Serratia marcescens: An Overview of an Opportunistic Pathogen
Serratia marcescens is a Gram-negative, facultative anaerobic bacillus belonging to the Yersiniaceae family within the Enterobacterales order. The organism has a long and colorful history, first described in 1819 by Bartolomeo Bizio, an Italian pharmacist. Bizio observed a blood-like pigmentation on polenta and rice, classifying the microorganism and naming it in honor of the physicist Serafino Serrati, with ‘marcescens’ coming from the Latin word ‘to decay.’ For many years, it was considered a benign saprophyte, or non-pathogen, due to its perceived low virulence. It was even used as a biological marker in medical and military experiments, such as those conducted by the U.S. government in the mid-20th century to study the spread of biological agents over civilian and military areas. However, by the mid-1960s, it was definitively recognized as a human pathogen. Today, *S. marcescens* is one of the six species within the *Serratia* genus associated with human infections and is the most commonly isolated, highlighting its transformation into a significant, globally distributed opportunistic pathogen.
Morphological and General Characteristics
As a member of the *Enterobacterales* order, *S. marcescens* is a motile, non-endospore-forming, rod-shaped bacterium (bacillus). Under the microscope, they are seen as short, straight, or slightly curved rods, often capsulated, which boosts their biofilm-forming ability and increases virulence. Its colonies are typically moist and smooth on standard laboratory media. A distinguishing and historically significant characteristic of many strains is the production of a striking, non-diffusible, insoluble pigment called prodigiosin, which ranges in color from dark red to pink depending on the age of the colonies and culture medium. This pigment production is what first drew attention to the species. However, non-pigmented strains are now predominant in clinical settings, especially among immunocompromised patients. The bacterium is mesophilic, capable of growth across a wide temperature range (5°C to 40°C), with ambient growth seen at 37°C, enabling it to thrive in diverse environments and survive in hospital settings.
Widespread Habitat and Clinical Epidemiology
*S. marcescens* is an organism of extensive ecological versatility, found ubiquitously in the environment, including soil, water, plants, and insects, demonstrating enormous metabolic versatility and an ability to survive in low-nutrient environments, such as distilled water, certain disinfectants, and moist damp places like bathrooms and sinks. In humans, it colonizes the upper respiratory tract, gastrointestinal tract, and urinary tract. This environmental resilience makes it a major causative agent of healthcare-associated infections (HAIs), or nosocomial infections, particularly in neonatology and Intensive Care Units (ICUs). The bacteria’s ability to survive on hospital surfaces, medical devices (catheters, ventilators, prefilled syringes), and the hands of healthcare workers facilitates its spread and contribution to outbreaks. Risk factors for infection include prolonged hospitalization, a weakened immune system, pre-existing conditions like cancer and diabetes, prior long-term antibiotic use, and the presence of invasive medical devices like indwelling urinary and central venous catheters, which are thought to be the main entry points for infection.
Spectrum of Infections and Virulence Factors
*S. marcescens* is responsible for a wide range of human infections. The most commonly reported include urinary tract infections (UTIs), bloodstream infections (bacteremia or sepsis), and respiratory tract infections (pneumonia). It can also cause severe and invasive infections such as meningitis (especially in pediatric wards), peritonitis, endocarditis (particularly in heroin-addicted persons), arthritis, osteomyelitis, and ocular infections like keratitis. Clinical management is challenging because bloodstream infections caused by *Serratia* can significantly raise the patient’s mortality rate compared to bacteremia caused by other pathogens. Its pathogenicity is underpinned by a battery of virulence determinants. It produces various extracellular enzymes, including hemolysin, lipase, protease, and nuclease, that aid in tissue invasion, immune modulation, nutrient acquisition, and host cell damage, such as the lysis of host cells by the pore-forming hemolysin ShIA. Furthermore, the capsule polysaccharide (CPS), especially in strains belonging to the KL1 and KL2 clades, is a critical fitness determinant during bloodstream infection. This capsule boosts its robust biofilm-forming capability and confers resistance to macrophage phagocytosis, protecting it from host antibacterial defenses and underpinning the persistent colonization of medical devices.
Antimicrobial Resistance and Treatment Challenges
A major clinical concern regarding *S. marcescens* is its complex profile of antimicrobial resistance (AMR). The organism is intrinsically resistant to several classes of antibiotics, including ampicillin, first and second-generation cephalosporins, macrolides, nitrofurantoin, colistin, and cationic antimicrobial peptides (CAPs). This intrinsic resistance is often due to the presence of chromosomally encoded antibiotic resistance genes (ARGs), such as $beta$-lactamases (like AmpC) and efflux pumps, along with modifications to its lipopolysaccharide (LPS). Furthermore, *S. marcescens* has a high capacity to acquire resistance genes, leading to the emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains. The misuse of antibiotics in intensive care units has accelerated this transformation. Treatment of invasive infections is challenging because many clinical isolates now show multiple forms of antimicrobial resistance to once-standard treatments like fluoroquinolones, aminoglycosides, and third-generation cephalosporins. Consequently, antibiotic sensitivity testing (AST) is indispensable to guide targeted therapy. Treatment of invasive *S. marcescens* infections often requires carbapenems or aminoglycosides in combination with third- or fourth-generation cephalosporins. For uncomplicated urinary infections, cotrimoxazole may be considered.
Biotechnological and Environmental Relevance
Beyond its role as a clinical pathogen, *S. marcescens* holds unique relevance in environmental and industrial contexts. Its ubiquitous nature and high metabolic plasticity enable it to degrade a wide variety of organic and inorganic compounds, including chitin and pollutants such as heavy metals, suggesting its potential utility as a biological control agent and in environmental bioremediation. Furthermore, the pigment prodigiosin, which caused the historical discoloration of food, possesses significant intrinsic bioactivity. Research has demonstrated that prodigiosin exhibits promising antimicrobial, antimalarial, anticancer, and immunomodulatory properties, opening avenues for its use in therapeutic development. The organism’s overall genetic plasticity and metabolic versatility highlight its importance not just as an opportunistic human pathogen but also as a versatile microbe with significant dual challenges and opportunities for microbiology and drug discovery.