69]. In the second study, a high-throughput sequencing platform was used for the assessment of gastric microbiota in the three groups, showing definitely different results: a greater bacterial diversity, a relative rise of *Bacilli* and *Streptococcaceae*, and a relative reduction of *Helicobacteraceae* were found in the cancer group compared to other groups [70].

It is possible that non-*H. pylori* species potentiate carcinogenesis through various mechanisms, such as promoting inflammation, stimulating cell proliferation, modifying stem cell dynamics, and producing toxic metabolites [71]. However, it is still unsure whether the different microbial structure is causative for the carcinogenesis or carcinogenesis itself causes a shift in the microbial communities, which subsequently promotes carcinogenesis further.

### **8. Gastric microbiota and extra-gastric diseases**

The stomach is part of the GI tract, and as such, possible relations between the gastric microbiota's composition and diseases of other parts of the GI tract, such as the esophagus (esophagitis and esophageal cancer), small intestines, and colon cannot be overlooked [72]. One study, using 16S rDNA analyses of duodenal aspirates, demonstrated lower diversity in irritable bowel syndrome patients compared to controls with significant alterations in 12 genera [73]. An increased risk for colorectal neoplasia in *H. pylori*-infected patients has been confirmed by many large-scale studies [74, 75].

Other studies have addressed the association between autoimmune hepatitis and altered microbiome of the upper GI tract and found this to be linked to increased intestinal permeability [76].

Regarding the extra-gastrointestinal involvement of gastric microbiota (especially *H. pylori*), studies find possible associations with hematological diseases like idiopathic thrombocytopenic purpura [77] and anemia [78] and cardiovascular [79], neurological [80], and endocrine [81] diseases.

Nevertheless, research in this field is far from sufficient to be conclusive.

### **9. Discussion**

It is undeniable that *H. pylori* is by far the most unique and important species that can colonize the gastric niche with clear pathogenic significance to

**61**

*Gastric Microbiota: Between Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.86926*

gastroduodenal diseases. However, culture-dependent and later culture-independent studies prove that the stomach harbors a complex microbiota with many other phyla (*Proteobacteria*, *Firmicutes*, *Bacteroidetes*, *Actinobacteria*, and *Fusobacteria*) and genera (*Lactobacillus*, *Streptococcus*, *Clostridium*, *Prevotella*, *Veillonella*, *Bifidobacterium*, and *Rothia*) being identified. By far, it is arguable which genera can be considered resident since results depend on a multitude of factors, among which gastric acidity stands out as the most significant. However, it is worth noting that *Streptococcus*, *Prevotella*, *Veillonella*, and *Rothia* seem to be the most abundant genera in *H. pylori*-negative subjects. Nevertheless, the significance of both transient and resident non-*H. pylori* microbiota lies in its possible role in the development and progression of gastroduodenal diseases, such as gastritis, peptic ulcer disease, and gastric cancer. So far, only a few and far from enough studies suggest that non-*H. pylori* microbiota can lead to gastric pathology in the absence of *H. pylori*. However, findings are convincing that the non-*H. pylori* microbiota can play an important role in modulating *H. pylori*-induced gastric inflammation, influencing an individual's risk of gastric diseases and consequently the severity of the resulting disease. Suspected pathophysiologic mechanisms involved in this include modulating immune cell responses, stimulating cell proliferation, modifying stem cell dynamics, and producing toxic metabolites. Although, substantial advancements in unrevealing the complexity of the gastric microbiota and its' role in health and disease have been made, studies are far from sufficient to suggest new strategies for prevention, diagnosis, and treatment of gastroduodenal diseases. However, this

potential remains and undoubtedly should be further explored.

The publication of this work was supported by KRKA.

**Acknowledgements**

**Conflict of interest**

There are no conflicts of interest.

*Gastric Microbiota: Between Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.86926*

*Gastrointestinal Stomas*

with *H. pylori* infection, colonized with artificial mouse intestinal microbiota, have shown increased incidence of gastric intraepithelial neoplasia by 69% [27, 32]. On the other hand, antibiotic treatments significantly delayed the onset of gastric neoplasia in

A study by Wang et al. found a similar number of bacterial species in the microbiota between gastric cancer and chronic gastritis, but by using a method to explore and visualize similarities or dissimilarities of the data, a pattern suggesting the presence of a diversified microbiota in gastric cancer was found [68]. Moreover, a 16S rRNA gene sequencing analysis of gastric mucosa of patients with gastric cancer showed a prevalence of the genera *Lactobacillus*, *Streptococcus* (among which the most common species were *S. mitis* and *Streptococcus parasanguinis*), *Prevotella*, and *Veillonella* [30]. Two other studies evaluated the gastric microbiota of subjects with non-atrophic gastritis, intestinal metaplasia, and gastric cancer. The first one showed a significantly lower diversity and a higher abundance of the genus *Pseudomonas* in the microbiota of neoplastic patients compared to patients with simple gastritis. Moreover, both the progressive decrease of six taxa and the progressive increase of two taxa were observed from the gastritis group to the neoplastic group, via the metaplastic group [69]. In the second study, a high-throughput sequencing platform was used for the assessment of gastric microbiota in the three groups, showing definitely different results: a greater bacterial diversity, a relative rise of *Bacilli* and *Streptococcaceae*, and a relative reduction of *Helicobacteraceae* were

It is possible that non-*H. pylori* species potentiate carcinogenesis through various mechanisms, such as promoting inflammation, stimulating cell proliferation, modifying stem cell dynamics, and producing toxic metabolites [71]. However, it is still unsure whether the different microbial structure is causative for the carcinogenesis or carcinogenesis itself causes a shift in the microbial communities, which

The stomach is part of the GI tract, and as such, possible relations between the gastric microbiota's composition and diseases of other parts of the GI tract, such as the esophagus (esophagitis and esophageal cancer), small intestines, and colon cannot be overlooked [72]. One study, using 16S rDNA analyses of duodenal aspirates, demonstrated lower diversity in irritable bowel syndrome patients compared to controls with significant alterations in 12 genera [73]. An increased risk for colorectal neoplasia in *H. pylori*-infected patients has been confirmed by many large-scale studies [74, 75].

Other studies have addressed the association between autoimmune hepatitis and altered microbiome of the upper GI tract and found this to be linked to increased

Regarding the extra-gastrointestinal involvement of gastric microbiota (especially *H. pylori*), studies find possible associations with hematological diseases like idiopathic thrombocytopenic purpura [77] and anemia [78] and cardiovascular

Nevertheless, research in this field is far from sufficient to be conclusive.

It is undeniable that *H. pylori* is by far the most unique and important species that can colonize the gastric niche with clear pathogenic significance to

*Helicobacter*-free and specific pathogen-free INS-GAS mice [67].

found in the cancer group compared to other groups [70].

subsequently promotes carcinogenesis further.

intestinal permeability [76].

**8. Gastric microbiota and extra-gastric diseases**

[79], neurological [80], and endocrine [81] diseases.

**60**

**9. Discussion**

gastroduodenal diseases. However, culture-dependent and later culture-independent studies prove that the stomach harbors a complex microbiota with many other phyla (*Proteobacteria*, *Firmicutes*, *Bacteroidetes*, *Actinobacteria*, and *Fusobacteria*) and genera (*Lactobacillus*, *Streptococcus*, *Clostridium*, *Prevotella*, *Veillonella*, *Bifidobacterium*, and *Rothia*) being identified. By far, it is arguable which genera can be considered resident since results depend on a multitude of factors, among which gastric acidity stands out as the most significant. However, it is worth noting that *Streptococcus*, *Prevotella*, *Veillonella*, and *Rothia* seem to be the most abundant genera in *H. pylori*-negative subjects. Nevertheless, the significance of both transient and resident non-*H. pylori* microbiota lies in its possible role in the development and progression of gastroduodenal diseases, such as gastritis, peptic ulcer disease, and gastric cancer. So far, only a few and far from enough studies suggest that non-*H. pylori* microbiota can lead to gastric pathology in the absence of *H. pylori*. However, findings are convincing that the non-*H. pylori* microbiota can play an important role in modulating *H. pylori*-induced gastric inflammation, influencing an individual's risk of gastric diseases and consequently the severity of the resulting disease. Suspected pathophysiologic mechanisms involved in this include modulating immune cell responses, stimulating cell proliferation, modifying stem cell dynamics, and producing toxic metabolites. Although, substantial advancements in unrevealing the complexity of the gastric microbiota and its' role in health and disease have been made, studies are far from sufficient to suggest new strategies for prevention, diagnosis, and treatment of gastroduodenal diseases. However, this potential remains and undoubtedly should be further explored.
