Copyright © 2017 American Academy of Pediatrics. From: Summaries of Infectious Diseases Red Book® 2015, 2015 Figure Legend: Serratia marcescens on MacConkey agar. This organism is a lactose fermenter, appearing light pink on this particular type of agar. Some strains of S marcescens may appear red due to the production of a red pigment called prodigiosin. Courtesy of Julia Rosebush, DO, Robert Jerris, PhD, Theresa Stanley M(ASCP) Date of download: 11/8/2017 Copyright © 2017 American Academy of Pediatrics. All rights reserved.