The Collaborative International Dictionary of English v.0.48:

actinomycetes \actinomycetes\ n.
   1. branched gram-positive bacteria, often found in soil, some
      of which are pathogenic for humans and animals.

   Note: some species have been found to produce metabolites
         which are useful in medicine. Many species of these
         bacteria have been isolated and extensively screened by
         pharmaceutical companies and university research groups
         for production of useful therapeutic agents. Among the
         agent produced by actinomycetes are tetracyclines,
         streptomycin, avermectin, and thienamycin
         [WordNet 1.5]