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Why is penicillin used to treat syphilis and streptococcal infections?

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Penicillin is used to treat syphilis and streptococcal infections because of its specific mechanism of action against bacteria.1

All penicillins function by inhibiting the enzymes responsible for cell wall synthesis in replicating microorganisms while simultaneously activating other enzymes that break down the bacteria's protective wall. Because human cells lack cell walls, penicillin can effectively target and destroy these pathogens without harming the host's own cells.1

Specifically, in the case of syphilis, which is caused by the spirochete bacterium Treponema pallidum, penicillin is highly effective at killing the causative organism. Similarly, for various streptococcal and other bacterial infections, the drug exploits the vulnerability of actively replicating microorganisms that rely on cell wall production for survival.21