Which mechanism explains the ability of Streptococcus pneumoniae to change its capsule type via DNA uptake, evidenced by DNase blocking capsule expression?

• A. Transduction
• B. Conjugation
• C. Transformation
• D. Natural transformation

Answer: (D)

The scenario highlights transformation, specifically natural transformation. Streptococcus pneumoniae is naturally competent and can take up naked DNA from the environment. When donor DNA containing a different capsule biosynthesis locus is taken up and recombines into the recipient’s genome, the capsule type can switch. The fact that DNase blocks capsule expression shows that extracellular DNA is required for this change—destroying the DNA prevents the transformation event from occurring. This distinguishes transformation from other gene transfer methods like transduction (phage-mediated) or conjugation (cell-to-cell transfer, often plasmid-based), which don’t depend on uptake of naked environmental DNA.