Microsporidia are a cause of emerging and opportunistic infections in humans and animals. Although two drugs are currently being used to treat microsporidiosis, concerns exist that albendazole is only selective for inhibiting some species of microsporidia that infect mammals, and fumagillin appears to have been found to be toxic. During a limited sequence survey of the Vittaforma corneae (syn. Nosema corneum Shadduck, Meccoli, Davis et Font, 1990) genome, a partial gene encoding for the ParC topoisomerase IV subunit was identified. The purpose of this set of studies was to determine if fluoroquinolones, which target topoisomerase IV, exert activity against Encephalitozoon intestinalis (syn. Septata intestinalis Cali, Kotler et Orenstein, 1993) and V. corneae in vitro, and whether these compounds could prolong survival of V. corneae-infected athymic mice. Fifteen fluoroquinolones were tested. Of these, norfloxacin and ofloxacin inhibited E. intestinalis replication by more than 70% compared with non-treated control cultures, while gatifloxacin, lomefloxacin, moxifloxacin, and nalidixic acid (sodium salt) inhibited both E. intestinalis and V. corneae by at least 60% at concentrations not toxic to the host cells. These drugs were tested in vivo also, where gatifloxacin, lomefloxacin, norfloxacin, and ofloxacin prolonged survival of V. corneae-infected athymic mice (P < 0.05), whereas moxifloxacin and nalidixic acid failed to prolong survival. Therefore, these results support continued studies for evaluating the efficacy of the fluoroquinolones for treating microsporidiosis and for characterizing the target(s) of these fluoroquinolones in the microsporidia.
The microsporidium Vittaforma corneae (Shadduck, Meccoli, Davis et Font, 1990) develops within the target cell cytoplasm. In the present study, green monkey kidney (E6) cells infected at 30°C, 35°C or 37°C with V. corneae developed enlarged multinucleate structures of up to 200 µm in any horizontal dimension made up either of a single cell or of multiple fused cells. A number of epithelial cell types (SW-480, HT-29, Caco-2 and HCT-8) were infected with V. corneae but did not induce the same highly organized structures, suggesting that for the structure to develop, the host cell must be capable of continued mitosis, and not be differentiated or be detaching from the surface matrix. Live cell imaging of infected E6 cells revealed large, multinucleate infected cells characterized by a central focus from which radiated parasite stages and host cell mitochondria. Immunocytochemistry identifying γ and α tubulin suggested that a single centrally-located microtubule organizing centre governed the distribution of parasite stages and host cell organelles, with mitochondria and parasites being eventually transported towards the periphery of the structure. Whole cell patch clamp analysis of infected cells indicated an average five-fold increase in total membrane capacitance, consistent with an enlarged single cell. Scanning electron microscopy revealed cell-like protrusions around the periphery of the structure with the intervening space being made up of parasites and cell debris. Clearly in the case of V. corneae-infected E6 cells the parasite-host cell relationship involves subverting the host cell cytoskeleton and cell volume control, providing the parasite with the same protected niche as does a xenoma.