A simple colourimetric method to determine anti-giardial activity of drugs.

A new colourimetric method to be used for the screening of compounds with giardicidal activity was developed. After the end of drug incubation, the remaining viable cells were fixed with methanol and stained by methylene blue. The inclusion of methylene blue by Guardia lamblia trophozoites was measured spectrophotometrically to determine the anti-giardial activity of metronidazole. The 50% inhibitory concentration (IC(50)) was 1.96 +/- 0.13 microM. The reliability of this method was assessed by comparison with the IC(50) obtained using a haemocytometer to get the number of viable cells (1.82 +/- 0.43 microM). This method provides a feasible, cheap and reliable method to determine the anti-giardial activity of drugs.