The pink stains often found on shower walls may look like mold, mildew or algae, but the red-tinged coloration is usually the sign of bacterial growth. It's probably not red algae -- also known as Rhodophyta -- which is generally found in marine environments rather than indoor bath and shower areas. A bacterium known as Serratia marcescens is the more likely culprit.
Conditions for Growth
Serratia marcescens thrives in a moist and humid environment and eats the residue of soap and shampoo. It is an airborne pathogen, so it can initially arrive through open windows during the warmer months and settle on the surfaces of showers, kitchen sinks and even the pet water dish if not washed regularly.
Conditions for Eradication
Total eradication of the bacteria will not be easy. If it has gained a foothold in your shower stall, you'll need to scrub it off with a cleaning product formulated with chlorine bleach. If the bacteria has settled deep into the grout, the grout may have to be replaced.
To keep S. marcescens under control, plan on consistent weekly cleaning using a non-abrasive household cleanser that includes bleach. A milder chemical, white vinegar, may also prove effective.