Question
A 70 year old man was brought to the emergency department with fever, cough, and dyspnoea. Physical examination revealed a dull note on percussion. X-ray chest revealed a patch on the right lung area. The gram stain of sputum sample revealed numerous pus cells and Gram positive lanceolate shaped diplococci.
- (a) What is the clinical diagnosis and what is the probable etiological agent. 2 mark(s)
- (b) What culture media is used to enable the growth of the organism suspected and how will you confirm the identity of the organism. 3 mark(s)
- (c) Discuss the virulence factors and pathogenesis of this etiological agent. 3 mark(s)
- (d) What are the vaccines available for this etiological agent. Discuss. 2 mark(s)
Answer
(a) Clinical diagnosis and probable etiological agent: Community-acquired lobar pneumonia, most probably caused by Streptococcus pneumoniae — an elderly patient with fever, cough, dyspnoea, a dull percussion note, a lobar chest X-ray patch, and Gram-positive lanceolate diplococci with pus cells on sputum Gram stain is the classical presentation.
(b) Culture media and confirmation: grown on blood agar (showing alpha-haemolytic, “draughtsman” colonies) and chocolate agar; confirmation is by optochin sensitivity (zone of inhibition around an optochin disc, distinguishing S. pneumoniae from other alpha-haemolytic streptococci/viridans group) and bile solubility (colonies lyse when exposed to bile/sodium deoxycholate, unlike viridans streptococci); the Quellung reaction (capsular swelling using type-specific antiserum) further confirms identity and allows serotyping.
(c) Virulence factors and pathogenesis: the major virulence factor is the polysaccharide capsule, which is anti-phagocytic, allowing the organism to evade host immune clearance — non-capsulated strains are essentially avirulent. Other virulence factors include pneumolysin (a pore-forming cytotoxin damaging host cell membranes and activating complement/inflammation), autolysin (releasing pneumolysin and other cell wall components), IgA1 protease (degrading secretory IgA, facilitating mucosal colonization), and surface adhesins (mediating attachment to respiratory epithelium). The organism typically colonizes the nasopharynx asymptomatically; under favourable conditions (e.g., viral URI, aspiration, impaired mucociliary clearance, extremes of age as in this patient), it descends to the lower respiratory tract, where the capsule resists phagocytosis, allowing local proliferation, inflammatory exudation into alveoli (producing the classical stages of lobar pneumonia — congestion, red hepatization, grey hepatization, resolution), and, in some cases, invasion into the bloodstream (bacteraemia) or CSF (meningitis).
(d) Vaccines available:
- Pneumococcal Conjugate Vaccine (PCV) — e.g., PCV13/PCV10/PCV15/PCV20, covering the most common invasive serotypes; a T-dependent, protein-conjugated vaccine inducing immunological memory, effective even in infants, part of the routine immunization schedule.
- Pneumococcal Polysaccharide Vaccine (PPSV23) — covers 23 serotypes, a T-independent vaccine, recommended for high-risk adults (elderly, as in this patient, chronic disease, immunocompromised, asplenia); less effective in children under 2 years given the T-independent response and lack of memory generation. Vaccination strategy targets high-risk groups (infants, elderly, immunocompromised) to reduce the burden of invasive pneumococcal disease.