A daily dose of a widely used antibiotic can prevent some infections with syphilis, gonorrhea and chlamydia, potentially a new solution to the escalating crisis of sexually transmitted infections, scientists reported on Thursday. Their study was small and must be confirmed by more research. Scientists still have to resolve significant questions, including whether S.T.I.s might become resistant to the antibiotic and what effect it could have on healthy gut bacteria in people taking it every day.