Serratia marcescens: Properties, Pathogenesis, Diagnosis

Last updated on June 21st, 2021

The genus Serratia currently contains >15 recognized species, seven of which have been isolated from human clinical specimens. S. marcescens is the most important member of the genus. Recognized as an important opportunistic pathogen, S. marcescens is the most frequently encountered species in clinical specimens. S. liquefaciens and S. odorifera are sometimes found in clinical materials.

Serratia marcescens in MacConkey Agar
Dark red colonies of Serratia marcescens in MacConkey Agar


  1. Member of the Enterobacteriaceae family;
  2. They ferment lactose very slowly (late lactose-fermenter).
  3. Small motile rod having peritrichous flagella
  4. Some strains produce a red diffusible pigment (prodigiosin) in culture and also in various foods.
  5. An important cause of nosocomial infection

Earlier, S marcescens was considered to be non-pathogenic saprophytes and was often used as a biological marker for studying the transmission of microorganisms.


Source: S. marcescens is found in tap water/shower, air conditioning units and mostly in medical equipment such as nebulisers and bronchoscopes.

Transmission: The predominant mode of spread of S. marcescens is thought to be hand-to-hand transmission by hospital personnel.


Biofilm formation by S. marcescens is responsible for its pathogenicity as well as drug resistance. Other probable virulence factors of S. marcescens include;

  1. Adherence and hydrophobicity mediated by mannose-resistant (MR) pili and mannose-sensitive (MS) pili.
  2. Endotoxic properties of lipopolysaccharide (LPS)
  3. These different enzymes produced by S. marcescens may act as virulence factors,
    1. chitinase,
    2. lipase,
    3. chloroperoxidase and
    4. an extracellular protein, HasA.


Serratia species are found mostly in soil and water. An opportunistic pathogen, Serratia causes urinary tract infections, wound infections and occasionally pneumonia. Septicemia, endotoxic shock and endocarditis due to S. marcescens have also been reported. Serratia also causes endocarditis in users of injection drugs. Infections due to S. marcescens are on increase in recent years, particularly among hospitalized patients, causing nosocomial surgical wound infections.

Red colonies of Serratia marcescens in Tryptic Soy Agar
Serratia marcescens grew as bright, glossy red colonies on the TSA plate (Image source)

Laboratory Diagnosis

Gram stain: Serratia species are Gram-negative rods.

Culture: S. marcescens grows well on blood agar and MacConkey agar. It is non-lactose fermenting. Some strains produce a red pigment in nutrient agar, tryptic soy agar, MacConkey agar at room temperature.

Identification: They are distinguished by biochemical tests from other coliforms.

Name of the testResult
Urease testvaries according to bacterial strains.
Voges-Proskauer testPositive
ONPG testPositive
Lactose fermentationvaries according to bacterial strains.
Mannitol fermentationPositive
Glucose fermentationPositive
Sucrose fermentationPositive
Oxidase testNegative
Citrate testPositive
Indole testNegative
Lysine decarboxylasePositive
TSI/KIA medium*Acid or Alkali/Acid, gas (-), H2S (-) *
*Some Serratia strains can ferment lactose slowly and give an acid reaction on the slant.

Antimicrobial Sensitivity

Serratia species are often resistant to multiple antibiotics. Quorum sensing (QS) plays important role in antibiotic resistance and biofilm production among Serratia marcescens. Serratia species are usually resistant to cephalosporins, polymyxins, and occasionally also to aminoglycosides.

About Acharya Tankeshwar 473 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.

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