Streptococcus pneumonia

  • The pneumococci (S. pneumoniae) are gram-positive diplococci.
  • Often lancet shaped or arranged in chains, possessing a capsule of polysaccharide that permits typing with specific antisera.
  • Pneumococci are normal inhabitants of theURT of 5–40%of humans and can cause pneumonia, sinusitis, otitis, bronchitis, bacteremia, meningitis, and other infections.

Morphology and Identification

  • With age, Pneumococcus rapidly become gram negative and tend to lyse spontaneously.Viridans streptococci do not lyse and are thus easily differentiated from pneumococci.
  • On solid media, the growth ofpneumococci is inhibited around a disk of optochin; viridans streptococci are not inhibited by optochin
  • “Capsule swelling test,” or quellung reaction is used for capsule
  • The exact makeup of the capsule is unique and distinctly antigenic for each of more than 90 serotypes.
  • Pneumococcal cell wall structure is similar to other streptococci.Teichoic acid, LPA,and phosphocholine are rooted in the peptidoglycan extending outward into the capsulewhere they provide binding domains for a variety of surface proteins.

Growth characteristic

  • On blood agar, pneumococci produce round, glistening colonies surroundedby a zone of ἀ-hemolysis.
  • Both colonies and broth cultures have a tendency toundergo autolysis due theaction of autolysins, a family of pneumococcal enzymes that degrade peptidoglycan.

Extracellular products

  • All pneumococci produce pneumolysin, which is a member of the family of transmembranepore-forming toxins.
  • The pneumococcus does not secrete pneumolysin but it is released on lysisof the organisms (autolysin's primary role was to release pneumolysin to an extracellular location). Pneumolysin has a number of other effects on the host cells,including its ability to stimulate cytokines and disrupt the cilia of human respiratoryepithelial cells.
  • Pneumococci also produce a neuraminidase, which cleaves sialicacid present in host mucin and involved in host colonization and involved in biofilm formation,

Pneumococcal Diseases

  • The most common form of infection with S. pneumoniae is pneumonia, whichbegins with fever and a shaking chill followed by signs that localize the disease tothe lung. These include difficulty breathing and cough with production of purulentsputum, sometimes containing blood. The pneumonia typically fills part or all of alobe of the lung with inflammatory cells, and the bacteria may spread to the bloodstreamand thus other organs. The most important of the latter is the CNS and leads to acute purulent meningitis.

EPIDEMIOLOGY

  • Infections are derived from colonization of the nasopharynx, where pneumococci canbe found in 5 to 40% of healthy persons depending on age, season, and other factors.
  • Respiratory secretions containingpneumococci may be transmitted from person to person by direct contact or from themicroaerosols created by coughing and sneezing in close quarters. Such conditions are favoredby crowded living conditions, particularly when colonized persons are mixed withsusceptible ones, as in child care centers, recruitment barracks, and prisons.
  • As with otherbacterial pneumonias, viral respiratory infection and underlying chronic disease are importantpredisposing factors.

Pathogenesis

  • Aspirationof respiratorysecretions containing these pneumococci is the initial event leading to pneumonia.
  • Normally, aspirated organisms are cleared rapidly by thedefense mechanisms of the lower respiratory tract, including the cough and other reflexes;the mucociliary “blanket;” and phagocytosis by alveolar macrophages.
  • Host factorsthat impair the combined efficiency of these defenses can allow pneumococci toreach the alveoli and multiply there.

These include

-chronic pulmonary diseases

-damageto bronchial epithelium from smoking or air pollution

-respiratory dysfunction fromalcoholic intoxication, narcotics, anesthesia, and trauma.

  • The polysaccharide capsule of S. pneumoniae is the major determinant of virulence. Unencapsulatedmutants do not produce disease in humans or laboratory animals.
  • Pneumolysin’stoxicity for pulmonary endothelial cells and direct effect on cilia contributes tothe disruption of the endothelial barrier and facilitates the access of pneumococci to thealveoli and eventually their spread beyond into the bloodstream.
  • Pneumococcal surface protein A (PspA) is found in virtually all pneumococci andhas been shown to interfere with complement deposition.
  • Initial alveolar multiplication produces aprofuseoutpouring of serous edema fluid, which is then followed by an influx of PMNs and erythrocytes.
  • By the secondor third day of illness, the lung segment has increased three- to fourfold in weightthrough accumulation of this cellular, hemorrhagic fluid typically in a single lobe of thelung.
  • A remarkable feature of pneumococcal pneumoniais the lack of structural damage to the lung, which usually leads to complete resolutionon recovery.\

IMMUNITY

  • Immunity to S. pneumoniae infection is provided by antibody directed against the specificpneumococcal capsular type. When antibody binds to the capsular surface, C3b is depositedby classical pathway mechanisms, and phagocytosis can proceed.

DIAGNOSIS

Gram smears of material from sputum and other sites of pneumococcal infection typicallyshow Gram-positive, lancet-shaped diplococci

S. pneumoniae grows well overnight on blood agar medium and is usually distinguished from viridans streptococci by susceptibility to (Optochin) or by a bile solubility

Quellung Reaction: When pneumococci of a certain type are mixed with specific antipolysaccharide serum of the same type—or with polyvalent antiserum (OMNISERUM)—on a microscope slide

Bacteremia is commonin pneumococcal pneumonia and meningitis, and blood cultures are valuable supplementsto cultures

Detection of pneumococcal capsular antigen inbody fluids is possible but valuable primarily when cultures are negative.

PREVENTION

A vaccine prepared from capsular polysaccharide extracted from the 23 most commonserotypes of S. pneumoniae is available.

However, this vaccine cannot be administered to infants below 2 years of age and to immunodeficient patients perhaps due to an immature or a sensitive immune system

It is recommended for patientswho are particularly susceptible to pneumococcal infection because of advancedage, underlying disease.

A seven-valent conjugatevaccine (PCV-7) is now available and recommended for use beginning at 2 months of age.