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Giardia lamblia predominantly colonizes the proximal small intestine where bile is plentiful. We have investigated interactions between bile and this parasite by (i) examining the specificity of the stimulatory effect of bile on parasite growth in vitro, (ii) studying the possible association between giardiasis and bile salt deconjugation in vivo, and (iii) quantifying bile salt uptake by Giardia and relating this to uptake by other microorganisms. Our findings indicate that the growth promoting effects of ox bile and pure bile salt (sodium glycocholate) are, as far as is known, specific for Giardia, since the growth of a related protozoon, Trichomonas vaginalis, and a variety of enteric bacteria was either unchanged or inhibited. We were unable to detect deconjugated ('free') bile salt in duodenal fluid from UK patients with giardiasis and found no evidence to suggest that Giardia deconjugated bile salts in vitro. However, Giardia avidly took up conjugated bile salt, apparently in a concentration-dependent manner and to a much greater degree than Trichomonas and enteric bacteria. Thus, bile specifically stimulates growth of Giardia, and bile salt is avidly consumed by the parasite. The mechanism by which bile stimulates parasite growth is unknown, but uptake of conjugated bile salt by Giardia could reduce intraluminal bile salt concentrations and possibly interfere with micellar solubilization of fat. This may contribute to the steatorrhoea which is well recognized in symptomatic patients with giardiasis.

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