M5: Streptococcus oral and non-oral infections Flashcards
Describe streptococci species
- Gram positive cocci forming chains
- Part of normal flora
- Catalase negative, facultative anaerobes
- Have cell wall carbohydrate antigens
- Grow well on blood agar (with glucose and serum)
Describe the A-haemolysis that occurs on blood agar with streptococci
Narrow zone of partial haemolysis and green discolouration around the colony produced = viridian’s streptococci
Describe the B-haemolysis that occurs on blood agar with streptococci
Wide, clear translucent zone of complete haemolysis around the colony = streptococcus pyogenes
What are the oral streptococci species
These are opportunistic so the pathogenic organism is usually commensal so exists without causing harm unless the host is immunocompromised and this can cause infection
- Mutans group
- Salivarious group
- Anginosus group
- Mitis group
Which streptococci species is associated with caries
S. mutans (highly cariogenic in presence of predisposing factors such as sucrose)
What characteristics of S. mutans are important in dental caries
- Surface antigens have - cell wall carbohydrates, lipoteichoic acid, lipoproteins, cell wall associated proteins
- Ability to make soluble/insoluble extracellular polysaccharides from sucrose
- Ability to make intracellular polysaccharides (reverse carbohydrate source)
- Acidogenic = ferment dietary carbs to produce lactic acid
- Acidoduric = ability to survive and grow at low pH
Which streptococci species is associated with infective endocarditis
Mitis group (causes septicaemia in neutropenic patients and are opportunistic pathogens for immunocompromised)
How do streptococci species lead to IE
- Patients with predispositions to IE have irregular blood flow through heart causing thrombus formation
- Dental manipulations introduce bacteria into patients bloodstream
- Bacteria can colonise thrombi and heart valves
- Bacterial vegetations arise on the valve
- Heart function is impaired
- Vegetations break off and can block supply
Describe S. Pyogenes
- B-haemolytic on blood agar
- Gram positive coccus
- Catalase negative
- Bacitracin sensitive
What do S. Pyogenes cause
- Tonsillitis or pharyngitis
- Erysipelas = acute cellulitis
- Impetigo
- Scarlet fever
- Septicaemia
- Rheumatic fever
- Acute glomerulonephritis
Describe S. Pneumoniae
- Gram positive
- A-haemolytic on blood agar
- Diplococci as seen in pairs
- Non-motile, non-spore forming
- Catalase negative
- Facultative anaerobe
- Colonies on blood agar
- Bile soluble and optochin sensitive
What do S. Pneumoniae cause
Pneumonia : when in lower respiratory tract they cause an inflammation reaction leading to lung congestion and consolidation - hypoxaemia, pleurisy and emphysema (lung necrosis and abscesses)
Meningitis : invasion via blood stream from pharynx and predisposing factors for bacteraemia causing mucous entrapment, aspiration and secondary infection of respiratory tract
