Serratia marcescens: Properties, Pathogenesis, Diagnosis

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.

Properties

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.

Pathogenesis

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

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

Virulence

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.

Infection

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.

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 test	Result
Urease test	varies according to bacterial strains.
Voges-Proskauer test	Positive
ONPG test	Positive
Lactose fermentation	varies according to bacterial strains.
Mannitol fermentation	Positive
Glucose fermentation	Positive
Sucrose fermentation	Positive
Oxidase test	Negative
Citrate test	Positive
Motility	Positive
Indole test	Negative
Lysine decarboxylase	Positive
TSI/KIA medium*	Acid or Alkali/Acid, gas (-), H2S (-) *

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.

References and further reading

1. Hirakata Y. (2002). Nihon rinsho. Japanese journal of clinical medicine, 60(11), 2156–2160.
2. Mahlen S. D. (2011). Serratia infections: from military experiments to current practice. Clinical microbiology reviews, 24(4), 755–791. https://doi.org/10.1128/CMR.00017-11
3. Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
4. Gupta, V., Sharma, S., Pal, K., Goyal, P., Agarwal, D., & Chander, J. (2021). Serratia, No Longer an Uncommon Opportunistic Pathogen – Case Series & Review of Literature. Infectious disorders drug targets, 21(7), e300821191666. https://doi.org/10.2174/1871526521666210222125215