Infection characteristics among Serratia marcescens capsule lineages

Area of Science:

• Microbiology and Immunology
• Infectious Diseases
• Bacterial Pathogenesis

Background:

• Serratia marcescens is a significant healthcare-associated pathogen causing severe infections like bacteremia and pneumonia.
• The capsule polysaccharide is crucial for S. marcescens survival during bacteremia, with diverse capsule locus (KL) types identified.
• KL1 and KL2 strains, producing sialylated polysaccharides, are prevalent in clinical isolates.

Purpose of the Study:

• To compare the infection characteristics and contributions to infection fitness of five different Serratia marcescens capsule types.
• To investigate the role of capsule polysaccharides in S. marcescens colonization and dissemination in animal and cellular models.
• To determine the impact of capsule type on resistance to host immune defenses, specifically macrophage phagocytosis.

Main Methods:

• Utilized murine models of primary bacteremia and bacteremic pneumonia to assess capsule-dependent colonization and dissemination.
• Employed tail vein inoculation in mice to study the impact of different KL types on spleen, liver, and kidney colonization.
• Examined the ability of various KL clade capsules to inhibit internalization by bone marrow-derived macrophages in vitro.

Main Results:

• Clinical isolates of multiple KL types demonstrated capsule-dependent colonization of key organs (spleen, liver, kidney) in a bacteremia model.
• All tested clinical S. marcescens strains required capsule for lung infection and subsequent bloodstream dissemination to secondary organs.
• Only sialylated KL1 and KL2 capsules significantly inhibited macrophage internalization, even when produced in a heterologous background.

Conclusions:

• Capsule polysaccharides are essential for Serratia marcescens virulence, mediating organ colonization and systemic spread.
• Lineage-specific resistance to macrophage phagocytosis, particularly by sialylated KL1 and KL2 capsules, enhances bacterial survival.
• Targeting critical genetic lineages with specific capsule types may offer effective strategies against life-threatening S. marcescens infections.