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Pneumonia I: Introduction

Pneumonia I: Introduction

Pneumonia is an infection of the lower respiratory tract that leads to inflammation of the lung parenchyma, often resulting in the accumulation of inflammatory exudate in the alveoli and airways. Unlike the watery, low-protein fluid exudate in pulmonary edema, the exudate in this case is a thick fluid rich in immune cells, proteins, and debris produced during infection and inflammation.This impairs gas exchange and can lead to consolidation of lung tissue. The infection may be caused by a...

Colonisation of Pathogens

Colonisation of Pathogens

Pathogen colonization of host tissues is a critical step in the development of infectious diseases. Various pathogenic microorganisms, including bacteria, fungi, viruses, and protozoa, have evolved complex strategies to attach to, invade, and persist within host environments. These mechanisms enable pathogens to establish infections, evade immune responses, and resist antimicrobial treatments.Attachment to Host CellsIn bacteria, colonization typically begins with adherence to host epithelial...

Atypical Pneumonia

Atypical Pneumonia

Atypical pneumonia, often caused by Mycoplasma pneumoniae, is a form of pulmonary infection that differs from the classical presentation of bacterial pneumonia in both its cause and clinical symptoms. Mycoplasma pneumoniae is a pleomorphic bacterium notable for its lack of a rigid cell wall. This structural characteristic imparts resistance to beta-lactam antibiotics and significantly influences the bacterium’s behavior within the human host.Other pathogens responsible for the disease include...

Microbiota of the Respiratory Tract

Microbiota of the Respiratory Tract

The human respiratory tract, comprising the upper and lower segments, serves as a critical interface with the external environment. The upper respiratory tract (URT)—including the nostrils, sinuses, pharynx, and oropharynx—is heavily colonized by microbes, while the lower respiratory tract (LRT), composed of the larynx, trachea, bronchi, and lungs, was long thought to be sterile. However, recent molecular studies have revealed that the lungs are not devoid of microbes but act more like...

Pneumonia II: Pathophysiology

Pneumonia II: Pathophysiology

The pathophysiology of pneumonia involves the following steps:

Surface Membrane Barriers

Surface Membrane Barriers

The skin and mucous membranes serve as the primary line of defense against pathogens by providing both physical and chemical protection. These barriers are essential in preventing the entry and establishment of microbes, thereby maintaining the integrity of the host.
The outer layer of the skin, the epidermis, is a robust barrier comprising layers of closely packed keratinized cells. This dense arrangement prevents microbes from penetrating the body. The periodic shedding of epidermal cells...

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Updated: Jul 19, 2026

Separating Bacteria by Capsule Amount Using a Discontinuous Density Gradient

Separating Bacteria by Capsule Amount Using a Discontinuous Density Gradient

Published on: January 7, 2019

Capsule enhances pneumococcal colonization by limiting mucus-mediated clearance.

Aaron L Nelson¹, Aoife M Roche, Jane M Gould

¹Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6076, USA.

Infection and Immunity

|November 8, 2006

Summary

The polysaccharide capsule of Streptococcus pneumoniae aids in nasal colonization by preventing bacterial entrapment in mucus. This mechanism is separate from immune evasion, highlighting a new role for the capsule in mucosal pathogens.

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Published on: February 23, 2014

Area of Science:

Microbiology
Immunology
Pathogenesis

Background:

Mucosal pathogens like Streptococcus pneumoniae utilize polysaccharide capsules for pathogenicity.
The capsule's role in immune evasion (opsonization, phagocytosis) is known for invasive infections.
The capsule's function during mucosal colonization, the commensal state, is not well understood.

Purpose of the Study:

To investigate the role of the polysaccharide capsule in Streptococcus pneumoniae nasal colonization.
To determine if capsule influences bacterial interaction with mucus and host tissues.

Main Methods:

Utilized a mouse model to compare encapsulated and unencapsulated Streptococcus pneumoniae strains.
Assessed bacterial colonization density and duration.
Performed in vitro binding assays with human airway mucus.
Investigated the effect of capsule charge and neuraminidase treatment on mucus binding.

Main Results:

Unencapsulated mutants showed reduced nasal colonization density and duration compared to encapsulated strains.
This colonization deficit was not due to increased opsonophagocytic clearance.
Unencapsulated bacteria aggregated in mucus, hindering epithelial surface transit.
Encapsulation inhibited in vitro binding to mucus, an effect dependent on capsule's negative charge (sialic acid).

Conclusions:

The polysaccharide capsule of Streptococcus pneumoniae facilitates mucosal colonization by preventing entrapment within lumenal mucus.
This anti-adhesive property is distinct from the capsule's known role in evading opsonophagocytosis.
Capsule-mediated electrostatic repulsion likely explains the prevalence of anionic polysaccharides in mucosal pathogens.