Science & Technology

That Pink Color in Your Bathtub Is a Soap-Eating Bacteria With a Tainted History

Even if you're a fastidious cleaner, you've probably noticed a light-pink hue appear somewhere in your bathtub, shower, or sink. No need to adjust your cleaning products: Those rose-colored streaks are colonies of bacteria chowing down on your soap scum. Thanks to their unusual color, these simple microbes have made some horrifying appearances in human history books.

Think Pink

Meet Serratia marcescens. It's an aerobic, gram-negative bacterium that thrives in a moist or wet environment, whether that's the soil in your backyard or the tiles in your shower. It loves to chow down on phosphates and fatty residues, both of which can be found in soaps and detergents. This bacteria can also travel through the air, which is why you may find more of it during the summer months when your windows are open. Wherever you live, you can't escape it; it's estimated that Serratia marcescens exists in similar levels in households all over the world.

You're constantly surrounded by this blushing bacteria, so here's the million-dollar question: Can it hurt you? If you're generally healthy, the answer is no — but that's not to say it's harmless. In hospitals especially, which are obviously full of people with compromised immune systems, it's a fairly common cause of blood infections, pneumonia, and urinary tract infections, along with a handful of other medical problems. Nonetheless, scientists believed it was harmless until the 1980s — and that, combined with the fact that its easy-to-spot color made it a great "tracer" microbe, led to more than a few ethical quagmires.

Related: How Bacteria Make a Grappling Hook for Propulsion

What Could Go Wrong?

The first "tracer" experiments were harmless enough. In 1906, Dr. M.H. Gordon was called into the House of Commons to investigate the ventilation in the building after a large number of its members caught influenza. To see how easily microorganisms could spread through the air, Dr. Gordon set up a bunch of petri dishes throughout the space, gargled a solution filled with the red-tinged bacteria — what can we say, science was a messy game back then — and recited Shakespeare in the debating chamber for a full hour. The scarlet bacterial colonies that grew on petri dishes a surprising distance away demonstrated that microbes could spread not only via coughing and sneezing, but also via talking. Dr. Gordon apparently didn't get sick from his experiment.

Serratia marcescens proved itself to be a quick and easy marker for bacterial contamination, so it isn't any wonder why the military wanted to use it to see how damaging biological warfare could be. Though the U.S. military wasn't the first to try the experiments — Germany had allegedly released the bacteria in Paris metro ventilation shafts in the 1930s — it was probably the most prolific. According to Senate hearings in the 1970s, the U.S. had tested Serratia marcescens in civilian areas at least seven times and military areas at least 29 times between 1949 and 1968. It allegedly led to the death of one hospital patient in 1950 and infections in at least five others. By the time of the hearings, scientists knew that this microbe wasn't as harmless as they thought — but the damage had already been done.

This might all sound scary, but again, healthy people likely have nothing to fear from Serratia marcescens. You can un-pink your bathroom sink with any cleaning product that contains bleach, but get ready for a fight; this salmon-colored colonizer is all around you, and it will inevitably return.

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Written by Ashley Hamer February 20, 2019

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