Complex sentence: Bacteria is involved in the intestinal immune system of a dog, as it influences the function of the intestinal mucosa; the levels of bacterial population in the gut regulates the immune system within the intestine.
Split into simpler sentences: Bacteria is involved in the intestinal immune system of a dog, as it influences the function of the intestinal mucosa. The levels of bacterial population in the gut is important; they regulate the immune system within the intestine.