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Transmission and Virulance of Streptococcus Pneumoniae

Info: 959 words (4 pages) Essay
Published: 17th Mar 2021 in Medical

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A leading cause of death in the developing world, Streptococcus pneumoniae has been classed as the single most prominent Gram-positive commensal to cause community acquired pneumonia over a vast demographic of young children, the elderly and immunocompromised individuals.

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S. pneumoniae is commonly found in the upper respiratory tract, where one of 91 strains can colonize to cause a variety of mild and severe respiratory tract infections (Henriques-Normark and Tuomanen, 2013). There is a surge of carrier-state related cases that involve children in community settings such as day cares where 60% of healthy infants are responsible for harbouring the pneumococcus pathogen in their upper respiratory tract asymptomatically (Henriques-Normark and Tuomanen, 2013). Annually, this causes 1.2 million infant deaths with an even higher mortality rate predicted in AIDS and immunocompromised patients (van der Poll and Opal, 2009).

Transmission of S. pneumoniae occurs when air droplets are released by an infected individual, in particular, members of the same household whenever coughing, sneezing or talking in close proximity. Although outbreaks of S. pneumoniae are mainly witnessed on a local scale such as in jails and military bases, endemic spread could also potentially occur as shown on a larger scale in Vancouver, Canada (van der Poll and Opal, 2009). After successful transmission, the nasopharyngeal area is colonized by S. pneumoniae which targets entry into the lower respiratory tract by overcoming mucosal defences, eventually migrating into the alveolus (Henriques-Normark and Tuomanen, 2013).

S. pneumoniae has a large variety of virulence factors that help with bacterial adherence, disruption of epithelium and dissemination into the bloodstream where the bacteria can infiltrate the rest of the body (Henriques-Normark and Tuomanen, 2013). The more invasive serotypes have been identified as 1 and 7F whereas 6B, 9V, 14, 19F and 23F are found to be the most common amongst young children (Henriques-Normark and Tuomanen, 2013).

Streptococcus pneumoniae has a plethora of virulence factors that are targeted by various therapies but without a doubt the only solid target is the polysaccharide capsule for which present vaccines are developed against (van der Poll and Opal, 2009). The polysaccharide capsule is thought to prevent mucosal clearance, possess antiphagocytic elements and the ability to transport the bacterium to the epithelial surface. Additionally, it also sterically inhibits the bond between phagocytic CR3 receptors and iC3b due to its high negative charge as well as hindering binding between the Fcy receptor and its complimentary counterpart on the IgG attached to pneumococci (van der Poll and Opal, 2009).

Restriction of autolysis and limited antibiotic exposure is also instigated by the capsule. The development of the most severe pneumococcal cases depends on what type of capsule the particular serotype has; more than 90 serotypes of S. pneumoniae have a polysaccharide capsule unique in virulence (Henriques-Normark and Tuomanen, 2013).

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One of the first pneumococcal serotype vaccines was a 23-valent polysaccharide vaccine administered to the elderly and immunocompromised individuals (Henriques-Normark and Tuomanen, 2013). The 23 valent vaccine is a polysaccharide vaccine containing 23 of the most common capsular serotypes that are prevalent across the globe and induces a T-cell independent B cell immune response (van der Poll and Opal, 2009). One major limitation of the 23 valent vaccine is its poor immunogenic performance in the individuals that need it the most; the elderly, immunologically compromised and infants younger than 2 years (van der Poll and Opal, 2009). New vaccines were invented to overcome this problem and widen the scope of the population that the vaccine covers. These vaccines used a protein conjugated with a polysaccharide to deliver a more T-cell dependant immune response (Henriques-Normark and Tuomanen, 2013). Due to external factors such as high vaccine development cost, only serotypes at the time most prominent in the USA were used (serotypes 7, 10 and 13) which meant that it was not suitable for use in other geographical locations. The 7 valent conjugate vaccine was introduced and made to target children younger than 2 years, and children younger than 5 years that were at risk. The serotypes covered by the vaccine include 4, 6B, 9V 14, 18C, 19F and 23F (van der Poll and Opal, 2009). The success of the vaccine is attributed to the T-cell dependant immune response which overcomes the innate lack of immunogenicity towards the polysaccharide vaccines in the first two years of life (van der Poll and Opal, 2009). A fortunate bonus to the conjugate vaccine was the widespread herd immunity it offered to non-vaccinated siblings and even adults in some cases. It all boils down to the fact that eliminating the carrier state in children reduces the transmission of S. pneumoniae to the rest of the population. The go to treatment of pneumococcal infections are antibiotics, however due to the rise of pneumococcal resistance to a range of antibiotics, this is no longer seen as an effective first line treatment. Instead, practitioners fall back onto vaccine, which despite also demonstrates

References

  • Henriques-Normark, B. and Tuomanen, E.I., 2013. The pneumococcus: epidemiology, microbiology, and pathogenesis. Cold Spring Harbor perspectives in medicine, 3(7), p.a010215.
  • van der Poll, T. and Opal, S.M., 2009. Pathogenesis, treatment, and prevention of pneumococcal pneumonia. The Lancet, 374(9700), pp.1543-1556.

 

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