Serratia marcescens

Table of Contents

Introduction

Serratia marcescens is a Gram-negative, non-motile, rod-shaped bacterium that belongs to the family Enterobacteriaceae. It is an opportunistic pathogen, known for causing infections in both immunocompromised and healthy individuals. While S. marcescens can be found in various environmental habitats, including soil, water, and food, it is also a common cause of nosocomial (hospital-acquired) infections, particularly in patients with compromised immune systems (Zong et al., 2020). It is notable for its ability to produce a distinctive red pigment, prodigiosin, which gives colonies a characteristic red appearance (Pratt et al., 2018).

Taxonomy and Classification

Domain: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Enterobacteriaceae
Genus: Serratia
Species: marcescens

Serratia marcescens was first described by the Italian biologist Bartolomeo Bizio in 1819, and its ability to produce prodigiosin has made it a subject of interest in microbiological studies (Wright et al., 2019).

Morphological Characteristics

Shape: S. marcescens is a rod-shaped bacterium (0.5–0.8 µm in diameter and 1.0–2.0 µm in length). It appears as a straight, non-spore-forming, and non-motile cell (Crispino et al., 2019; Zong et al., 2020).

Gram Staining: It is a Gram-negative organism, which appears pink/red under the microscope following Gram staining.

Pigmentation: One of the distinctive features of S. marcescens is its ability to produce a red pigment, known as prodigiosin, under favorable growth conditions. This pigment contributes to its colony's reddish appearance (Pratt et al., 2018).

Cultural Characteristics

Serratia marcescens exhibits distinct cultural characteristics that are useful in laboratory identification.

Growth Media:

Serratia marcescens can grow on standard microbiological media like MacConkey agar, nutrient agar, and tryptic soy agar (TSA).

MacConkey agar: It is a lactose non-fermenter, so it forms pale, colorless colonies on MacConkey agar.

Blood Agar Plate (BAP): S. marcescens typically shows beta-hemolysis on blood agar, where it breaks down red blood cells, producing a clear zone around the colonies (Zong et al., 2020).

Nutrient Agar: Colonies on nutrient agar exhibit the distinctive red pigmentation, though the color may vary depending on environmental conditions (Pratt et al., 2018).

Temperature Range:

Serratia marcescens grows well at 37°C, the body temperature of humans, which makes it particularly adept at causing hospital-acquired infections.

It can also grow at temperatures as low as 25°C, and it produces the red pigment (prodigiosin) more prominently at lower temperatures, such as room temperature (25–30°C) (Crispino et al., 2019; Zong et al., 2020).

Pigment Production:

Prodigiosin, a red pigment, is produced under ambient temperatures but can be diminished or lost when grown at higher temperatures (Pratt et al., 2018). This pigment is a key identifying feature in some strains of S. marcescens.

Oxygen Requirements:

Serratia marcescens is a facultative anaerobe, capable of growing in both the presence or absence of oxygen. However, it generally exhibits better growth in aerobic conditions (Zong et al., 2020).

Biochemical Properties:

Oxidase Test: Serratia marcescens is oxidase-negative, which helps distinguish it from other Gram-negative rods such as Pseudomonas aeruginosa (Pratt et al., 2018).

Catalase Test: It is catalase-positive, meaning it produces the enzyme catalase, which breaks down hydrogen peroxide (H_2O_2) into water and oxygen.

Indole Test: Serratia marcescens is indole-positive, which means it can hydrolyze the amino acid tryptophan to produce indole (Crispino et al., 2019).

Nitrate Reduction: Serratia marcescens is typically nitrate-positive, reducing nitrate to nitrite or nitrogen gas.

Urease Test: It is urease-negative, which can help differentiate it from other Enterobacteriaceae members.

Carbohydrate Fermentation: It is generally non-fermentative for carbohydrates such as glucose, lactose, and sucrose, which is characteristic of Serratia species (Wright et al., 2019).

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Virulence Factors

Serratia marcescens possesses several virulence factors that contribute to its pathogenicity, especially in the context of hospital-acquired infections.

Biofilm Formation:

Serratia marcescens has the ability to form biofilms on medical devices such as catheters and prosthetic devices, contributing to chronic infections and resistance to antibiotic treatment (Zong et al., 2020). Biofilm formation protects the bacteria from the immune system and antibiotics.

Hemolysin Production:

It produces hemolysins, enzymes that break down red blood cells, contributing to tissue invasion and the ability to establish infections in the host (Crispino et al., 2019).

Proteases and Lipases:

Serratia marcescens produces extracellular enzymes like proteases and lipases, which degrade host tissues and promote invasion (Zong et al., 2020). These enzymes break down proteins and lipids, facilitating bacterial spread and infection.

Antibiotic Resistance:

It is naturally resistant to a number of antibiotics and has acquired resistance to multiple other agents, including beta-lactams, aminoglycosides, and fluoroquinolones. Serratia species are known for producing extended-spectrum beta-lactamases (ESBLs) and other beta-lactamases, contributing to their antibiotic resistance profile (Wright et al., 2019; Pratt et al., 2018).

Pathogenesis

Serratia marcescens is an opportunistic pathogen that can cause a wide range of infections, particularly in immunocompromised patients or those with underlying medical conditions.

Common Infections:

Urinary Tract Infections (UTIs): Often associated with the use of urinary catheters.

Respiratory Infections: Includes pneumonia, particularly in hospitalized patients on ventilators (VAP).

Wound Infections: Can occur in surgical wounds or after trauma.

Bloodstream Infections (BSIs): Occur in patients with compromised immune defenses, particularly through the use of intravenous lines or catheters.

Host Factors:

Serratia marcescens is particularly associated with hospital-acquired infections, as it can thrive on surfaces, medical devices, and in hospital water systems. It can survive in various hospital environments and on contaminated equipment for long periods, contributing to its ability to cause outbreaks (Wright et al., 2019).

Diagnosis

Microbiological Culture:

The gold standard for diagnosis of Serratia marcescens infections is culture. It is grown on MacConkey agar (non-lactose fermenter), nutrient agar (for pigment production), and blood agar plates (for hemolytic activity).

Biochemical Testing:

Oxidase test: Serratia marcescens is oxidase-negative.

Indole test: Indole-positive, a critical differential test.

Nitrate reduction: Nitrate-positive.

Molecular Methods:

Polymerase chain reaction (PCR) and whole-genome sequencing (WGS) are increasingly used for rapid and accurate identification of Serratia marcescens strains, including their resistance mechanisms (Zong et al., 2020).

Treatment

Antibiotic Therapy:

Treatment of Serratia marcescens infections is complicated by its natural antibiotic resistance. However, several options exist for managing these infections:

First-line treatment: Typically, carbapenems (e.g., imipenem, meropenem) are used for serious infections, although resistance to these antibiotics is becoming more common.

Alternative therapies: Colistin, tigecycline, and aminoglycosides (such as gentamicin) may be used in multi-drug resistant strains.

Combination therapy: For severe infections, combination therapies, such as carbapenems plus aminoglycosides or colistin plus tigecycline, may be required (Zong et al., 2020).

Antibiotic Resistance:

Resistance profiles should be determined by antibiotic susceptibility testing before treatment. Resistance to beta-lactams, fluoroquinolones, and aminoglycosides is frequently reported.

Prevention and Control

Preventing Serratia marcescens infections in healthcare settings involves stringent infection control measures:

Hand hygiene and the use of personal protective equipment (PPE).

Disinfection of medical equipment and environmental surfaces, especially in areas such as ICUs and ventilated patient rooms.

Antibiotic stewardship: Limiting unnecessary antibiotic use to reduce the development of resistance.

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