In vitro susceptibility of Giardia lamblia trophozoites to metronidazole and tinidazole.

The number of motile Giardia lamblia trophozoites approximately doubled in axenic culture in 24 hr when 50% fresh medium was provided every 24 hr. In a microculture modification involving no change of medium, similar multiplication took place during the first 24 hr, and trophozoites remained motile for four days or longer. Metronidazole and tinidazole were active against G. lamblia in vitro, an observation suggesting that their effect in therapy of giardiasis is a direct one. The effect was rapid--high drug concentrations immobilized most trophozoites within 3.5 hr. There was an inverse relationship between duration of exposure and concentration of drug needed for significant immobilization: the effect of 0.1 microgram/ml was detectable after two to four days. Tinidazole was 4.4-fold as active as metronidazole; the minimal concentrations that immobilized 100% of trophozoites by 24 hr ranged from 0.2 to 12.5 microgram/ml and from 1.6 to 50 microgram/ml, respectively.