Viridans Streptococci are gram positive spherical cells that occur in chains. Most of members in this group are α haemolytic. But they may non haemolytic as well. They lack the both carbohydrate antigen of pyogenic streptococci and the capsular polysacharides of the pneumococci.


They can be found in upper respiratory tract and mouth as normal flora. And it is important to maintain the healthy state of the mucous membrane. But if they reach the blood stream, it causes endocarditis especially after invasive dental procedures and also causes dental caries, gingivitis and periodontal diseases. They multiply on cardiac valves and cause systemic and local diseases which are fatal if untreated.


 Virulence factors of them include,

  • glucosyl transferase , a enzyme which hydrolyse glucose
  • production of insoluble glucans leads to dental  plaque enhance their attachment to cardiac valves

How to Diagnose bacteria at lab ?

If we consider the colony morphology, small colonies with α- haemolysis  can be seen on blood agar. but sometimes this may be non haemolytic as well.  This viridians streptococcus do not Grow MacConkey Agar, as we know only group B,D and enterococci of steptococcus spp give positive growth on macconkey agar. ( see below video for photos )

Then we have to prepare smear for gram stain, and we can see Gram positive cocci in chain under light microscopy.

If we perform the Catalase test it is catalase negative. Now we are confident enough to say this one as α- hemolytic strep , based on above findings along with clinical suspicion. Next we can direct out tests towards the alpha hemolytic strep identification panel.

first we can perform the optochin disk test. It is resistant to optochin (5μg) disk. usually we do not perform bile solubility test for optochin resistant spp. but if any one has performed , viridans streptococci  gives negative results in bile solubility test ( bile insoluble)

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