**Core Concept**
The normal vaginal microbiota is predominantly composed of *Lactobacillus* species, which maintain a low pH environment and inhibit the growth of pathogenic bacteria through lactic acid production and competition for resources.

**Why the Correct Answer is Right**
*Lactobacillus* is the dominant protective bacterium in the healthy vagina. It produces lactic acid via fermentation of glucose, lowering vaginal pH to 3.8–4.5, which creates an acidic environment hostile to harmful microorganisms. This acidification is mediated by the enzyme lactate dehydrogenase and involves the conversion of pyruvate to lactate. *Lactobacillus* also produces bacteriocins and competes for nutrients, preventing colonization by pathogens like *Gardnerella* or *Escherichia coli*.

**Why Each Wrong Option is Incorrect**
Option A: *Peptostreptococcus* is a normal commensal but not the primary protective organism; it is often found in abnormal or postpartum vaginal flora and is not dominant in healthy states.
Option C: *Gardenella vaginalis* is associated with bacterial vaginosis, not normal protection; it is a pathogen in dysbiosis, not a protective bacterium.
Option D: *E. coli* is a common commensal in the gut and urogenital tract but is not a dominant protective organism in the vagina and can cause infection when present in excess.

**Clinical Pearl / High-Yield Fact**
In normal vaginal flora, *Lactobacillus* is the key bacterium maintaining microbial balance and preventing infections. A shift in this balance (e.g., due to antibiotics or douching) leads to bacterial vaginosis or increased risk of STIs.

✓ Correct Answer: B. Lactobacillus