Leptonema illini is a species of bacteria. It belongs to the spirochetes and it is the only species of the genus Leptonema. It can be found in water and soil media. Leptonema are Gram-negative bacteria, very thin, helical bacteria of about 0.1-0.2 μm in diameter and 13 to 21 μm. length. They are usually unicellular, but can also be seen as short chains in growing cultures.