Medical Bacteriology I: Cocci Bacteria

Lecture 6: Medical Bacteriology (I) - Cocci Bacteria

• Lecture's focus: Medical Bacteriology (I), specifically cocci bacteria
• Instructor: Dr. Rasha Mokhtar Elnagar. Credentials: M.B.B.C, MSc, PhD, MD Medical Microbiology and Immunology, Associate Professor of Medical Microbiology & Immunology, Consultant Microbiology & Immunology
• Two main types of bacteria:
  • Pathogenic bacteria: Cause diseases in humans
  • Commensal bacteria (normal flora): Normally inhabit the human body
• Staphylococci (Gram-positive cocci):
  • Morphology: Gram-positive cocci arranged in grape-like clusters
• Culture and biochemical characteristics of Staphylococci:
  • Facultative anaerobes
  • Grow at 37°C
  • Grow on Nutrient agar and Blood agar
  • All Staphylococci are Catalase-positive
• Classification of Staphylococci:
  • Coagulase-positive Staphylococci: Staphylococcus aureus (most pathogenic)
  • Coagulase-negative Staphylococci: 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) infections: Skin infections (boils, folliculitis, impetigo), Bacteremia, Endocarditis, Osteomyelitis, Pneumonia, Urinary tract infection.
  • Toxigenic diseases: Scalded skin syndrome (By Epidermolytic toxin), Food poisoning (By Enterotoxin), Toxic Shock Syndrome (By Toxic shock syndrome toxin).

2- Streptococci

• Morphology: Gram-positive cocci, arranged in pairs (e.g., S. pneumoniae) or chains (e.g., S. pyogenes)
• Some strains are capsulated.
• Culture and biochemical characteristics of Streptococci:
  • Facultative anaerobes
  • Grow at 37°C
  • Catalase-negative
  • Grow on Blood agar
• Classification of Streptococci (based on hemolytic activity on blood agar):
  • 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 (Streptolysin S & Streptolysin 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 (fever, skin rash and strawberry tongue), Streptococcal toxic shock-like syndrome
  • Post-streptococcal sequelae: Acute rheumatic fever, Acute glomerulonephritis

Other Streptococci and Cocci

• Streptococcus agalactiae: Normal flora of the vagina, causes neonatal meningitis and pneumonia
• Viridans Streptococci: Normal flora of the mouth, causes infective endocarditis and dental caries
• S. pneumoniae: Causes Pneumonia and Meningitis
• Enterococcus faecalis: Normal flora in the gastrointestinal tract (GIT), can cause urinary tract infections (UTIs)
• Neisseria species (Gram-negative cocci): Arranged in pairs (diplococci)
  • Neisseria meningitidis: Causes meningitis, transmitted via inhalation of infected droplets
  • Neisseria gonorrhoeae: Sexually transmitted disease, causes gonoccocal urethritis or cervicitis, ophthalmia neonatorum

Case Study

• A 1-year-old girl with fever, generalized convulsion and stiff neck has a CSF sample with Gram-negative diplococci
• Most likely pathogen: Neisseria meningitidis

References

• Lippincott Illustrated Microbiology 4th edition, Chapters 8, 9, 11