Gram positive
Streptococcus species

Author: Christopher Hale, M.D. (see Authors page)

Revised: 27 April 2016, last major update August 2013

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PubMed Search: Streptococcus species[title]

Cite this page: Streptococcus species. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/microbiologystreptococci.html. Accessed July 21st, 2018.
Definition / general
  • Gram stain: positive
  • Morphology: cocci in Chains
  • Facultative anaerobe
  • Non - pathogenic Streptococcal species are normal flora of mucosal surfaces
  • Greek "Strepto" (chain) + coccus, referring to chain - like morphology on Gram stain
  • Grouped broadly according to hemolytic pattern on blood agar: alpha, beta

  • Alpha (α) hemolytic: Strep. pneumoniae, viridans group Strep

  • Beta (β) hemolytic: further divided serologically using Lancefield antigens into Groups A through V (some letters not used)

  • Gamma (γ) hemolytic: formerly Group D Streptococci, re-classified as Enterococcus faecalis and Enterococcus faecium

  • "Streptococcus viridans" (viridans = "green") not a species, but a group of non - S. pneumoniae alpha hemolytic species including S. mutans, S. mitis, S. anginosus and others (J Clin Microbiol 2010;48:3829)
  • "Pyogenic Streptococci" refers not only to S. pyogenes, but all beta hemolytic streptococci and a few non - beta hemolytic streptococci (S. dysgalactiae)
  • Lancefield Group A Streptococci (GAS) = S. pyogenes
  • Lancefield Group B Streptococci (GBS) = S. agalactiae
  • "pneumococcus" = Streptococcus pneumoniae
  • S. pyogenes (Group A Streptococci): pharyngitis, necrotizing fasciitis, cellulitis, erysipelas, scarlet fever, streptococal toxic shock syndrome; non - infectious immune sequelae includes post - infectious glomerulonephritis, acute rheumatic fever
  • S. agalactiae (Group B streptococci): pneumonia, sepsis, meningitis in newborns
  • S. pneumoniae: community - acquired pneumonia, meningitis, otitis media
  • Viridans Group streptococci: endocarditis, bacteremia (particularly in setting of oral mucosal disruption)
  • Culture conditions:
    • Blood agar (non-contaminated sites), colistin nalidixic acid agar (contaminated sites)
    • 35 - 37C
    • Ambient air or 5% CO2, anaerobic conditions enhance growth (particularly Viridans group)

  • Gram positive cocci in chains (S. pneumoniae classically diplococci)
  • Catalase negative
  • Alpha or beta hemolytic on blood agar, depending on species (gamma reclassified as Enterococcus)
  • Bile solubility test (sodium deoxycholate) to distinguish alpha hemolytic species: Viridans group Strep. (bile insoluble) from S. pneumoniae (bile soluble)
  • Latex agglutination card test for Lancefield serotyping of beta hemolytic strains
  • S. pneumoniae: Quellung (Neufeld's quellung) reaction positive, optochin ("P" disc) sensitive
  • S. pyogenes: positive PYR test (L-pyrrolidonyl-beta-naphthylamide), bacitracin sensitive, CAMP (Christie Atkins Munch-Petersen) test negative
  • S. agalactiae: CAMP test positive, hippurate positive
  • Biochemical identification especially useful for Viridans group
  • Rapid antigen detection used clinically for Group A Strep; culture is gold standard
Clinical images

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Viridans group Streptococci

Group A Strep. from throat culture

Group B Strep. culture

Strep. pneumoniae, encapsulated strain

Gross description
  • Gray - white glistening, alpha or beta hemolysis
  • S. pneumoniae capsule gives colonies mucoid appearance, may have central depression
Microscopic (histologic) images

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Gram stain of Strep. pyogenes

Molecular / cytogenetics description
Differential diagnosis
  • Distinctive morphology on Gram stain when chains are seen, distinction more difficult among alpha strep species
  • MALDI-TOF may discriminate often misdiagnosed alpha hemolytic streptococci (Infect Genet Evol 2011;11:1709)
  • Other Gram positive cocci