I would like to know on whether oxygen levels increase on blood has affected the Gut bacteria or not,
Can you rephrase that? From my understanding, you're asking if we breathe more oxygen, does it affect the bacteria that live in our gut. If that's the question, there's no relationship between breathing more oxygen and increased/decreased levels of bacteria in the gut. Bacteria in the gut originates from our diet alone, and it grows based on our diet. A bad diet and increased use of antibiotics disrupts this balance between our intestinal health and beneficial bacteria in the gut.
Furthrmore, I would like to know on how gut bacteria produce metabolites influencing mood.
Bacteria found in the intestines produces metabolites that aid in food digestion, oxidation and reduction of molecules, and the synthesis of essential amino acids (D-Met, D-Trp, or D-Phe). In addition, many studies have reported that the intestinal metabolites regulate pathogen infection in intestines, preventing bad bacteria from taking over. Some metabolites, such as
nisin, and several lantibiotics directly kill pathogens by disrupting bacterial cell structures (a full list is shown in the PDF below).
Among these is Vitamin K2 and B vitamins. These vitamins stimulate proliferation of neutrophils, monocytes, and macrophages (host immune system), and regulate lymphocyte proliferation, natural killer cell activity, and T
reg cells (again, enhance the immune system).