Streptococcus Pneumoniae Pathogensis Humoral Immunity

Streptococcus pneumoniae is a species of Streptococcus that is a major human pathogen.

The organism was originally named Diplococcus pneumonia in 1926 because of its characteristic appearance in Gram-stained sputum. It was renamed Streptococcus pneumoniae in 1974 because of its growth in chains in liquid media. Because of its role as the etiologic agent of pneumonia, it has long been known informally as the pneumococcus.

Despite the name, the organism causes many types of infection other than pneumonitis, including acute sinusitis, otitis media, meningitisMeningitis is inflammation of the membranes ( meninges) covering the brain and the spinal cord. Although the most common causes are infection (either bacterial or viral), chemical agents and even tumor cells may cause meningitis. Encephalitis and brain ab, osteomyelitisOsteomyelitis is an infection of bone, usually caused by pyogenic bacteria or mycobacteria. It can be usefully subclassifed on the basis of the causative organism, the route, duration and anatomic location of the infection. Generally microorganisms may be, septic arthritisSeptic arthritis is the proliferation of bacteria in joints and resultant inflammation. Bacteria are either carried by the bloodstream from an infectious focus elsewhere or are introduced by a skin lesion that penetrates the joint. Septic arthritis should, endocarditisEndocarditis is an inflammation of the inner layer of the heart, the endocardium. The most common structures involved are the heart valves. Endocarditis can be classified as either infective or non-infective depending on whether a foreign organism is caus, peritonitisPeritonitis is infection (or inflammation) of the peritoneum, which is a two-layered membrane covering both the surfaces of the organs that lie in the abdominal cavity and the inner surface of the abdominal cavity itself. It is frequently life-threatening, pericarditisIn medicine ( cardiology) pericarditis is inflammation of the pericardium, the "bag" of tissue enveloping the heart. It is a lethal condition, but can be treated dependant on the cause with pericardiocentesis, antibiotics, steroids and surgery., cellulitisCellulitis is an inflammation of the connective tissue underlying the skin that can be caused by a bacterial infection. Cellulitis can be caused by normal skin flora or by exogenous bacteria, and often occurs where the skin has previously been broken: cra, and brain abscessBrain abscess (or cerebral abscess) is an abscess caused by inflammation and collection of infected material coming from local (ear infection, infection of paranasal sinuses, infection of the mastoid air cells of the temporal bone, epidural abscess) or re.

S. pneumonia is the most common cause of bacterial meningitis in adults, and is one of the top two isolates found in otitis media. Pneumococcal pneumonia is more common in the very young and the very old.

1 Pathogensis

S. pneumoniae is normally found in the nasopharynx of 5-10% of healthy adults, and 20-40% of healthy children. It attaches to nasopharyngeal cells through interaction of bacterial surface adhesin s and epithelial cells . This normal colonization can become infection if the organisms are carried into areas, such as the Eustachian tube or nasal sinus es, where their clearance is hindered (as it might be if allergies or viral infection is present). Pneumonia occurs if the organisms are inhaled into the lungs and not cleared (again, viral infection, or smoking-induced ciliary paralysis might be contributing factors). Once the organism makes its way to a site where it is not normally found, it activates the complement protein group, stimulates cytokine production, and attracts white blood cells (specifically polymorphonuclear neutrophils). The organism's polysaccharide capsule makes it resistant to phagocytosis, and if there is no pre-existing anticapsular antibody, alveolar macrophages cannot adequately kill the pneumococci. The organism spreads to the blood stream and is carried to the meninges, joint spaces, bones, and peritoneal cavity, and may result in meningitis, brain abscess, septic arthritis, or osteomyelitis.

The risk of pneumococcal infection is much increased in persons with impaired IgG synthesis, impaired phagosytosis, or defective clearance of pneumococci. In particular, the absence of a functional spleen, through congenital asplenia , splenectomy, or sickle cell disease predisposes one to a more severe course of infection.

2 Humoral immunity

In the 19th century, it was demonstrated that immunization of rabbits with killed pneumococci protected them against subsequent challenge with viable pneumococci. Serum from immunized rabbits or from humans who had recovered from pneumococcal pneumonia also conferred protection. In the 20th century, the efficacy of immunization was demonstrated in South African miners.

It was discovered that the pneumococcus's capsule made it resistant to phagocytosis, and in the 1920s it was shown that an antibody specific for capsular polysaccharide aided the killing of S. pneumoniae. In 1936, a penumococcal capsular polysaccharide vaccine was used to abort an epidemic of pneumococcal pneumonia. In the 1940s, experiments on capsular transformation by pneuococci first identified DNA as the material that carries genetic information.

In 1900, it was recognized that different serotype s of pneumococci exist, and that immunization with a given serotype did not protect against infection with other serotypes. Since then over ninety serotypes have been discovered, each with a unique polysaccharide capsule. Because some of these serotypes are more commonly the cause of disease, it is possible to provide reasonable protection by immuniving with less than 90 serotypes; the current vaccine contains 23 serotypes (i.e., it is "23-valent").

The serotypes are numbered according to two systems: the American system, which numbers them in the order in which they were discovered, and the Danish system which groups them according to antigenic similarities.

3 Treatment

Treatment is usually with β-lactam antibiotics. In the 1960s, nearly all strains of S. pneumoniae were susceptible to penicillin, but since that time, there has been an increasing prevalence of resistance, especially in areas of high antibiotic use. A varying proportion of penicillin-resistant strains may also be resistant to erythromycin, macrolides, and clindamycin and the quinolones. Most remain susceptible to vancomycin, which is a less desirable antiboitc because of dosing and tissue penetration issues. Susceptibility testing is routine, with empiric antibiotic treatment, guided by resistance patterns in the community in which the organism was acquired, pending the results.

4 Prevention

Pneumococcal vaccine (Pneumovax™ is one brand) gives at least 85% protection in those under 55 years of age for five years or longer. Immunization is suggested for those at highest risk of infection, including those 65 years or older, and generally should be a single lifetime dose (high risk side effects if repeated). The standard 23-valent vaccines are ineffective for children under two years old; a conjugated 13-valent vaccine is effective for them and should be reinforced with the standard 23-valent vaccine upon reaching the age of 2 years. It should also be administered before a splenectomy.

The current guidelines of the American College of Physicians call for administration of the immunization between ages 2 and 65 when indicated, or at age 65. If someone received the immunization before age 60, the guidelines call for a one-time revaccination.

Revaccination at periodic intervals is also indicated for those with other conditions such as asplenia or nephrotic syndrome. Product license suggests every 5-10 years and with immunity testing generally being unhelpful clinically, revaccination every six years has gained ground; aiming for 5 yearly risks greater side-effects if revaccination is performed prematurely due to a miscalculated interval.

<Hide>