Monovalent Cations Induce Microsporidian Spore Germination In Vitro

ABSTRACT The relative capacity of Na⁺, K⁺ and Cl^- to stimulate germination of spores of the microsporidian Nosema algerae, a pathogen of mosquitoes, was examined by ion substitution experiments. Sodium at 0.1 M was ineffective to produce the high percentage of germination that typically occurs with 0.1 M NaCl (the normal stimulation solution) if Cl^- was substituted with the usually impermeant anions SO₄^2-, HPO₄^2-, or the organic acids oxalate, cacodylate, EGTA, MES and HEPES. However, substantial concentration- and pH-dependent germination was seen with Na₂SO₄ in the 0.2-0.8 M Na⁺ range. Similar results were obtained with solutions of K⁺ accompanied by impermeant anions. In contrast, the chloride salts of usually impermeant cations, like choline and triethanolamine, failed to germinate spores even at 0.8 M unless Na⁺ or K⁺ was independently added. The presence of 0.5 M choline chloride in the medium reduced the levels of Na₂SO₄ required to produce germination down to equivalence with those of Na⁺ in the normal stimulation solution. Monensin, a Na⁺ ionophore, facilitated the germination induced by a medium-level stimulus (0.04 M NaCl) in sonicated samples. These findings indicate that N. algerae spores germinate in response to the alkali metal cations, while CI^- plays a passive role by diffusing to maintain internal electroneutrality during cation influx. A possible mechanism of cation action in spore germination is suggested on the basis of these results and observations on other systems of intracellular motility.