Inhibition of Growth ofGiardia lambliaby Difluoromethylornithine, a Specific Inhibitor of Polyamine Biosynthesis1

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Abstract
Difluoromethylornithine (DFMO) is a specific and irreversible inhibitor of ornithine decarboxylase, an enzyme which catalyzes the 1st step in the biosynthetic pathway of the polyamines. The effect of DFMO on the growth of G. lamblia, Entamoeba histolytica, and Trichomonas vaginalis was tested. Growth of G. lamblia was inhibited by DFMO at concentrations of .gtoreq. 1.25 mM. Culture doubling time increased with increasing DFMO concentration. Growth inhibition was reversed if spermidine was added within 53 h of addition of DFMO; no growth was observed if spermidine was added later, indicating eventual parasite death. The growth of E. histolytica and T. vaginalis, 2 unrelated mucosal-dwelling parasites of humans, was not inhibited by 20 mM DFMO. Apparently, polyamine biosynthesis from ornithine is required for growth of G. lamblia.