streptococci

• Streptococci

  • Gram-positive cocci (spherical bacteria)
  • Non-motile
  • Catalase-negative
  • Ovoid to spherical
  • Occur in pairs or chains
  • Aerotolerant anaerobes
  • Require blood enriched medium

  
• 2 Ways of Classifying Streptococci

  1. Hemolytic properties on blood agar

  2. Serologic (Lancefield) groupings

  
• α-Hemolytic

  Chemical change in the hemoglobin in blood agar, resulting in a green pigment that forms a ring around the colony
• β-Hemolytic


  Gross lysis of RBC, resulting in a clear ring around the colony

  
• γ-Hemolytic

  Streptococci that cause no color change or lysis of RBC

  
• Lancefield Groupings


  Classifies primarily β-hemolytic streptococci into groups A through U based on their C-substance (polysaccharide in cell wall)

  
• Group A β-Hemolytic Streptococci (S. pyogenes)


  • Most clinically important member
  • Can invade intact skin or mucous membranes
  • Causes rheumatic fever and acute glomerulonephritis
  • Nasopharyngeal carriage is common
  • Spread person to person by skin contact and respiratory tract
  • Forms long chains in liquid culture
• Capsule


  Made of hyaluronic acid, identical to human connective tissue, non-immunogenic, antiphagocytic

  
• Cell Wall

  • Contains M protein (not infectious if absent, highly variable with 80 antigenic types, antiphagocytic)
  • Contains Protein F (mediates attachment to pharyngeal epithelium)
• Extracellular Products

  • Secretes a wide range of exotoxins that vary between strains
• Group B β-Hemolytic Streptococci (S. agalactiae)


  • Found in vaginocervical tract, urethral mucous membranes, and GI tract

  Leading cause of meningitis and septicemia in neonates

  Occasional cause of infections in postpartum women and immunocompromised individuals

  Larger colonies and less hemolysis than Group A

  
• Streptococcus pneumoniae (Pneumococcus)


  • Gram-positive, non-motile, encapsulated cocci

  Lancet-shaped, tend to occur in pairs

  Most common cause of community-acquired pneumonia and adult bacterial meningitis

  Fastidious, cultured on blood agar

  Releases α hemolysin, causing α-hemolytic colonies

  
• Streptococcus genus was coined in 1877 by Albert Theodor Billroth, combining "strepto-" (easily twisted) and "-coccus" (grain, seed, berry)

  
• Streptococci are oxidase-negative, catalase-negative, and many are facultative anaerobes
• In 1984, many bacteria formerly grouped in Streptococcus were separated into Enterococcus and Lactococcus genera

  
• Streptococcus genus has been found to be part of the salivary microbiome

  
• Streptococci pyogenes


  Most clinically important member of this group of gram(+) cocci, is one of the most frequently encountered bacterial pathogens of humans worldwide
• Streptococci pyogenes


  • Can invade apparently intact skin or mucous membranes, causing some of the most rapidly progressive infections known
  • Causes rheumatic fever and acute glomerulonephritis
  • Nasopharyngeal carriage is common especially in colder months and particularly among children
  • Does not survive well in the environment (unlike staph)
  • Its habitat is infected patients and also normal human carriers in whom the organism resides on skin and mucous membranes
  • Usually spread person to person by skin contact and via the respiratory tract
  • Usually form long chains when recovered from liquid culture but may appear as individual cocci, pairs, or clusters of cells in Gram stains of samples from infected tissue
• Capsule


  Hyaluronic acid, identical to that found in human connective tissue, forms the outermost layer of the cell - Not recognized as foreign by the body and, therefore, is non-immunogenic - Antiphagocytic
• Cell wall

  • Contains a number of clinically important components: M Protein - not infectious if absent - highly variable, w/ 80 different antigenic types - antiphagocytic
  • Protein F (fibronectin-binding protein) - mediates attachment in the pharyngeal epithelium
• Extracellular products


  • Secretes a wide range of exotoxins that often vary from one strain to another
• The only known reservoir is the skin and mucous membranes of the human host
• Respiratory droplets or skin contact spreads from person to person, especially in crowded environments (classrooms & play areas)
• Clinical significance

  • Major cause of Cellulitis
  • Acute pharyngitis / pharyngotonsilitis > most common type of infection > "strep throat" is associated with severe, purulent inflammation of the posterior oropharynx and tonsillar areas
  • Impetigo > classic cause > begins on any exposed surface (legs) > extensive lesions on face & limbs > treated with a topical agent such as mupirocin, or systemically with penicillin or a f first-generation cephalosporin (cephalexin)
  • Erysipelas > affecting all age groups > fiery red, raised plaques, especially on the face or lower limbs
  • Puerperal sepsis > initiated during, or following soon after, delivery caused by exogenous transmission (nasal droplets from an infected carrier or from contaminated instruments) or endogenously, from the mother's vaginal flora
  • Streptococcal toxic shock syndrome > mediated by production of pyrogenic exotoxins > initially flulike symptoms, followed shortly by necrotizing soft tissue infection, shock, acute respiratory distress syndrome, and renal failure > Tx: penicillin G + clindamycin
• Post-streptococcal sequelae

  • Acute rheumatic fever after 2-3 weeks of pharyngitis characterized by fever, rash, carditis, & arthritis
  • Acute glomerulonephritis after a week of impetigo or pharyngitis causing renal failure
• Rapid latex antigen kits


  (+) test: latex particles clump together (-) test, they stay separate, giving the suspension a milky appearance
• Treatment

  1. Penicillin

  2. Clarithromycin or Azithromycin

  3. Plus Clindamycin to inhibit protein (toxin) synthesis released from rapidly dying bacteria