The pink rings found in these locations are caused by an airborne bacteria called Serratia marcescens. It thrives in the moist, dark conditions often found in bathrooms. Serratia marcescens is not an indicator of poor water quality in the distribution system. Rather, the residue is thought to result from airborne bacteria and is also affected by a homeowner’s cleaning habits. There are several methods that homeowners can use to control the development of this pink residue. Products containing chlorine, such as common household bleach, will both remove and control the proliferation of Serratia marcescens. Also, keep bathroom fixtures dry and free of standing water, paying special attention to the rim underneath the toilet.