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What is the pink deposit that is sometimes seen in toilets and around sinks and bathtubs?

The pink colour is due to a pigment secreted by a species of bacteria known as Serratia marcescens, found naturally in soil, air and water. They can cause infections, but that generally happens in hospitals and affects people with compromised immune systems.

Serratia marcescens love a moist environment but cannot survive in chlorinated water. However, when tap water sits around, like in a toilet bowl, or on grout, or on a shower curtain, the chlorine evaporates and bacteria can take root and multiply, especially if soapy deposits are present which they use as food. When they multiply, Serratia marcescens produces a reddish pigment called prodigiosin. A household cleaner that contains bleach or hydrogen peroxide usually solves the problem.

There are a couple of interesting little footnotes to Serratia marcescens. In 1263, a miracle is said to have occurred in the Italian town of Bolsena, when during the celebration of mass,  “blood” appeared on the ceremonial bread. The theory is that the bread had become contaminated with the pigment-producing bacteria. A fresco in the Vatican painted by Raphael commemorates this event.

The occurrence of that event is questionable, but the use of Serratia marcescens by the US Navy as part of a cold war experiment is factual. The bacteria, believed to be harmless, were released from balloons over the San Francisco area to simulate the use of pathogenic bacteria as biological warfare agents. The plan was to track the Serratia marcescens bacteria by the pigment it produces in order to get a feel for how large an area can be contaminated by bacteria released in such a fashion. As it turned out, the bacteria may not have been harmless since an unusual number of infections were noted in area hospitals, although a connection was never proven.


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