erythromycin, any of several related antibiotic drugs produced by bacteria of the genus Streptomyces (see antibiotic). Erythromycin is most effective against gram-positive bacteria such as pneumococci, streptococci, and some staphylococci (see Gram's stain). The antibiotic also has some effect on gram-negative bacteria and some fungi. Erythromycin inhibits protein synthesis in susceptible microorganisms. It is used to treat such diseases as pneumonia caused by fungi, and streptococcus and syphilis infections, especially where the patient is allergic to penicillin.