Biology: Gram-Positive Bacteria

Gram-PoBacteria

• Genera: Staphylococcus, Streptococcus, Enterococcus, Corynebacterium, Listeria

Staphylococcus

• Gram-positive cocci, arranged in grape-like clusters, facultative anaerobic and non-fascidious bacteria
• Classification:
  • Coagulase positive: S. aureus
  • Coagulase negative: S. epidermidis, S. saprophyticus
  • Pathogenicity: S. aureus (common pathogen), S. epidermidis, S. saprophyticus (opportunistic pathogens)

Staphylococcus aureus

• Most virulent and well-known member of the genus
• Virulence factors:
  • Cell wall-associated structures
  • Extracellular toxins
  • Enzymes for invasion
  • Protein A, TSST, coagulase, capsule, exfoliative toxin, hyaluronidase, clumping factor, enterotoxin, lipase, leukocidin, protease, haemolysin, DNAse

Epidemiology of S. aureus Infections

• Infection sources: human carriers (10-30% carry S. aureus in their nose), infected animals
• Transmission: direct contact (person-to-person), fomites, dust, airborne droplets, or food
• Nosocomial cross-infection is important

Hemolysis on Agar Plates

• Types of hemolysis:
  • γ (gamma) complete (beta) hemolysis
  • α (alpha) incomplete hemolysis
  • No (gamma) hemolysis
• α-hemolysis is characterized by a greenish zone around the bacterial colony

Enterococcus

• Classification: non-hemolytic
• Normal flora: colonize the gastrointestinal and genitourinary tracts
• Pathogens: β-hemolytic streptococci (S. pyogenes, S. agalactiae), Streptococcus pneumoniae

Streptococcus pyogenes

• β-hemolytic, group A
• Normal flora: 5-15% of individuals harbor the bacterium in the respiratory tract without signs of disease
• Most serious streptococcal pathogen
• Infections:
  • Suppurative: pharyngitis, pyoderma (impetigo), erysipelas, necrotizing fasciitis
  • Nonsuppurative: streptococcal toxic shock syndrome, bacteremia, pneumonia

Streptococcusal Infections

• Pharyngitis:
  • Highest incidence in ages 5-15
  • Person-to-person transmission via droplets or secretions
• Impetigo:
  • Superficial lesions that break and form highly contagious crust
  • Often occurs in epidemics in school children
  • Associated with insect bites, poor hygiene, and crowded living conditions
• Erysipelas:
  • Acute infection of the skin
  • Pathogen enters through a break in the skin and spreads to the dermis and subcutaneous tissues

Streptococcus pneumoniae

• Normal inhabitant (5-50%) of the throat and nasopharynx of healthy people
• Diseases:
  • Pneumonia (bacteria multiply in alveolar spaces)
  • Meningitis (high mortality)
  • Bacteremia (25-30% of patients with pneumococcal pneumonia, >80% of patients with meningitis)
  • Sinusitis
  • Otitis media (middle ear infection)

Treatment and Prevention of S. pneumoniae

• Traditionally treated with penicillin G or V
• Increased drug resistance (PRSP)
• Two vaccines available for high-risk individuals:
  • Capsular antigen vaccine for older adults and other high-risk individuals (effective for 5 years)
  • Conjugate vaccine for children 2 to 23 months

Viridans Streptococci

• α-hemolytic streptococci
• Members of the normal oral and nasopharyngeal flora
• Large complex group: Streptococcus mitis, Streptococcus mutans, Streptococcus oralis, Streptococcus sanguis, Streptococcus salivarius (K)
• Diseases:
  • Dental caries
  • Subacute endocarditis

Enterococcus

• Previously classified as group D streptococcus
• Most commonly isolated and clinically important: Enterococcus faecalis, Enterococcus faecium
• Colonize the gastrointestinal tract
• Associated infections:
  • Bacteremia
  • Urinary tract infections
  • Wound infections
  • Endocarditissitive