Intrinsic susceptibility of Giardia duodenalis assemblage subtypes A(I), A(II), B and E(III) for nitric oxide under axenic culture conditions.

The antigiardial effects of nitric oxide (NO·) have been reported in vitro, but only for assemblage A(I) lab strains. This study investigated the intrinsic NO· susceptibility of different assemblage subtypes. The susceptibility (IC₅₀) for NO· released by MAHMA NONOate was studied for three lab (WB, G1 and GS/M-83-H7) and six field isolates of assemblage subtypes A(I), A(II), B and E(III). Tests were performed in phosphate-buffered saline supplemented with L-cysteine HCl, trypticase peptone, powder bovine bile and 20% inactivated foetal calf serum (for assemblages A and E) or human serum (for assemblage B), adjusted to pH 7.3, to support adequate trophozoite survival. Flow cytometry with fluorescein diacetate and propidium iodide as viability indicators was used to determine trophozoite viability. This study indicated that the NO· susceptibilities of assemblage A lab and field strains (subtypes A(I) and A(II)) were fully comparable, indicating that the NO· susceptibility of the lab strains remained representative for their genotype. The trophozoites of assemblages B and E(III) showed comparable NO· susceptibilities that were markedly higher than the susceptibilities of assemblage subtypes A(I) and A(II). This study suggests a role for the assemblage subtype in defining NO· susceptibilities. The underlying mechanisms still need to be elucidated, but assemblage-linked differences in the expression of the genes coding for flavohemoglobin or A-type flavoprotein may certainly deserve further attention.