Mareitta was born on 20 March 1850 to Joseph Hannon, a professor of zoology and comparative anatomy at Brussels University.

Marietta said of Elisa: "The conformity of our tastes, the similarity of certain aspects of our characters, formed the basis of a complete agreement and a collaboration whose activity could never be slowed down.

Together they published a paper on Costa Rican fungi focused on with material collected from 1887 by Henri François Pittier,[2] from which they discovered several new species.

After Elisa Bommer died in 1910, Marietta was not longer involved in active collecting, but she still led naturalist excursions in Brussels and the surrounding countryside.

[4] She was friends with the artist James Ensor with whom she corresponded, and she appeared occasionally as the subject of his paintings and sketches.