Overview
Serratia marcescens is a Gram-negative, facultatively anaerobic, rod-shaped bacterium belonging to the family Yersiniaceae (within the order Enterobacterales). Historically notable for producing prodigiosin, a distinctive red pigment, S. marcescens was once considered a harmless environmental saprophyte. However, it has emerged as an important opportunistic pathogen responsible for a wide range of nosocomial infections. The organism is ubiquitous in the environment, found in soil, water, and on plant surfaces, and can transiently colonize the human gastrointestinal tract, though it is not considered a typical component of the healthy gut microbiome.
Classification
S. marcescens belongs to the phylum Pseudomonadota, class Gammaproteobacteria, order Enterobacterales. Pan-genome analyses of Serratia isolates have revealed important distinctions between nosocomial and environmental strains. Hospital-associated strains form predominantly distinct multilocus sequence types and carry unique antibiotic resistance gene profiles compared to environmental isolates. Notably, environmental strains may serve as reservoirs of antibiotic resistance and virulence genes that can be transferred to clinical strains through horizontal gene transfer, blurring the line between environmental and pathogenic populations.
Key Characteristics
A key virulence factor of S. marcescens is the ShlA hemolysin/cytotoxin, which causes rapid, contact-dependent destruction of epithelial cells. Research has demonstrated that ShlA can destroy intestinal epithelial cell microvilli and induce a tenfold increase in LDH cytotoxicity within six hours of infection. The organism also triggers robust IL-8 production, initiating a pro-inflammatory cascade in intestinal tissues. S. marcescens possesses intrinsic AmpC beta-lactamase activity and can acquire extended-spectrum beta-lactamase genes, contributing to multidrug resistance. Interestingly, studies using insect gut models have shown that a healthy, intact commensal microbiota can rapidly clear S. marcescens, demonstrating the importance of colonization resistance.
Health Significance
S. marcescens is responsible for a range of nosocomial infections including septicemia, urinary tract infections, respiratory tract infections, wound infections, meningitis, and eye infections, particularly keratitis associated with contaminated contact lens solutions. The organism has also been associated with necrotizing enterocolitis (NEC) in neonates, where its cytotoxic effects on intestinal epithelium are thought to contribute to disease pathology. The emergence of multidrug-resistant strains in hospital settings is a growing concern. From a microbiome perspective, a robust and diverse gut microbial community appears to provide significant colonization resistance against S. marcescens. Maintaining microbial diversity through appropriate dietary habits and avoiding unnecessary antibiotic exposure may help reduce susceptibility to opportunistic colonization by organisms like Klebsiella pneumoniae and Serratia. This organism is not routinely included in gut microbiome panels but is identified through clinical cultures when infection is suspected.