Streptococci: Gram Stain, Introduction, Pathogenecity and Differential Lab Diagnosis

streptococci in gram stain showing gram positive cocci in chains

Streptococci in Gram 

Streptococci in gram stain showing gram-positive cocci in chains as shown above image. The possible organisms may be  Streptococcus pyogenes, Streptococcus agalactiae, Enterococcus species, Streptococcus bovis, viridans streptococci.

Introduction of Streptococci

Streptococci are Gram-positive, non-motile, non-spore-forming, catalase test negative cocci that occur in pairs or chains.

Older cultures may lose their Gram-positive properties. Most of them are facultative anaerobes, and some are strict anaerobes. Most require enriched media i.e. blood agar. They are subdivided into groups by antibodies that recognize surface antigens. These groups may include one or more species. Serologic grouping is based on antigenic differences in cell wall carbohydrates (groups A – V), in cell wall pili-associated protein, and in the polysaccharide capsule in group B streptococci (GBS). Rebecca Lancefield developed the serologic classification scheme in 1933. β-hemolytic strains possess group-specific cell wall antigens, most of which are carbohydrates. These antigens can be detected by immunologic tests and have been useful for the rapid identification of some important streptococcal pathogens. The most important groups of streptococci are A, B, and D.  Pharyngitis is caused by group A and they have a hyaluronic acid capsule. S. pneumoniae causes of pneumonia and S. mutans and other so-called viridans streptococci (among the causes of dental caries) do not possess group antigen. S. pneumoniae has a polysaccharide capsule that acts as a virulence factor for the organism. More than 90 different serotypes are known, and all these types differ in virulence.

Pathogenicity of Streptococci

Group A streptococci (S. pyogenes) are responsible for-

  • Strep throat – a sore, red throat, sometimes with white spots on the tonsils
  • Scarlet fever – an illness that follows strep throat. It causes a red rash on the body.
  • Impetigo – a skin infection
  • Toxic shock syndrome
  • Cellulitis and necrotizing fasciitis (flesh-eating disease)
  • Rheumatic fever is a non-suppurative complication of S. pyogenes pharyngitis.
  • Rheumatic fever is an inflammatory disease affecting primarily the heart and joints. Although severe, it can take an extended period of time to develop.

Group B streptococci (Streptococcus agalactiae)

It can cause blood infections, pneumonia, and meningitis in newborns.
Adults can also get group B strep infections, especially if they are elderly or already have health problems. It can cause urinary tract infections, blood infections, skin infections, and pneumonia in adults.

Group D streptococci (Enterococcus) –

Enterococci are distantly related to other streptococci and have been moved into the genus Enterococcus; the most commonly isolated are Enterococcus faecalis and Enterococcus faecium. E.  faecalis can cause nosocomial infections, urinary tract infections, bacteremia, endocarditis, meningitis, and can be found in wound infections along with many other bacteria.

Streptococcus pneumoniae

It causes pneumonia, acute sinusitis, otitis media, meningitis,
bacteremia, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess. It is the most common cause of bacterial meningitis in adults and children and is one of the top two isolates found an ear infection, otitis media.

Differential Laboratory Diagnosis of Streptococci


  • Streptococci on blood agar

Most species of streptococci are facultative anaerobes. Some grow only in an atmosphere enhanced by carbon dioxide. The classification of species within the genus is complicated because three different schemes are used:
Hemolytic patterns:

Complete (β) hemolysis: e.g.  Streptococcus pyogenes, Streptococcus agalactiae,

incomplete (α) hemolysis: Streptococcus pneumoniae, Streptococcus salivarius, viridans are referred to collectively as viridans streptococci, a name derived from Viridis (Latin for “green”), referring to the green pigment formed by the partial, α-hemolysis of blood agar. Encapsulated, virulent strains of S. pneumoniae often forming highly mucoid, glistening colonies (production of capsular polysaccharide) surrounded by a zone of α -hemolysis.

and no hemolysis (γ):  Enterococcus faecalis

  • Serologic properties

Bacitracin test (0.04 U) for Streptococcus pyogenes

The bacitracin test is used to determine the effect of a small amount of bacitracin on an organism. S.pyogenes is inhibited by the small amount of bacitracin in the disk (visible zone of inhibition of growth; other beta-hemolytic streptococci usually are not.

CAMP test for Streptococcus agalactiae

CAMP test showing the arrow-shaped zone of enhanced hemolysis (positive)
of S. agalactiae  and the negative result of S.pyogenes
when tested against Staphylococcus aureus.

  • Biochemical (physiologic) properties

Optiochin test for S. pneumoniae

Optochin i.e. ethylhydrocupreine test is a chemical used in cell culture techniques for S. pneumoniae, which is optochin-sensitive (positive ), from other alpha-hemolytic streptococci such as Streptococcus viridans which are resistant. Optochin Differentiation Disks are recommended for use in the presumptive identification of  S. pneumoniae from other alpha-hemolytic streptococci.

Bile solubility test for S. pneumoniae

The bile (sodium deoxycholate) solubility test distinguishes S. pneumoniae from all other alpha-hemolytic (viridans) streptococci. It  is bile soluble
whereas all other alpha-hemolytic streptococci are bile resistant.

Bile esculin test  for Enterococcus 

Genus, Enterococcus is bile esculin test positive.

Further Readings

  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Handbook Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Manual of Clinical Microbiology. Editors: P.R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover and R. H. Yolken, 7th ed 2005, Publisher ASM, USA
  7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
  8. Topley & Wilsons Principle of Bacteriology, Virology, and immunology Vol I, II, III, IV & V. Editors: M.T. Parker & L.H. Collier, 8th ed 1990, Publisher Edward Arnold publication, London.
  9. Medical Microbiology-The Practice of Medical Microbiology Vol-2-12th Edn. –Robert Cruickshank
  11. District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update


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