Which organism cannot produce hydrogen peroxide due to lack of the SpxB gene?

• A. Veillonella
• B. Lactobacillus casei
• C. Streptococcus sanguinis
• D. Streptococcus mutans

Answer: (D)

SpxB encodes pyruvate oxidase, an enzyme that produces hydrogen peroxide as a byproduct of pyruvate metabolism in certain oral bacteria. This H2O2 production is a way some commensals help suppress competing microbes in dental plaque, such as Streptococcus mutans. Streptococcus mutans lacks SpxB, so it cannot generate hydrogen peroxide through this pathway. In contrast, Streptococcus sanguinis uses SpxB to produce H2O2, illustrating how the presence or absence of SpxB determines the ability to make hydrogen peroxide. Veillonella and Lactobacillus casei aren’t defined by this SpxB-mediated H2O2 production in the same way, making Streptococcus mutans the one that cannot produce H2O2 due to lacking SpxB.