In vitro susceptibility of Giardia lamblia trophozoites to metronidazole and tinidazole

The nU~ber of motile Giardia lamblia trophozoites approximately doubled in axenic cult~r ~ m.24?r wh~n 50 % fresh medium was provided every 24 hr. In a microculture mod~fICatlOn involving no change of medium, similar multiplication took place during the fI~S ~ 24 hr, and trop~ozoites remained motile for four days or longer. Metronidazole an ~ tlmdaz?le were active against G. lamblia in vitro, an observation suggesting that their e~fect ~n ther~~y of giardiasis is a direct one. The effect was rapid-high drug con-ce~tratIons Immob~lIzed most trophozoites within 3.5 hr. There was an inverse relation-ship ~.etw~en duration of exposure and concentration of drug needed for significant im-mobilization: the e~fect of 0.1 J.ig/ml was detectable after two to four days. Tinidazole was 4.4-fold as active as metronidazole; the minimal concentrations that immobilized 1oolJ,10 ~f trophozoites by 24 hr ranged from 0.2 to 12.5 ug/ml and from 1.6 to 50 ug/ml, respectively, Giardia lamblia was one of the first microorga-nisms seen by the human eye [1], and giardiasis (infection of the small intestine with Giardia) i