Drosophila have been used to model infection by Serratia marcescens, a Gram-negative, quorum-sensing bacterium found in the soil, water, and associated with plants and animals. Although S. marcescens is not a very commonly isolated human pathogen, it has a long history of being used in epidemiology research (some of it on uninformed participants), due to the convenience of its bright-red color, and the mistaken impression that it was non-pathogenic. It can cause meningitis, septic arthritis, and infect the urinary tract, respiratory tract, eye, and bloodstream. Its biofilm-forming capacity makes it a threat to patients using catheters, central lines, and similar devices. While it has been associated with outbreaks in hospitals, it is not a strictly nosocomial infection.
In Drosophila, S. marcescens infection increases the production of antimicrobial peptides (FBgg0001101, FBgg0001102) regulated by both the Toll and Imd pathways, unlike many other Gram-negative bacteria that stimulate only the Imd pathway. Macrophage-like plasmatocytes also contribute to fighting S. marcescens infections. The transmembrane phagocytosis receptor eater specifically binds to S. marcescens, and infected eater-null flies die more quickly than wild-type.
[updated December 2019 by FlyBase; FBrf0222196]
Since many strains of S. marcescens have red pigment, and the organism was assumed to be nonpathogenic, it was used as a tracer organism in medical experiments and as a biological warfare test agent. In a now-famous exposé, the U.S. government released S. marcescens over both civilian population centers and military training areas from the late 1940s to the mid-1960s in the hopes of gathering data on the potential spread of bioterrorism agents used against the United States. These experiments were unearthed by investigative journalism in the mid-1970s, prompting a congressional investigation that studied U.S. government testing on the public. In the meantime, S. marcescens was revealed to be a pathogen capable of causing a full spectrum of clinical disease, from urinary tract infections (UTIs) to pneumonia. (Mahlen 2011, pubmed:21976608)
Serratia marcescens is a member of the genus Serratia, which is a part of the family Enterobacteriaceae. Currently 14 species of Serratia are recognized within the genus, eight of which are associated with human infection. Of the eight species implicated in clinical infection S. marcescens, S. liquefaciens and S. odorifera are best known . Of all Serratia species, S. marcescens is the most common clinical isolate and the most important human pathogen. (http://www.antimicrobe.org/b26.asp)
S. marcescens produces a hemolysin, ShlA, that functions as a pore-forming toxin in concert with another protein, ShlB; together, these proteins cause cytotoxicity in red blood cells and in other eukaryotic cells, such as epithelial cells and fibroblasts. (Mahlen 2011, pubmed:21976608; Kurz et al. 2003, pubmed:12660152)
