- Capsular polysaccharides (CPSs) are complex carbohydrate polymers that form a protective, often slimy, outer layer—known as the capsule—surrounding the cell walls of many bacteria.
- They are typically composed of repeating units of monosaccharides, uronic acids, and occasionally non-carbohydrate substituents such as acetyl or pyruvyl groups. The structures of CPSs vary widely between bacterial species and even among strains, resulting in significant antigenic diversity. This variability underlies the basis of many serotyping schemes, such as the classification of Streptococcus pneumoniae, Neisseria meningitidis, and Escherichia coli strains.
- The primary function of capsular polysaccharides is to provide protection against host defenses. Capsules help bacteria evade phagocytosis by immune cells, resist complement-mediated lysis, and shield underlying surface antigens from detection. In addition, CPSs can act as physical barriers against desiccation, antimicrobial peptides, and toxic environmental factors. By reducing immune recognition and clearance, capsulated bacteria often exhibit enhanced pathogenicity compared to non-capsulated counterparts.
- Capsular polysaccharides also play critical roles in host–pathogen interactions. Many CPSs are poorly immunogenic on their own, particularly in infants and young children, because they are T-cell-independent antigens. However, they can elicit strong immune responses when conjugated to protein carriers, a strategy used in conjugate vaccines. Vaccines based on capsular polysaccharides have been among the most successful interventions in infectious disease prevention, dramatically reducing cases of bacterial meningitis, pneumonia, and sepsis caused by H. influenzae type b (Hib), N. meningitidis, and S. pneumoniae.
- In addition to immune evasion, CPSs contribute to bacterial survival and colonization. They facilitate biofilm formation by promoting adhesion to surfaces and protecting microbial communities within the biofilm matrix. This enhances persistence in both environmental niches and host tissues, contributing to chronic infections and antibiotic tolerance.
- From a biomedical perspective, capsular polysaccharides are both a challenge and an opportunity. On one hand, they are major virulence factors, making infections harder to treat and control. On the other hand, their surface exposure and strain-specific variability make them excellent vaccine targets. Research into capsular biosynthesis pathways has also uncovered potential targets for novel antimicrobial strategies. Disrupting capsule production can attenuate bacterial virulence, rendering pathogens more susceptible to immune clearance and antibiotics.
- Capsular polysaccharides also have biotechnological applications. Certain CPSs, such as those produced by Klebsiella or Enterobacter species, are explored for use as biothickeners, emulsifiers, and hydrocolloids in industrial and pharmaceutical formulations. Their stability, water-retention capacity, and ability to form protective layers provide functional benefits in a variety of applied contexts.
