Mould behind plasterboard wall

Sounds like a condensation issue, was the bathroom wall insulated before you dry lined it?

Stachybotrys chartarum, also known as black mould, is a variety of microfungus that produces its conidia in slime heads. It is most often detected in cellulose-rich building materials from damp or water-damaged buildings. It requires very high moisture levels in order to grow and is associated with wet gypsum material and wallpaper. It is manifest most commonly in homes with significant condensations issues and can usually be controlled by changing one or more of the 3 elements which facilitate its existence. Heating, Insulation & Ventilation. Often ventilation alone can solve the problem if addressed correctly. We routinely install the Nuaire Drimaster Eco PIV unit in lofts to remedy problems like this.

In case the brick walls in the house were originally bedded on lime mortar: It is possible that the moisture exchange within the wall(s) is out of balance because hard cement pointing and / or cement render is present on exterior and cement based plasters on interior side. This should be removed and re pointed / plastered using lime mortar in spring / summer. The wall(s) will gradually dry out, this can take several weeks. Before this is done: The black mould should be treated with a water and bleach mix and removed. In case it is a modern build: The previous answer will apply. This can be tricky to resolve if the building design has not been thought through and executed properly.

Spray in were you took brick out with a anti fungus solution with spray pump ...lance ...bottle..put brick back and make good...no need to disturb bathroom