Final answer:
Lactobacillus is the resident flora of the mouth, large intestine, and vagina (from puberty to menopause), where it plays a key role in maintaining a healthy microbial environment by producing lactic acid.
Step-by-step explanation:
The organism that is a resident flora of the mouth, the large intestine, and from puberty to menopause the vagina is Lactobacillus. This bacterium is imperative for human health, as it occupies ecological niches that may be invaded by pathogenic microorganisms if not for their presence. In the vagina, Lactobacillus species are predominant and are crucial for maintaining an acidic pH through the production of lactic acid, which inhibits the growth of pathogens such as yeasts. Lactobacillus is also a common resident of the human mouth and the large intestine, contributing to the healthy microbiota in these areas.