In oral biofilms, Streptococcus sanguinis inhibits Streptococcus mutans through hydrogen peroxide production. What type of interaction is this?

• A. Cooperation through nutrient exchange
• B. Mutualism
• C. Commensalism
• D. Antagonism via inhibitory metabolite

Answer: (D)

Antagonistic interactions in a biofilm often hinge on one species releasing a chemical that harms a competitor. Here, Streptococcus sanguinis emits hydrogen peroxide that inhibits Streptococcus mutans in the same dental plaque. This negative effect on a rival fits an interference-type competition: an inhibitory metabolite reduces the fitness of another species rather than providing a shared or reciprocal benefit. It’s not a cooperative or mutual relationship, since there’s no mutual gain, and it’s not commensalism, where the other organism would be unaffected. The hydrogen peroxide acts as an antimicrobial agent that curtails the rival’s growth, helping S. sanguinis gain a competitive edge in the biofilm. In short: antagonism via inhibitory metabolite.