Untitled Quiz

Lecture 6: Medical Bacteriology (I) - Cocci Bacteria

• Types of Bacteria:
  • Pathogenic: Cause disease in humans
  • Commensal: (Normal flora/human microbiome): Normally inhabit the human body

Gram-Positive Cocci (Staphylococci)

• Morphology: Gram-positive cocci arranged in grape-like clusters
• Culture & Biochemical Characteristics:
  • Facultative anaerobes
  • Grow at 37°C
  • Grow on nutrient agar and blood agar
  • All Staphylococci are catalase-positive

Classification of Staphylococci

• Coagulase-positive:

  • Staphylococcus aureus (Most pathogenic)

• Coagulase-negative:

  • Staphylococcus epidermidis (Normal skin flora)
  • Staphylococcus saprophyticus

Virulence Factors of Staphylococcus aureus

• Toxins:

  • Epidermolytic (exfoliative) toxin
  • Enterotoxin
  • Toxic shock syndrome toxin
  • Hemolysin

• Enzymes:

  • Coagulase
  • Staphylokinase
  • Catalase
  • Proteases, Lipase, DNase

Diseases Caused by Staphylococcus aureus

• Suppurative (pyogenic):

  • Skin infections: Boils, Folliculitis, Impetigo
  • Bacteremia
  • Endocarditis, Osteomyelitis, Pneumonia, Urinary tract infections

• Toxigenic Diseases:

  • Scalded skin syndrome
  • Food poisoning
  • Toxic Shock Syndrome

2- Streptococci

• Morphology: Gram-positive cocci arranged in pairs (e.g., S. pneumoniae) or in chains (e.g., S. pyogenes)
• Some strains are capsulated
• Culture & Biochemical Characteristics:
  • Facultative anaerobes
  • Grow at 37°C
  • Catalase-negative.
  • Grow on blood agar

Classification of Streptococci (hemolytic activity)

• Alpha-hemolytic: (partial hemolysis) - S. pneumoniae, S. viridans
• Beta-hemolytic: (complete hemolysis) - Group A Streptococci (S.Pyogenes), Group B Streptococci (S. agalactiae)
• Gamma-hemolytic: (no hemolysis) - Enterococcus faecalis

Virulence Factors of Streptococcus pyogenes

• M protein: Adherence to cells; resistance to phagocytosis
• Enzymes: Streptokinase, Streptodornase, Hyaluronidase, Streptolysins (S & O)
• Exotoxins: Erythrogenic toxin

Diseases Caused by Streptococcus pyogenes

• Suppurative infections: Pharyngitis, Tonsillitis, Sinusitis, Otitis media, Impetigo, Cellulitis, Bacteremia, Puerperal sepsis, Endocarditis, Osteomyelitis
• Toxigenic diseases: Scarlet fever, Streptococcal toxic shock-like syndrome, (due to erythrogenic toxin)
• Post-streptococcal sequelae: Acute rheumatic fever, Acute glomerulonephritis

Other Streptococci and Bacteria

• Streptococcus agalactiae: (normal flora of the vagina); Neonatal meningitis, Neonatal pneumonia

• Viridans Streptococci: (normal flora of the mouth); Dental caries, Infective endocarditis

• S. pneumoniae: Pneumonia, Meningitis

• Enterococcus faecalis: Normal flora in GIT → Urinary tract infection (UTI)

• Gram-negative cocci (Neisseria species):

  • Morphology: Gram-negative cocci arranged in pairs (diplococci)
  • Culture conditions: Facultative anaerobes, grow at 37°C, Capnophilic (need 5-10% CO2), grow best on chocolate agar and Thayer-Martin agar (selective media)

• Important species of Neisseria:

  • Neisseria meningitidis: Mode of transmission - inhalation of infected droplets; Diseases: Meningitis
  • Neisseria gonorrhoeae: Mode of transmission - sexually transmitted disease; Diseases: Gonorrheal urethritis or cervicitis, Ophthalmia neonatorum

Case Study

• Case: 1-year-old with fever and generalized convulsion, mild throat inflammation, neck rigidity, clear lungs, Gram-negative diplococci in CSF
• Most likely Causative Pathogen: Neisseria meningitidis