Streptococcus Pneumoniae Virulence Factors

By Acharya Tankeshwar •  Updated: 07/04/21 •  3 min read

Streptococcus pneumoniae (pneumococcus) is one of the most common causes of pneumonia, septicemia, and meningitis in children and adults both in developing and developed countries. S. pneumoniae is the most common causative agent in community-acquired pneumonia (CAP). It produces several virulence factors including polysaccharide capsules, toxin pneumolysin, and enzyme IgA protease that are involved in the disease process.

Autolysin (peptidoglycan hydrolase)

Autolysin is a cell wall degrading enzyme that has the ability to enzymatically disrupt and disintegrate S. pneumoniae. The main autolysin in the pneumococcus is N-acetyl-muramoyl-1-alanine amidase, commonly known as Lyt A. It is thought that when Lyt A is activated the pneumococcal virulence factors such as pneumolysins are released. Autolysin is responsible to kill the entire culture of S. pneumoniae during its stationary phase. Autolysis usually begins within 18-24 hours of culture with colonies collapsing in the centers.

Schematic diagram of the virulence factors of Streptococcus pneumoniae

IgA1 protease

Immunoglobulin A1 (IgA1) is the predominant immunoglobulin isotype involved in the protection of mucosal membranes of the upper respiratory tract in humans. IgA1 proteases of S. pneumoniae cleaves, thus facilitating colonization and invasion in those surfaces. It is an extracellular bacterial enzyme that specifically cleaves human IgA1 in the hinge region at one of the several postproline peptide bonds (absent in the IgA2 molecule).

A number of other human pathogens such as Haemophilus influenzae, Neisseria meningitidis, Neisseria gonorrhoeae and Ureaplasma urealyticum which infect and invade mucosal membranes also have IgA1 cleaving activity.

Pneumolysin (PLY)

Pneumolysin is a pore-forming toxin of 53 kDa composed of 471 amino acids produced by virtually all clinical isolates of the pneumococcus. At high concentration (above 50 hemolytic units), it lyses all cells having cholesterol (receptor for this toxin) in its surface e.g. it damages respiratory epithelium.

Pneumolysin also may cause a range of effects, including induction of apoptosis, activation of host complement and induce proinflammatory reactions in immune cells.

Polysaccharide capsule

Polysaccharide capsule completely encloses the cell of Streptococcus pneumoniae and serves as one of the major virulence factor (probably the most important virulence factor). The importance of the S. pneumoniae capsule for virulence is demonstrated by these facts:

The capsule may inhibit complement activity (capsule impaired bacterial opsonization with C3b/iC3b by both the alternative and classical complement pathways and also inhibited conversion of C3b bound to the bacterial surface to iC3b) and phagocytosis.  Capsule also prevents mechanical removal by mucus and reduce exposure to antibiotics

For S. pneumoniae strains, there are >93 antigenically distinct capsular serotypes and antibody to the polysaccharide capsule provides type-specific immunity.

Teichoic acids

Pneumococcal wall teichoic acid is involved in pneumococcal infection of sepsis and adherence to epithelial and endothelial cells. Pneumococcal wall teichoic acid enhances pneumococcal colonization and dissemination.

Other virulence factors of S.pneumoniae includes pneumococcal surface protein A (PspA); protection against complement system, neuraminidase; (may enhances colonization due to its action on glycans) and Pneumococcal surface antigen A etc.

References and Further Reading

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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6 responses to “Lancefield Classification of Streptococci”

  1. Anonymous says:

    The last table states “medically important fungi”. Streptococci are bacteria not fungi.

    • Tankeshwar Acharya says:

      Dear, Thank you so much for your remark. This is a clear guidance for me to focus on proof reading activity. That’s the blunder, I am amazed how i did such a typo. It has been corrected. Thank you once again.

  2. Stalin.P says:

    Sir, is there any reason
    why there is no group I and J in Lancefield classification ?

  3. Augusta says:

    Sir ,what is meaning of PYR

    • Tankeshwar Acharya says:

      Pyrrolidonyl Arylamidase (PYR) test is a rapid test which is used for the presumptive identification of group A beta-hemolytic Streptococci and Enterococci.

  4. great article

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