Serratia marcescens: Introduction, Morphology, Pathogenicity, Lab

Introduction

Table of Contents

Classification: Belongs to the family Enterobacteriaceae.
Habitat: Widely found in soil, water, plants, and damp environments (like bathrooms).
Nature: Opportunistic pathogen that thrives in moist hospital environments.
Significance: Frequently causes nosocomial (hospital-acquired) outbreaks.

Gram Stain: Gram-negative.
Shape: Bacillus (rod-shaped).
Motility: Motile via peritrichous flagella.
Pigmentation: Some strains produce a dark red/pink pigment called prodigiosin.
Temperature: Pigment production is optimal at room temperature (25°C) and decreases at 37°C.
Spores: Non-spore-forming.

Transmission: Spread via hand-to-hand contact by healthcare workers or contaminated medical devices.
Infections: Cause urinary tract infections (UTIs), respiratory tract infections, wound infections, and sepsis.
Vulnerable Groups: Affects intensive care unit (ICU) patients, newborns, and the immunocompromised.
Biofilms: Forms strong biofilms on indwelling medical devices like catheters and intravenous lines.
Virulence Factors: Secretes extracellular enzymes including DNase, lipase, and gelatinase.

Microscopy: Gram-negative rods with no specific structural arrangement.
Culture: Grows well on Nutrient Agar and MacConkey Agar.
Colony Appearance: Non-lactose-fermenting colonies on MacConkey; may show red/pink coloration if prodigiosin is produced.

Biochemical Profile: Catalase positive, Oxidase negative, Citrate positive, and Indole negative.
Key Differentiation: Rapidly hydrolyzes DNA (DNase positive), gelatin (gelatinase positive), and lipids (lipase positive).

Resistance: Intrinsic resistance to many common antibiotics, including ampicillin, first-generation cephalosporins, and macrolides.
AmpC Beta-Lactamase: Carries inducible chromosomal AmpC genes that can lead to resistance during treatment.
First Line: Fluoroquinolones (like ciprofloxacin), carbapenems (like meropenem), or fourth-generation cephalosporins (cefepime).
Susceptibility: Strict reliance on culture and sensitivity testing due to multi-drug resistance (MDR) risks.

Hand Hygiene: Strict adherence to handwashing and sanitizing protocols in healthcare settings.
Sterilization: Thorough disinfection of medical equipment, respirators, and endoscopes.
Catheter Care: Timely removal and aseptic management of indwelling urinary and central venous catheters.
Environmental Control: Keeping sinks, showers, and liquid soap dispensers clean, as bacteria thrive in damp areas.

Red Pigment: Prodigiosin acts as a visual marker for environmental colonization, often resembling “pink mold” in bathrooms.
Historical Fact: Once considered non-pathogenic, it was used in the 1950s biological warfare experiment “Operation Haze” to track aerosol dispersion.
Serratia-Induced UTI: Often associated with long-term catheterization.
Enzyme Triad: Uniquely positive for DNase, lipase, and gelatinase, differentiating it from most other Enterobacteriaceae.