Pink growth (aka pink slime) in bathtub or shower
I live in St. Louis, MO and get this in my bathtub over time, but not in my other sinks. I actually think @Christiaan is correct here, but with a few additional observations:


I don't think it's bacteria. When dry, it's not slimy, nor does it grow back quickly when wiped away. It feels much more like a mineral deposit. 
It was no longer pink when using a different shampoo. Upon returning to an older brand, the color returned to pink.
It's only directly around the drain, it isn't on the walls or anywhere else. 


These lead me to think it's a mix of mineral deposits from the hard water and potential reactions with certain soaps/shampoos, at least in my case. These are only based on my own observations though, there very well could be a pink bacteria/fungi that tends to inhabit damp areas.