fermentation broths or impure solutions containing thienamycin , a substance having antibiotic activity against gram - negative and gram - positive microorganisms , are purified using a sequence of ion exchange resins , starting with an anion exchange resin of the polystyrene - trimethylammonium type in the hco 3 - cycle , and ending with absorption on and elution from a polymeric cross - linked polystyrene - type resin adsorbent . variations in the intermediate steps , e . g . different resins and eluates are possible and are illustrated within .