Describe Streptococcus pneumoniae with its identification, diseases and prevention.

Morphology and Identification

• Gram-positive, lanceolate (flame-shaped) diplococci with their broad ends apposed, typically surrounded by a clear capsule; may occur in short chains
• Alpha-haemolytic on blood agar, forming small colonies that become draughtsman or carrom-coin shaped as autolysis creates a central depression
• Catalase negative; optochin sensitive and bile soluble — the two tests that distinguish it from the viridans streptococci, which are optochin resistant and bile insoluble
• Quellung reaction — type-specific antiserum causes apparent capsular swelling, allowing rapid identification and serotyping
• Over 90 capsular serotypes, and the polysaccharide capsule is the principal virulence factor, being antiphagocytic. Non-capsulated strains are avirulent

Diseases

• Pneumonia — the commonest cause of community-acquired lobar pneumonia, with rusty sputum and often preceded by a viral illness
• Meningitis — the commonest cause of bacterial meningitis in adults, with a high mortality and a high rate of neurological sequelae
• Otitis media and sinusitis — the commonest bacterial cause in children
• Bacteraemia, empyema, endocarditis, peritonitis, septic arthritis
• Predisposing factors — preceding viral infection, splenectomy or hyposplenism (including sickle cell disease), HIV, myeloma, alcoholism, smoking, chronic lung and heart disease, extremes of age, and complement or antibody deficiency

Laboratory Diagnosis and Treatment

• Specimens — sputum, blood, cerebrospinal fluid, pleural fluid, ear swab
• Gram stain is often diagnostic in cerebrospinal fluid and good-quality sputum; culture on blood agar with 5 to 10% carbon dioxide
• Identification — optochin sensitivity, bile solubility, inulin fermentation, Quellung reaction, and mouse pathogenicity (historical)
• Antigen detection — latex agglutination and urinary pneumococcal antigen, which remains positive after antibiotics have sterilised the cultures
• Treatment — penicillin or amoxicillin where sensitive; third-generation cephalosporins with vancomycin for meningitis where penicillin resistance is prevalent, since even intermediate resistance matters at that site. dexamethasone given with or just before the first antibiotic dose reduces hearing loss and neurological sequelae in pneumococcal meningitis
• Resistance arises from altered penicillin-binding proteins acquired by transformation from related streptococci — so beta-lactamase inhibitors are useless against it

Prevention and Applied Aspects

• Conjugate vaccine (PCV) — polysaccharide linked to a protein carrier, which converts a T-independent response into a T-dependent one with memory, so it works in infants under 2. It also reduces nasopharyngeal carriage, producing herd immunity that protects the unvaccinated. Included in India's Universal Immunisation Programme
• Plain polysaccharide vaccine (PPSV23) — broader serotype coverage but T-independent, so no memory, no herd effect, and ineffective under 2 years; used in adults at risk and in the elderly
• Vaccinate before elective splenectomy, and at least 2 weeks beforehand so that a response develops; also vaccinate against meningococcus and Haemophilus
• Pneumococcal pneumonia and meningitis remain leading causes of childhood death in India, and conjugate vaccination is among the highest-value additions to the national programme
• Serotype replacement occurs after vaccination, as non-vaccine serotypes expand into the vacated niche, so surveillance must continue and vaccine composition may need revision
• Take blood cultures and a urinary antigen before antibiotics in suspected pneumococcal disease; the antigen test rescues the diagnosis when cultures are already sterile