Final answer:
The majority of the human gut microbiota consists of the phyla Firmicutes and Bacteroidetes, with Bacteroidetes alone comprising about 30% of the gut microbiome.
Step-by-step explanation:
The bacterial phylum that constitutes approximately 90% of the gut microbiota in humans is a combination of two main phyla: Firmicutes and Bacteroidetes. However, Bacteroidetes is specifically known for making up a significant proportion, about 30% of the entire gut microbiome. Members of the genus Bacteroides, which belong to the Bacteroidetes phylum, are prevalent inhabitants of the human large intestine.
Proteobacteria is another phylum present in the human gut, which includes many bacteria that are part of the normal human microbiota as well as many pathogens. However, it does not represent the majority of the gut microbiota.