Microbiology Chapter on Micrococcus and Streptococcus

Micrococcus

• Gram-positive cocci arranged in tetrads (groups of four)
• Non-motile, non-capsulated, and non-sporulated
• Found in the upper respiratory tract and skin, often as commensals or opportunistic pathogens in immunocompromised individuals
• Notable species include Micrococcus varians, Micrococcus luteus (Milk Micrococci), and Micrococcus roseus (Dust of the air)
• Strictly aerobic, growing at 37°C
• Grows on ordinary media (Nutrient agar) and Blood agar (Gamma hemolysis)
• Produces rose or pink colonies on mannitol salt agar, except for M. luteus which produces yellow colonies
• Previously classified into the genus Micrococcus, but now reclassified into Kocuria species based on phylogenetic and chemotaxonomic analysis

Streptococcus

• Gram-positive cocci arranged in chains
• Non-motile and non-sporulated
• May or may not be capsulated
• Facultative anaerobic, growing poorly on ordinary media but better in the presence of blood, serum, and glucose
• Notable species include Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae, and Enterococcus faecalis

Classification of Streptococcus

• Brown's classification: based on hemolytic reactions on blood agar
  • Beta-hemolytic (β): complete lysis of red blood cells, clear zone around colonies; most pathogenic
  • Alpha-hemolytic (α): partial hemolysis, greenish discoloration around colonies; mild pathogenic
  • Gamma-hemolytic (γ): non-hemolytic; saprophytic
• Lancefield classification: based on carbohydrate antigens in the cell wall; groups A-H, K-V, and untippable groups
  • Group A: S. pyogenes - pharyngitis, skin infections
  • Group B: S. agalactiae - neonatal infections
  • Group D: Enterococcus faecalis (previously Streptococcus faecalis) - nosocomial infections, endocarditis
  • Untippable group: S. pneumoniae - pneumonia in humans
• Sherman's classification: based on hemolytic reaction, Lancefield group, and specific physiological characteristics including fermentation and tolerance tests

Streptococcus pyogenes

• Causes pharyngitis (strep throat), scarlet fever (complication of strep throat), skin infections (impetigo, necrotizing fasciitis, cellulitis)
• Associated with post-streptococcal diseases like rheumatic fever and rheumatic kidney, which are autoimmune reactions triggered by similarities between heart muscle antigens and Streptococcus antigens
• Virulence factors include:
  • M protein: evades phagocytosis by immune cells, helps adherence to host tissues
  • Hyaluronic acid capsule: resists phagocytosis by immune cells
  • Protein F: adherence to host cells
  • Lipoteichoic acid: found in gram-positive cell walls
  • Hyaluronidase: breaks down connective tissue, facilitates bacterial spread
  • Streptokinase: dissolves blood clots, facilitating bacterial spread
  • DNase: breaks down DNA
  • C5a peptidase: inactivates complement system for immune evasion
  • Hemolysins (Streptolysin O and S): lyse red and white blood cells, causing tissue damage and infection spread
  • Exotoxins (e.g., SpeA, SpeB): cause toxic shock syndrome and scarlet fever

Diagnostic Tests for Streptococcus pyogenes

• Samples: blood, throat swabs, or other infected sites
• Gram stain: Gram-positive cocci in chains
• Blood culture: Streptococcus pyogenes is Beta-hemolytic
• Antibiotic sensitivity: Bacitracin-sensitive
• PYR test: positive
• Rapid antigen detection: latex agglutination tests from throat swabs
• Serological tests: detect antibodies against Streptococcus pyogenes including Lancefield grouping

Enterococcus faecalis (Group D Streptococcus)

• Often arranged in pairs or short chains
• Commensal bacterium found in the human large intestine
• Can tolerate high salt concentrations (6.5% NaCl) and bile presence
• Usually non-hemolytic or alpha-hemolytic
• Naturally resistant to many antibiotics
• Major cause of nosocomial infections, particularly in patients with indwelling medical devices
• Causes urinary tract infections (UTIs)
• Diagnostic tests:
  • Samples: blood, urine, CSF, wound swabs, catheter tips
  • Biochemical tests: PYR test positive, growth in 6.5% NaCl
• Lancefield grouping is not reliable for identifying E. faecalis because it lacks typical group-specific carbohydrates.

Viridans Group Streptococci (VGS)

• Heterogeneous group of organisms; can be commensals or pathogens
• Alpha-hemolytic or non-hemolytic
• Optochin-resistant
• Not soluble in bile salts
• Notable species: Streptococcus mutans (dental caries), Streptococcus salivarius
• Causes endocarditis and bacteremia

Streptococcus pneumoniae (Diplococcus pneumoniae)

• Untippable group in Lancefield classification - lacks Lancefield antigens
• Differentiated from VGS through specific tests and characteristics.

Explore the fascinating characteristics of Micrococcus and Streptococcus, including their morphology, growth conditions, and clinical significance. This quiz covers the classification, notable species, and the environments where these bacteria thrive. Perfect for microbiology students looking to test their knowledge on these important genera.