mi·to·my·cin (m??t?-m??s?n)n. Any of a group of antibiotics produced by the soil actinomycete Streptomyces caespitosus that inhibit DNA synthesis and are used against bacteria and cancerous tumor cells.[Probably mito(sis) + -mycin.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.mitomycin (?ma?t???ma?s?n) n (Pharmacology) pharmacol any of the antibiotics that come from Streptomyces caespitosusCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014