Thrush is a mixed bacterial infection in the grooves along the side of the frog, in the central cleft of the frog and/or in the crease between the heel bulbs. It usually involves the organism Spherophorus necrophorus (aka Fusobacterium necrophorus), which requires a very low oxygen environment to grow. Although this organism is capable of invading skin, it usually needs other bacteria to do so effectively. Exactly how this works is incompletely understood, but one theory is that the other bacteria will keep the oxygen level low enough for S. necrophorus to survive.