Hygromycin A is a neutral aminoglycoside produced by Streptomyces hygroscopicus, reported by researchers at Eli Lily in the early 1950s. The much rarer hygromycin A is structurally unrelated to hygromycin B and the metabolites display quite different biological profiles. Hygromycin A inhibits bacterial ribosomal peptidyl transferase, binding to the ribosome in a distinct but overlapping manner with other antibiotics. Hygromycin A is active against Gram positive and Gram negative bacteria, including Mycobacteria, Leptospira and Treponema. Hygromycin A has immunosuppressant and anthelmintic activity. Most recently, hygromycin A has been reported as having unexpectedly specific activity against Borrelia burgdorferi, the causative agent of Lyme disease.