Microbiology: Streptococci Quiz

Gram-positive Cocci Bacteria (Streptococci) - Overview

• Gram-positive cocci are spherical bacteria arranged in chains or pairs
• Streptococci are a diverse group of bacteria, often classified by carbohydrate antigens on their cell walls, using the Lancefield system (e.g., group A, B, C). Another classification system is based on hemolysis reactions
• Medically important streptococci include S. pyogenes (Group A), S. agalactiae (Group B), S. pneumoniae, viridans streptococci, and Enterococcus faecalis
• These bacteria have various characteristics, which include being non-spore-forming, non-motile, and facultative anaerobes

Streptococci: General Characteristics

• Typically gram-positive, spherical, and arranged in chains or pairs; often small, non-pigmented colonies, sensitive to drying, heat, disinfectants.
• Can form capsules and slime layers
• Do not produce catalase, but possess a peroxidase system.
• Most parasitic forms require enriched media for growth due to their fastidious nature.

Classification of Streptococci

• Lancefield classification is based on the type of carbohydrate antigens present on the cell wall.
• Another system is based on hemolysis, distinguishing between alpha-hemolytic (e.g., S. pneumoniae) and beta-hemolytic (e.g., S. pyogenes).

Streptococcus pyogenes (Group A)

• Most serious and common streptococcal pathogen; strict parasite of the throat and nasopharynx, occasionally skin.
• Laboratory characteristics: Culture on blood agar shows small, typically matt or dry colonies surrounded by beta-hemolysis. Some strains produce a hyaluronic acid capsule, evident in mucoid colonies on blood agar. Also catalase negative and bacitracin-sensitive.

Virulence Factors of S. pyogenes

• Surface antigens, such as C-carbohydrates and lipoteichoic acid (protect against lysozyme, aid in adherence), M proteins (antiphagocytic, major virulence factor), and hyaluronic acid capsules (protect against phagocytosis)
• Extracellular enzymes: streptokinase (lyses fibrin), hyaluronidase (breaks down hyaluronic acid), DNAases (deoxyribonucleases), NADase (kills leukocytes)
• Extracellular toxins: streptolysins O and S (hemolysins), pyrogenic toxin (erythrogenic) (causes scarlet fever), and superantigens (stimulate immune response)

Pathogenesis and Infections (S. pyogenes)

• Can cause suppurative infections (e.g., impetigo, pharyngitis, erysipelas, cellulitis; necrosing fasciitis, and streptococcal toxic shock syndrome)
• Can lead to nonsuppurative sequelae, like rheumatic fever and acute glomerulonephritis (immune response to bacterial antigens)

Epidemiology (S. pyogenes)

• Primarily occurs in childhood.
• Infections are spread by asymptomatic carriers.
• Rheumatic fever and acute glomerulonephritis were historically associated with poverty, implying a genetic predisposition to susceptibility.
• Entry portals are generally the skin or pharynx.

Host Defenses (S. pyogenes)

• Antibodies to M protein provide type-specific immunity.
• Antibodies to erythrogenic toxin prevent scarlet fever rash.
• Immune mechanisms are critical to acute rheumatic fever and glomerulonephritis development.

Laboratory Identification (S. pyogenes)

• Grown on blood agar with pronounced hemolytic activity, enhanced anaerobically. Catalase-negative.
• Bacitracin sensitivity, testing for group (A)-specific carbohydrates, identification of L-pyrrolidonyl arylamidase enzyme (PYR).
• Anti-streptolysin O (ASO) test to confirm rheumatic fever or glomerulonephritis.
• Anti-DNAse B test used for glomerulonephritis.

S. pneumoniae

• Causes 60-70% of bacterial pneumonias. Gram-positive diplococci, with short chains and lancet shapes
• Culture requires blood or chocolate agar; improved growth with 5-10% CO2.
• All pathogenic strains form large capsules (a major virulence factor).
• Causes lobar pneumonia, acute exacerbation of chronic bronchitis often with Haemophilus influenzae, meningitis, otitis media, sinusitis, and conjunctivitis (especially in splenectomized patients).

S. pneumoniae: Epidemiology

• 5-50% of people carry as normal flora in the nasopharynx.
• Infections are typically endogenous in origin. Pneumonia occurs when cells are aspirated into lungs.
• Susceptibility is influenced by factors such as young age, immune compromise, viral infections, and living conditions.

S. pneumoniae: Laboratory Diagnosis

• Gram stain of specimen; a hemolytic, optochin-sensitive bacteria; Quellung test (capsular swelling reaction)
• Culture on blood agar, showing alpha hemolysis, sensitive to optochin disk.
• Definitive diagnosis with culture and microscopic confirmation of characteristic diplococcal morphology.

S. pneumoniae: Treatment and Prevention

• Traditionally treated with penicillin G or V; increased drug resistance, making alternative antibiotics necessary.
• Two vaccines available: one targeting older adults (PPSV23) and another for children (PPV13), to decrease mortality risk.

Alpha-hemolytic Streptococci (Viridans Group)

• Alpha-hemolytic (viridans = green).
• Lack Lancefield group-specific carbohydrates.
• Normal microbiota of the mouth, pharynx, stomach, and urinary tract.
• Opportunistic pathogens causing dental caries, and dental plaques; capable of causing meningitis and endocarditis

Group D Enterococci and Groups C and G Streptococci

• Enterococcus faecalis, E. faecium, E. durans are normal intestinal flora
• Cause opportunistic urinary, wound and skin infections, particularly in debilitated patients.
• Group C and group G are frequently found in upper respiratory tract infections, pharyngitis, glomerulonephritis and bacteremia.
• Some group D non-enterococci (e.g., S. bovis) have been reclassified and are taxonomically significant, distinguished by being PYR negative and bile esculin positive.

Streptococci: Treatment and Prevention

• Streptococcal infections are traditionally treated with penicillin, but resistance to penicillin has become a concern. Erythromycin is used if penicillin is contraindicated.
• Resistance to erythromycin can also occur but is less prevalent.
• Vaccines are available to prevent Streptococcus pneumonia and some forms of Group A Streptococcus infections.