“…Saliva in the mouth contains bacteriolytic enzymes (lysozyme), and gastric juice in the stomach contains hydrochloric acid and digestive enzymes, while the small and large intestine contain antimicrobial peptides (defensins, cathelicidin, cryptdin, elafin, etc. ), bile, natural microflora, mucus, secretory IgA, oxygen-limiting environment, epithelial barrier, and submucosal immune cells ( Brogden, 2005 ; Wesche et al, 2009 ; Garrett et al, 2010 ; Jäger et al, 2010 ; Sleator and Hill, 2010 ; Becattini and Pamer, 2017 ; Bhunia, 2018 ). Pathogens encounter multiple host-induced stresses in the intestine from acidic pH, nutrient limitation, low iron, oxidative and nitrosative stress, bile salts, free fatty acids, DNA damage, oxygen-limitation, and temperature in the gut ( Louis and O’Byrne, 2010 ; Fang et al, 2016 ).…”