Role of the Gut Microbiome in the Modulation of Cancer Immunotherapy Response

The gut microbiome or gut flora is a vast community of microorganisms such as bacteria, viruses, protozoa, and fungi that inhabit the digestive tract of the human and other animals [1,2]. In the human body, bacterial species colonize into the oral cavity, skin, vagina, and placenta, however, the largest population of microorganisms resides in the intestine. The majority of gut microbiota belong to the phyla Firmicutes, Bacteroidetes, Proteobacteria and Actinobacteria [2]. Colonization of the gut microbiota begins during or after the birth when the neonates get exposed to the vaginal microbes as he or she passes through the birth canal of the mother [3,4]. The growth and population of the gut microbiome are influenced by multiple factors including gestational age, mode of delivery (vaginal/cesarean), infant feeding method, diet, environment, medications, exposure to antibiotics, and comorbid diseases [2,5-8]. Gut microbiota has co-evolved with humans for millions of years to form a mutually beneficial relationship. They play important role in food digestion, nutrient and mineral absorption, synthesis of amino acids, enzymes and vitamins, and production of short-chain fatty acids, thus crucial for health and wellbeing [9,10]. Also, the gut microbiota is involved in immune regulation, brain function, and neuroendocrine responses [11,12]. The human gastrointestinal tract harbor both unhealthy and healthy microbiota, that arise through a complex combination of genetic, environmental, and lifestyle factors. Their imbalance contributes to high blood sugar, high cholesterol, weight gain, and other pathological conditions [11,13]. Change in the population of normal microbiota has been suggested to associate with the development and progression of many diseases [13]. Several species of bacteria including Helicobacter pylori and Coriobacteriaceae have been identified as potential candidates associated with carcinogenesis [14,15].