Serratia marcescens

Introduction

Serratia marcescens is a Gram-negative, facultatively anaerobic bacterium belonging to the Enterobacteriaceae family. It is widely distributed in the environment and is recognized as an opportunistic pathogen in humans. This organism is associated with a variety of healthcare-related infections, particularly in immunocompromised patients.

Taxonomy and Classification

Serratia marcescens has a well-defined taxonomic position within bacterial nomenclature. Understanding its classification helps in distinguishing it from closely related species and in guiding diagnostic and therapeutic approaches.

• Kingdom, Phylum, Class, Order, Family, Genus, Species:
  • Kingdom: Bacteria
  • Phylum: Proteobacteria
  • Class: Gammaproteobacteria
  • Order: Enterobacterales
  • Family: Enterobacteriaceae
  • Genus: Serratia
  • Species: S. marcescens
• Historical Classification and Nomenclature: Initially described in the 19th century, S. marcescens was first noted for its red pigment, prodigiosin, which led to early misclassification as a harmless environmental bacterium.
• Related Species and Strains: The genus Serratia includes other species such as S. liquefaciens and S. rubidaea, which differ in virulence, environmental distribution, and pigment production.

Microbiological Characteristics

Morphology

Serratia marcescens is a rod-shaped, Gram-negative bacterium that exhibits motility due to peritrichous flagella. Its distinctive colony pigmentation and structural features aid in laboratory identification.

• Gram-negative Bacillus Structure: Straight rods, approximately 0.5–0.8 µm in width and 0.9–2.0 µm in length, with a cell wall typical of Gram-negative bacteria.
• Motility and Flagella: Motile via peritrichous flagella, which contribute to its ability to colonize various surfaces.
• Colony Characteristics: Forms circular, smooth colonies that may produce a characteristic red pigment called prodigiosin at room temperature; colonies are non-pigmented at 37°C.

Growth and Culture

S. marcescens is a facultatively anaerobic organism capable of growing in diverse environmental conditions. Its growth characteristics are essential for laboratory isolation and identification.

• Optimal Growth Conditions: Grows well at temperatures between 25–37°C, pH 6–8, and can tolerate both aerobic and anaerobic environments.
• Media Types and Colony Morphology: Readily grows on MacConkey agar producing pink colonies due to lactose fermentation variability; also grows on nutrient agar and EMB agar, showing distinctive colony morphology and pigment production under certain conditions.

Biochemical Properties

Biochemical tests help confirm identification and differentiate S. marcescens from other Enterobacteriaceae members.

• Enzyme Production: Produces DNase, lipase, gelatinase, and urease, which contribute to its pathogenic potential.
• Metabolic Characteristics: Facultative anaerobe with variable lactose fermentation; reduces nitrate to nitrite and utilizes various carbohydrates.
• Pigment Production (Prodigiosin): The red pigment is a secondary metabolite produced under specific environmental conditions, often used as a diagnostic feature.

Virulence Factors

Serratia marcescens possesses multiple virulence factors that enable colonization, tissue invasion, and evasion of host defenses. These factors contribute to its pathogenicity, particularly in immunocompromised patients.

• Adhesins and Fimbriae: Surface structures that facilitate attachment to epithelial cells, medical devices, and biofilm formation.
• Exotoxins and Enzymes: Hemolysins, proteases, and other secreted enzymes degrade host tissues and promote invasion.
• Biofilm Formation: Allows persistence on surfaces, including catheters and hospital equipment, enhancing resistance to antimicrobial agents.
• Capsule and Surface Polysaccharides: Protects bacteria from phagocytosis and complement-mediated killing, contributing to immune evasion.

Pathogenesis

The pathogenic mechanisms of Serratia marcescens involve complex interactions between bacterial virulence factors and host defenses. Infection often occurs in hospital settings or in patients with underlying comorbidities.

• Mechanisms of Infection: Entry through disrupted skin, mucosa, or indwelling devices; colonization is facilitated by adhesins and biofilms.
• Host Immune Evasion Strategies: Capsule production, biofilm formation, and secretion of enzymes allow the bacterium to evade neutrophil and complement-mediated killing.
• Common Sites of Colonization: Urinary tract, respiratory tract, bloodstream, wounds, and medical devices such as catheters and ventilators.

Clinical Manifestations

Healthcare-associated Infections

Serratia marcescens is a common cause of nosocomial infections, particularly in patients with compromised immunity or invasive devices.

• Urinary Tract Infections: Frequently associated with catheterization and can lead to cystitis or pyelonephritis.
• Respiratory Tract Infections: Occurs mainly in ventilated patients, causing pneumonia or tracheobronchitis.
• Bloodstream Infections and Sepsis: Bacteremia may arise from colonized devices or wounds, potentially leading to septic shock.
• Wound and Surgical Site Infections: Postoperative infections, particularly in burns or surgical wounds, can result in delayed healing and abscess formation.

Other Infections

In addition to common nosocomial infections, S. marcescens can cause less frequent but severe infections.

• Endocarditis: Infection of heart valves, especially in intravenous drug users or patients with prosthetic valves.
• Meningitis: Occurs mainly in neonates or immunocompromised individuals, often with high morbidity.
• Ocular Infections: Conjunctivitis, keratitis, or endophthalmitis, typically following trauma or surgical procedures.

Laboratory Diagnosis

Specimen Collection

Proper specimen collection is critical for accurate diagnosis and appropriate treatment.

• Blood for culture in suspected bacteremia or sepsis.
• Urine samples for urinary tract infections.
• Respiratory secretions for pneumonia or tracheobronchitis.
• Wound swabs or pus for surgical site or wound infections.

Culture and Identification

Standard microbiological methods are used to isolate and identify S. marcescens from clinical specimens.

• Growth on MacConkey agar producing pink to red colonies depending on pigment production.
• Biochemical tests including oxidase negative, catalase positive, and fermentation of specific sugars.
• API strips or automated systems for species-level identification.

Molecular Methods

Advanced techniques provide rapid and accurate identification of S. marcescens and its resistance genes.

• PCR targeting species-specific genes for confirmation.
• MALDI-TOF mass spectrometry for rapid proteomic identification.

Antimicrobial Susceptibility and Resistance

Serratia marcescens exhibits intrinsic resistance to several antibiotics and has the ability to acquire additional resistance mechanisms, complicating treatment.

• Intrinsic Resistance Patterns: Naturally resistant to ampicillin, first-generation cephalosporins, and colistin due to chromosomal beta-lactamases and outer membrane permeability.
• Acquired Resistance: Production of extended-spectrum beta-lactamases (ESBLs), carbapenemases, and aminoglycoside-modifying enzymes leading to multidrug resistance.
• Guidelines for Antibiotic Therapy: Treatment is guided by susceptibility testing; commonly used agents include third-generation cephalosporins, fluoroquinolones, and carbapenems in resistant infections.

Prevention and Control

Preventing S. marcescens infections is critical in healthcare settings due to its ability to colonize surfaces and medical devices.

• Hospital Infection Control Measures: Strict adherence to infection control protocols, including isolation procedures for infected or colonized patients.
• Hand Hygiene and Equipment Sterilization: Proper handwashing and disinfection of medical equipment reduce transmission.
• Environmental Reservoirs and Contamination Prevention: Regular cleaning of sinks, water sources, and surfaces; monitoring and decontamination of hospital fluids and solutions.

Prognosis and Clinical Outcomes

The prognosis of infections caused by Serratia marcescens varies depending on the site of infection, patient comorbidities, and timeliness of appropriate therapy.

• Factors Affecting Morbidity and Mortality: Immunocompromised status, presence of indwelling medical devices, and multidrug-resistant strains increase the risk of severe outcomes.
• Complications in Immunocompromised Patients: Bacteremia, septic shock, organ failure, and prolonged hospital stay are more common in vulnerable populations.
• Outcomes with Appropriate Therapy: Early identification and targeted antimicrobial therapy generally result in favorable recovery, especially in localized infections.

References

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