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Biology LibreTexts Viridans Streptococci

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    Figure \(\PageIndex{1}\): Viridans Streptococci adhering to oral epithelial cells. Crystal violet stain, 1000X magnification. (2019; Jeanne Kagle)


    •  Group of Streptococcus species with similar characteristics
    • Some common species are Strep. mutansStrep. mitis, and Strep. sanguis
    • Gram-postivie cocci in pairs or short chains
    • No capsule
    • Catalase negative
    • Facultative anaerobe
    • Use lactic acid fermentation
    • Alpha-hemolytic (Figure \(\PageIndex{1}\))



    Figure \(\PageIndex{1}\): The viridans streptococci are alpha-hemolytic. They can only partially degrade blood in blood agar. (Rebecca Buxton. 2005.


    • Some of the most common normal oral flora


    • Can produce dental disease in the mouth
    • Can be introduced into the bloodstream by dental procedures or poor oral health


    • Most people have minor disease caused by viridans streptococci (e.g. dental caries)
    • More severe disease usually occurs in people with underlying conditions

    Clinical Disease

    • Dental caries (cavities):
      • Normal oral Streptococci metabolize through fermentation, producing acid which degrades tooth enamel
    • Subacute bacterial endocarditis (SBE):
      • Tends to affect people with damaged, malformed, or artificial heart valves
      • Viridans Streptococci enter the bloodstream through cuts in the mouth or during dental work
      • Once at the heart, they can attach to heart valve tissue and form a fibrin-embedded biofilm called a vegetation
      • Develops slowly (over weeks or months)
      • Symptoms include: flu-like symptoms (such as low-grade fever, fatigue, shortness of breath), small, tender nodules on the fingers or toes, and tiny broken blood vessels on the whites of the eyes, the palate, inside the cheeks, on the chest, or on the fingers and toes
      • Good prognosis with timely treatment (antibiotics)

    Primary Virulence Factors

    • As part of the normal oral flora, the viridans Streptococci convert glucose into a sticky substance called dextran which is necessary for the formation of dental plaque
    • Virulence factors involved in SBE include biofilm formation, interaction with platelets (adherence, aggregation, and/or activation), and adhesins (

    Additional Information:

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