actinomyces archaea bacteria bacterial vaginosis biotin body odor carbohydrate dental plaque dietary supplement digestion digital object identifier escherichia coli folic acid gut flora human body human microbiome project immune system immunodeficiency lactobacillus lactobacillus acidophilus lactose large intestine malassezia methanobrevibacter methanogen methanosphaera microorganism model organism oxygen pathogen pathogenic bacteria probiotic sebaceous gland small intestine tooth enamel vitamin vitamin k yeast