Serratia marcescens is a species of rod-shaped gram-negative bacteria in the family Enterobacteriaceae. The bacteria is pathogenic to humans and is responsible for hospital-acquired infections (HAIs), particularly through catheters, urinary tract infections, and wound infections.

Due to its abundant presence in the environment, and its preference for damp conditions, S. marcescens is commonly found growing in bathrooms, particularly on tile grout, in shower corners, toilet water line, and basin. It is identifiable as a pink, pink-orange, or orange discoloration and slimy film feeding off phosphorus-containing materials or fatty substances such as soap and shampoo residue. S. marcescens infections are also known to be transmitted through hand-to-hand contact with medical personnel.