Judith Swann, of Dunedin, asks :-

What is the proper name for the pink mould that grows in bathrooms? Why does it like the bathroom so much and what is the easiest way to get rid of it, preferably for a decent length of time?

Corinna Richter, a microbiologist at the University of Otago, responded.

While most common moulds are caused by fungi, the cause of the so called pink “mould” is a bacterium. The name of the bacterium in question is Serratia marcescens. It is related to well known bacteria such as Escherichia coli or Salmonella which all belong to a family called Enterobacteriaceae.

Serratia marcescens is ubiquitious. That means it is not adapted to a certain niche but can be found almost everywhere in the environment e.g. in soil, on plants, etc. However, the bacterium prefers moist and damp surroundings. Therefore, it finds perfect living conditions in the bathroom and especially the shower and basins where it feeds on fatty acids from residue of soap.

Serratia marcescens produces a red pigment called prodigiosin which is responsible for the pink color. Apart from being colorful, prodigiosin has some other features which are quite useful to the bacterium. For example it acts as an antibiotic that might act against other types of bacteria that are living in close proximity and competing for the same nutrients.

Getting rid of Serratia marcescens once it is in the bathroom can be a difficult task. It can help to withdraw the basic survival needs by removing residues of soap and shampoo as well as drying the shower after each use. There are some bleach-based disinfectants, too, which can be efficient, however, these should not be used without previous consultancy of an expert.