Suimitsutou (吸蜜糖, Suimitsutō?, lit. Nectar sucker) are microscopic mushi found in plants. They act similar to yeast and metabolize sugar, which turns into luminous sake after the fermentation process.
According to Ginko, some Mushishi thought the Suimitsutou could be used to brew Kouki, but none had succeeded until Rokusuke, who brewed a batch so convincing that even the Shoujounohige were deceived by it. The sake lights up, but it's still not real Kouki.
Sake brewed with Suimitsutou makes whoever drinks it hallucinate, giving them the temporary ability to see mushi.