SUMMARY The human enteric protozoan, Giardia lamblia, has surface membrane lectin activity which mediates parasite adherence to erythrocytes. To determine whether an intestinal binding site exists for this lectin we have studied the interaction in vitro between axenically cultured Giardia trophozoites and isolated rat intestinal epithelial cells. Scanning electron microscopy showed that Giardia attached to the apical microvillus membrane and basolateral membrane of rat enterocytes. Any location on the parasite surface could mediate attachment without predeliction for the ventral disc. Trophozoites attached more avidly to jejunal compared with colonic epithelial cells. Attachment was inhibited at 4°C, by sugars and glycoproteins containing D-mannosyl residues and by subagglutinating concentrations of anti-Giardia rabbit serum and two monoclonal antibodies, all with reactivity to parasite surface membrane determinants. Trypsinisation of trophozoites also reduced attachment but the ability to attach was rapidly restored after returning trophozoites to TYI-S culture medium for 4 h at 37°C. Attachment was unaltered by the presence of the microfilament inhibitor cytochalasin B and in the absence of Ca++ and Mg++ ions. These findings support previous work that Giardia possesses a surface membrane mannose binding lectin and indicate that appropriate binding sites are present on rat intestinal epithelial cells. This lectin may play a part in mediating adherence of Giardia to mammalian intestine and could be a target for host immune defence.Adherence of microbial enteropathogens to host intestinal epithelium is considered to be a critical step in the pathogenesis of many intestinal infections.' Giardia is commonly found in close proximity to the small intestinal mucosa and when in the ventral surface down position, it is generally considered that the parasite attaches to intestinal epithelial cells by either a suction force generated beneath the ventral disc by the propulsive efforts of the ventral flagella (hydrodynamic theory),' by mechanical processes related to contractile protein elements of the ventral disc and ventrolateral flange," or by a combination of the two mechanisms.Giardia may be found in other orientations with respect to the gut mucosa such as the dorsal surface down position' as seen in one of our patients (Fig. 1). This suggests that parasite surface membrane determinants may also be involved in the attachment process. We have shown previously using mammalian erythrocytes as a model target for attachment, that Giardia lamblia, like some bacteria and other protozoa, has lectin activity associated with its surface membrane with specificities for D-glucosyl and D-mannosyl residues.7 To discover the possible relevance of this attachment mechanism in the pathogenesis of giardiasis, we have now investigated the interaction between Giardia and isolated mammalian intestinal epithelial cells to determine whether an intestinal receptor exists for this putative lectin in gut epithelium.

No abstract

Susceptibility of Giardia lamblia trophozoites to 3 antigiardial drugs was determined morphologically by 24h parasite counts and radiometrically by uptake of [3H]thymidine and by rapid 4h microassays of motility and viability. Growth of Giardia trophozoites in liquid culture correlated well with uptake of [3H]thymidine during a 72h period (r = 0.91, P less than 0.01). ED50 and MIC for metronidazole, mepacrine and sodium fusidate were similar in both morphological and radiometric growth assays and, except for mepacrine, results were similar to previously reported drug efficacies obtained with more labour intensive methods. Motility and viability 4h microassays were insensitive and time-consuming and cannot be recommended for routine screening of antigiardial drugs. The radiometric growth assay is therefore a simple, objective measure of antigiardial drug activity which could be used to determine drug sensitivity profiles of clinical isolates or for screening new antigiardial drugs.

Propranolol inhibits sperm motility and has been considered as a spermicide contraceptive. In view of the inhibitory effects of D-propranolol on sperm flagellar activity, we have investigated its effect on motility and growth of two human flagellate, protozoan parasites. D-propranolol had a dose-dependant inhibitory effect on motility of Giardia lamblia and Trichomonas vaginalis with ED50s of 0.38 and 0.66 mmol/l respectively, D-propranolol also inhibited growth of both parasites with ED50s of 0.18 and 0.23 mmol/l for Giardia and Trichomonas respectively. D-propranolol, unlike DL-propranolol is devoid of the unwanted effects of beta-blockade, and thus may be a useful antiprotozoal drug particularly after vaginal placement when it is concentrated in vaginal mucus.