Grout is organic because its binder cement is organic [or
inorganic plaster, lime or plastic]- and ballast –often sand which has an organic content of 4%, and water which contains organic compounds. These give the spores nutrition. The affection starts to grow.
The grout surface pores have different sizes, which with gathered dirt is binding damp. This mixture supplies the lichen with more nutrition. The rate and spread of affection increases and the lichens endurance makes them very hard to prevent and maintain.
The lichen's spores and old remainders of lichen enables new affections to constantly recur –as long as there is any remainders left on or in the grout. The affections and the surface must be sterilized and disinfected.