Inhibition of clonal growth of Giardia lamblia and Entamoeba histolytica by metronidazole, quinacrine, and other antimicrobial agents

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Abstract
A convenient new method has facilitated precise determination of the susceptibility of the parasitic protozoa Giardia lamblia and Entamoeba histolytica to a number of antimicrobial agents. Inhibition of clonal growth in semisolid media was used to quantitate killing of parasites as a function of drug concentration and exposure time. The minimum lethal concentration of metronidazole for G. lamblia (2–4 mg/l) was similar to that determined by microscopic observation and subculture of organisms in liquid medium. Both parasites were killed rapidly by metronidazole: 50% in 5–7 h at 2 mg/l or in 1 h at 10 mg/l. L-Cysteine decreased the susceptibility of both parasites to metronidazole and exerted a lesser protective effect on Giard. lamblia with quinacrine, emetine, and chloramphenicol. When metronidazole and quinacrine were tested in combination, no strong synergistic or antagonistic interactions were observed. The susceptibilities of these unrelated intestinal pathogens to a variety of drugs were similar. However, Giard, lamblia was more sensitive to quinacrine and less sensitive to several inhibitors of protein synthesis.