Streptococcus Overview and Characteristics

Streptococcus Overview

• Streptococcus is a genus of Gram-positive, spherical bacteria often appearing in chains.
• Many Streptococcus species are important human pathogens causing a wide range of infections.
• The genus is well-studied, providing insights into bacterial pathogenesis and immunity.

Intended Learning Outcomes (ILOs)

• Describe the main species of clinical importance.
• Know the morphology and culture characteristics of Streptococcus species.
• List the virulence factors of streptococci.
• Know the laboratory diagnosis of streptococcal infections.

Streptococcus Species: Characteristics

• Agalactiae: Primarily affects newborns and pregnant women. Associated with neonatal, invasive infections. Key virulence factors include capsule and toxins. Antibiotic resistance is variable.
• Pyogenes: Common cause of strep throat and severe invasive infections. Associated with pharyngitis, scarlet fever, impetigo, cellulitis, necrotizing fasciitis, rheumatic fever, and post-streptococcal glomerulonephritis. Key virulence factors include M protein and exotoxins. Antibiotic resistance is increasing.
• Pneumoniae: Leading cause of bacterial pneumonia and invasive disease. It also causes meningitis and otitis media. Key virulence factors include capsule and toxins. Antibiotic resistance is significant.

Group A Streptococcus (Streptococcus pyogenes)

• Characterized by beta-hemolysis (clear zone).
• Colonies are small, translucent, and white.
• Catalase-negative.
• Bacitracin-sensitive.
• Associated with various infections, including pharyngitis, scarlet fever, impetigo, cellulitis, necrotizing fasciitis, rheumatic fever, and post-streptococcal glomerulonephritis.

Group B Streptococcus (Streptococcus agalactiae)

• Characterized by beta-hemolysis (narrow clear zone).
• Colonies are larger than Group A, greyish-white.
• CAMP test-positive.
• Common cause of neonatal sepsis, pneumonia, and meningitis in newborns.
• Colonizes the gastrointestinal and genital tracts.

Streptococcus pneumoniae

• Characterized by alpha-hemolysis (greenish zone).
• Colonies are mucoid or flattened due to capsule production.
• Optochin-sensitive.
• Bile solubility-positive.
• A major cause of pneumonia, meningitis, and otitis media (middle ear infections).

Viridans Group Streptococci

• Includes species like Streptococcus mutans, S. mitis, S. sanguinis.
• Commonly associated with dental caries (tooth decay).
• Can cause subacute bacterial endocarditis (especially in individuals with damaged heart valves).
• Hemolysis: alpha or gamma.
• Colonies are small, rough, or smooth, non-capsulated.
• Optochin: resistant.
• Bile solubility: negative.

Streptococcus Growth Requirements

• Require nutrient-rich media (e.g., blood agar, Todd-Hewitt broth).
• Most are facultative anaerobes, growing in both aerobic and anaerobic conditions.
• Thrive at 35-37°C (human body temperature).
• Prefer a neutral to slightly acidic pH range (6.5-7.5).

Streptococcus Taxonomy and Classification

• The genus Streptococcus includes a diverse group of Gram-positive bacteria, encompassing commensal and pathogenic species.
• Typically arranged in pairs or chains (streptococci).
• Lancefield grouping classifies streptococci based on carbohydrate antigens.

Cell Structure and Morphology

• Gram-positive cocci.
• Typically arranged in pairs or chains.
• Thick, rigid cell walls composed of peptidoglycan and teichoic acid.
• Pili and other surface proteins for adherence and virulence are present.

Sample Collection and Transport

• Collect samples from appropriate sites (throat, skin, blood) using sterile technique.
• Transport in appropriate media (Stuart's, Amies) to maintain viability during transit.
• Deliver samples to the lab as soon as possible (within 2 hours).

Diagnosis of Streptococcus Infections

• Sample Collection: Appropriate specimens based on infection site.
• Preliminary Tests: Gram staining, catalase, rapid ID tests.
• Culture Identification: Selective/differential media, biochemical tests.
• Serological Assays: Antibody detection for acute vs. convalescent.
• Diagnose using a combination of clinical history, sample collection, laboratory tests, and interpretation.

Susceptibility Testing

• Obtain pure isolates, use standardized inoculum.
• Apply antibiotic disks, incubate, and interpret.
• Measure zones of inhibition and compare to breakpoints.
• Report susceptible, intermediate, or resistant for each antibiotic.

Streptococcus Infections: Clinical Manifestations

This section details the clinical presentations and sequelae for different Streptococcus species (A, B, and Pneumoniae).

• Skin and soft tissue infections
• Respiratory tract infections
• Invasive infections
• Post-infectious sequelae (e.g., rheumatic fever, glomerulonephritis)

Additional Information

• Clinical manifestations of different Streptococcus species (A, B, and Pneumoniae)
• Laboratory identification of specific species.
• Methods for diagnosis (e.g., culture, rapid antigen tests, PCR).
• Importance of accurate and timely diagnosis for effective treatment and public health surveillance.