**2. Understanding the human gut microbiome**

The term microbiome has been derived from two words, "micro" and "biome", meaning, a specific microbial community with distinct physiological and chemical properties, residing in a well-defined habitat which is their "theatre of activity". This definition was proposed by Whipps while working on mycoparasites [20]. The term "gut microbiome" or "human microbiome" was coined by Joshua Lederberg in 2001 and since then it has been a topic for debate among researchers [21]. The human microbiome can be defined as a specific community of commensal, symbiotic and pathogenic micro-organisms that reside within our body spaces [22]. These include gut bacteria, eukaryotes, archaea, and specific viruses [23, 24]. In a healthy individual, these bacteria are responsible for various synthetic and metabolic functions and detoxification of various xenobiotics [25]. They form an integral part of the "gut-brain axis" which is bidirectional communication between the gut and the

### *Role of Gut Microbiome and Enteric Bacteria in Gallbladder Cancer DOI: http://dx.doi.org/10.5772/intechopen.104404*

### **Figure 1.**

*Importance of gut bacterial microflora. The figure illustrates the role of enteric bacteria in the maintenance of homeostasis. The "eubiotic" bacteria display a complex interaction with the various synthetic and metabolic functions of our body as well as in the "gut-brain crosstalk". Alteration or "dysbiosis" due to any factor (diet, chemicals, antibiotics, inflammatory conditions) may lead to "blooms" of harmful bacteria. The dysbiotic bacteria have now been linked to various cardiovascular, metabolic, neuropsychiatric diseases, including cancer.*

cognitive and emotional centers of the brain. This link is responsible for satiety and appetite regulation, elevation of mood, cognitive development, and neuroprotection [26]. Studies have also found a link between the gut microbiome and immune homeostasis. The complex and bidirectional interaction between the gut microbiota and the host immune system is responsible for the development of both innate and adaptive immunity, thus preventing the body from pathogenic organisms [27, 28]. Moreover, the microbiota is also responsible for the maintenance of gut mucosal integrity and prevents the overgrowth of pathogenic organisms, thus maintaining the first line of defense against the pathogens [29]. Therefore, any imbalance in the gut microbiota may lead to the development of various autoimmune diseases. This concept of "dysbiosis" or "imbalance" in the gut microbiota may result in relative "blooms" of harmful bacteria, especially Enterobacteriaceae [30, 31]. Dysbiosis can be caused by a variety of factors, namely, dietary changes, inflammatory conditions, exposure to drugs, and toxins [32, 33] (**Figure 1**). The gut bacteria have been linked to a wide variety of cardiovascular diseases [34, 35], obesity [36, 37], inflammatory bowel disease [38], irritable bowel syndrome [39], and some neuropsychiatric diseases like depression [40]. But what has intrigued the researchers is the role of gut microbiota in the development of cancer.
