Dr Sabrina Moyo Department of microbiology and immunology
Definition: - Gram positive diplococci, often lancet shaped or arranged in chains, possessing a capsule of polysaccharides.
- Normal inhabitant of the throat and nasopharynx in healthy people.
- Leading cause of morbidity and mortality in persons of all ages.
- It is the most common cause of bacterial pneumonia and an important cause of otitis media, meningitis and septicaemia.
- S. pneumoniae - encapsulated gram-positive cocci, oval or spherical: - 0.5 - 1.2 µm diameter. - Lancet shaped, in pair or singles. - Pneumococci are very sensitive to the product of their fermentative metabolism, which result in Gram-negative reaction in older cultures. - Capsule readily demonstrated using India ink or use of homologous type-specific antibody in the Quellung reaction. - Smooth to rough variation (transformation) occurs in culture in presence of homologous antisera.
Has fastidious nutritional requirements
Optimum pH - 7.4 - 7.8
5 - 10% of all pneumococcal strains require increase CO2 conc for primary isolation
Can grow only on enriched media supplemented with blood products
Colonies of encapsulated strains are round, mucoid and larger 1-3mm in diameter on blood agar but smaller on chocolate agar
Colonies of non capsulated strains are smaller and appear flat
All colonies undergoes autolysis with aging leaving dimpled appearance
Colonies appear alpha hemolyitic due to production of pneumolysin
Cell wall has peptidoglycan + teichoic and capsular antigens.
The major component of the cel wall is teichoic acid which is rich in galactosamine, phosphate and choline
- The capsule consists of complex polysaccharides that form hydrophilic gels on the surface of the organism and is responsible for the pathogenicity.
- They are antigenic and forms the basic of separation of pneumonia into 90 different serotypes
- The polysaccharrides stimulates antibody productions that are protective against subsequent infection with pneumococci of the homologous type.
- Purified capsular polysaccharides from the most commonly isolated serotypes are used in a polyvalent vaccine.
The virulent strains of S.pneumo are covered with capsule polysacharide
This has been used for serologic classification of strains
83 serotypes are currently recognized
Not so well understood. The disease manifestations are caused primarily by host response to infection mainly in a form of inflammatory reaction mediated by pneumococcal antigens stimulating the complement system.
- Polyssacharide capsule inhibit phagocytosis
Smooth strains are highly virulent compared to rough strains which are easily phagocytosed.
Adherence - S pneumoniae attaches to the nasopharynx or the inner ear by interacting with the N-acetyl-glucosamine-galactose moiety of the cell surface glycolipid.
- In studies on the adhesin of S. pneumonia to human pharyngeal cells, differences observed between the adhesive capacities of the various strains could be correlated with the clinical origin of the strain.
1. Pneumococcal pneumonia
Is the most common form bacterial pneumonia
-Pneumococcal are carried in the nasopharynx of healthy contacts carriers who constitute the major reservoir for pneumococcal infections.
- Carrier rate varies with age, environment, and the presence of URTI.
- Pneumococcal pneumonia is rarely a primary infection and results only when the normal defense barriers of the RT are disturbed e.g anaesthesia, morphine, alcohol intoxication.
- Viral infection of URT is the major contributor.
Pneumonia lesion: - Invasion of alveolar tissues
- Out-pour of edema fluid, - Rapid multiplication + spread of organism
- PMN + RBC accumulation in infected alveoli
- Complete consolidation of the lobe/segment.
- patients presents with - acute onset with chills, fever. - History of mild URTI, severe pleuritic pain. - cough - rusty mucopurulent sputum.
-In untreated cases, recovery can be dramatic in 5-10 days.
- Recovery can be also be gradual.
Complications : - Pleural effusion, empyema, meningitis, pericarditis and endocarditis. - Bacteremia in 25 - 30 % of cases.
The pneumococcus is the most common cause of bacterial meningitis in adults and recurrent meningitis in all age groups.
Specimen: Sputum, blood, csf
1) Direct examination: Gram stain, Quellung reaction on sputum
2) Culture - Brain heart infusion or trypticase soya agar with 5% blood.
Alpha-heamolytic colonies that are bile soluble, optochin sensitive and positive quellung reaction.
i) Detection of pneumococcal antibody by RIA - very sensitive
ii) Detection of soluble capsular polysaccharides in serum, body fluids and urine.
1) Optochin sensitivity (Ethyl hydrocuprein hydrochloride) disk method.
2) Bile solubility - amidase is produced by pneumococci when stimulated by surface agents e.g. bile or bile salts - cleaves the bond between alanine and muramic acid in the peptidoglycan. This leads to lysis of the organism. Needs a neutral pH, 1% desoxycholate and viable young organisms.
- Loopful of specimen/culture + antisera + methylene blue.
The capsule becomes refractile and greatly swollen.
Are killed by moist heat at 55 ºC in 10 minutes
Killed readly by most disinfectants
Differ from other strptoccocci by being highly sensitive to killing by Optochin at a concentration of 0.0001 IU
Most strains are highly sensitive to benzyl penicilline, cephalosporins, erthromicin etc
- Penicillin (im/iv depending on the severity
Cephalosporin, Erthromycin for pneumonia
- Chloramphenicol for meningitis.
- Penicillin resistance of intermediate type has been reported (MIC of 0
Immunity: In adults natural immunity is high, without treatment 70% of pts recover. Type specific immunity to pneumococcal infection is long lasting.
Pneumococcal conjugate vaccine - using conjugate polyvalent vaccine with capsular polysacharides (23 types) which protects against 90% of pneumococcal infections. Efficacy 80-95%.
Indications -All pts at an increased risk of developing pneumococcal pneumonia
Those who have poor immunologic ability to respond to polysaccharides Ag to produces homotypic antibodies e.g chronic cardiac or respiratory diseases, SCD, Spleen hypofunction or asplenia, liver cirrhosis, DM, and chronic renal disease and Elderly.