Bacteria and fungi, naturally present in air and water, can attach to damp surfaces and multiply to form a visible black slime or stain in various colours (black, red, pink). These growths are known as biofilms or ‘microbial slime’ and are most commonly seen in bathrooms and kitchens. Some biofilms can grow into certain materials and become a permanent feature. Although these biofilms can appear very unpleasant they are generally harmless and do not pose a risk to health.
For more information, including details of how to prevent biofilm growth and how to remove them, please read our factsheet on slime and staining (PDF 168 KB opens in a new window).
If you are at all concerned about the quality of your water please call us on 0345 672 3723. Dial 18001 first if you have hearing or speech difficulties.