Medical Bacteriology I: Cocci Bacteria

Study Notes

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