From the American Water Works Association, Opflow Article: Question of the Month, page 3, November 2000. Water Quality Bulletin, Woodinville Water District, Woodinville
Each year, a few customers call the water treatment plant to ask about a slimy pink substance that sometimes forms in moist areas around their homes. They most frequently observe it in the toilet bowls, on surfaces in shower stalls and bathtub enclosures, on tiles, in sinks and in pet water dishes.
A red or pink pigmented bacteria known as Serratia marcescens is thought to be the cause of the pink "stuff". Serratia bacteria are common inhabitants of our environment and can be found in many places, including human and animal feces, dust, soil, and in surface waters. The bacteria will grow in any moise location where phosphorous containing materials or fatty substances accumulate. Sources of these substances include soap residues in bathing areas, feces in toilets, soap and food residues in pet water dishes. Many times, the pinkish film appears during and after new construction or remodeling activities. Others have indicated the pink "stuff" occurs during a time of year that their windows are open for the majority of the day.
These airborne bacteria can come from any number of naturally occurring sources, and the condition can be further aggravated if customers remove the chlorine from their water by way of an activated carbon filter. Serratia can also grow in tap water in locations such as toilets in guest bathrooms where the water is left standing long enough for the chlorine residual disinfectant to dissipate. Serratia will not survive in chlorinated drinking water.
Serratia marcescens is not known to cause any waterborne diseases. Members of the Serratia genus were once known as harmless organisms that produced a characteristic red pigment. More recently, Serratia marcescens has been found to be pathogenic to some people, having been identified as a cause of urinary tract infections, would infections, and pneumonia in hospital environments.
Once established, the organism usually cannot be eliminated entirely. However, periodic and thorough cleaning of the surfaces where the pink slime occurs, followed by disinfection with chlorine bleach appears to be the best way to control it.
Cleaning and flushing with chlorine will not necessarily eliminate the problem, but will help to control these bacteria. Keep bathtubs and sinks wiped down and dry to avoid this problem. Using a cleaning solution that contains chlorine will help curtail the onset of the bacteria.