Pseudomonas aeruginosa is an adaptable bacterium known for causing infections, particularly in individuals with compromised immune systems. Its success as a pathogen is partly due to its ability to protect itself within the human body. This survival is often linked to whether it possesses a protective outer layer, a feature that allows it to persist and cause long-term illness.
The Bacterial Capsule Explained
Many bacteria surround themselves with a gelatinous outer layer called a capsule. This structure is composed of long chains of sugar molecules known as polysaccharides, which form a coating around the bacterial cell wall. The capsule functions as a protective barrier, helping the organism survive in harsh conditions like dehydration.
Within a host, the capsule serves as a defense mechanism. Its slippery nature makes it difficult for immune cells, such as phagocytes, to engulf and destroy the bacterium. This layer also helps the bacterium attach to various surfaces, like medical devices and tissues, which is often a first step in establishing an infection.
The composition and thickness of the capsule can vary widely among different bacterial species. These variations influence the capsule’s specific functions and its effectiveness in protecting the bacterium. The presence of a capsule contributes to a bacterium’s ability to cause disease.
The Mucoid Phenotype of Pseudomonas
Not all Pseudomonas aeruginosa bacteria have a capsule, but many strains can develop one under certain conditions. This ability is known as the mucoid phenotype, which describes the visible, mucus-like colonies these bacteria form in a lab. This change to an encapsulated state is an adaptive strategy for the bacterium.
The capsule of P. aeruginosa is made of an exopolysaccharide called alginate. This substance gives the bacterial colonies their slimy and moist appearance. The production of alginate is not constant and is a response to environmental triggers.
The switch to a mucoid state is often triggered by stresses within a host. This transition is a genetic event, frequently caused by a mutation in the mucA gene. This mutation leads to the overproduction of alginate, wrapping the bacterium in a protective shield and allowing it to thrive.
Role in Disease and Biofilm Formation
The alginate capsule plays a direct role in the bacterium’s ability to cause disease. It acts as a physical barrier, shielding P. aeruginosa from the host’s immune system. The capsule makes it difficult for immune cells to clear the bacteria and can also block certain antibiotics from reaching their target. This protection is a major reason why mucoid strains are associated with chronic infections that are notoriously difficult to treat.
Alginate is also a building block for biofilms. A biofilm is a structured community of bacteria encased in a self-produced matrix of slimy extracellular substances. In P. aeruginosa, this matrix is predominantly composed of alginate. Within a biofilm, bacteria are more resistant to antibiotics and immune defenses than they are as individual, free-swimming organisms.
The clinical impact of this is most clearly seen in chronic lung infections, particularly in patients with cystic fibrosis (CF). In the CF lung environment, P. aeruginosa frequently converts to the mucoid, alginate-overproducing phenotype. This conversion allows the bacteria to form dense biofilms in the airways, leading to a cycle of chronic infection and inflammation that causes progressive lung damage. The presence of these mucoid, biofilm-forming strains is linked to a decline in lung function and a poorer prognosis for the patient.