Ulcer serpens is caused by:

Ulcer serpens is a characteristic ulcer caused by pneumococci, it has a tendency to creep over cornea in a serpiginous fashion. Greyish white/ yellowish disc like lesion near centre of cornea, opacity is greater at the edges than at the centre paicularly well marked in one direction. Violent iritis is present and there may be a definite hypopyon which vary in size from hour to hour. Ref: Parson's diseases of eye, 21st edn, page:199