Potential of beta-galactosidase-expressing Toxoplasma gondii for in situ localization and observation of rare stages of the parasite life cycle.

A cyst-forming strain of Toxoplasma gondii was transfected with the Escherichia coli LacZ gene and expressed beta-galactosidase constitutively. This strain has been used to localize and analyze the early stages of development and reactivation of T. gondii in mice. The chromogenic detection of the enzyme allows an easy detection of the parasites after light fixation and therefore allows a submacroscopic analysis of tissue distribution within the organism. Also, it allows further embedding and retrieval of rare stages for electron microscopic observation. that detect the presence of the parasite and initiate the response, and (2) the early stages of reactivation, when the cysts are supposed to break open and release the infectious bradyzoïtes. We have taken advantage of the possibility of detecting the enzymatic activity of beta-galactosidase (beta-gal) in transfected parasites to show that one could perform a semi-macroscopic detection and that this was compatible with further analysis by histological or electron microscopic techniques, being therefore able to detect the rare events and then to analyze them further with more refined morphological techniques.