VIRIDANS STREPTOCOCCI

INTRODUCTION: This is a heterogenous group of very poorly defined alpha haemolytic streptococci that are present in abundance in the mouth and pharynx. About 50% of bacterial endocarditis is caused by these bacteria. Since they do not have a defined group carbohydrate antigen, no correlation exists between serologic and physiologic characteristics of these strains.  The viridans streptococci comprise of six species viz. Strep. pneumoniae, Strept. mutans, Strept. sanguis, Strept. salivarius, Strept. milleri and Strept. mitior.

CLINICAL SIGNIFICANCE: When isolated from mouth, throat or respiratory tract, viridans streptococci are generally regarded as harmless commensals. The medical importance of this group is because of its association with dental caries, sepsis and bacterial endocarditis. S. milleri is associated with deep sepsis, including liver and brain abscesses. S. mutans, S. mitior and S. sanguis are involved in the production of dental caries. Endocarditis can be caused by S. mitior and S. sanguis. Nothing much is known about the pathogenicity of S. salivarius but is believed to be capable of causing dental caries.  Streptococcus MG group is one of the viridans group which share certain antigens with Mycoplasma pneumoniae

Dental Caries: It is a multifactorial disease that results only when all dependent factors coincide. Three important factors that cause it are:

1. Susceptibility of host and teeth. Pits, fissures and spaces tend to entrap food and let microflora flourish.

2. Excessive dietary sucrose intake which can be fermented by bacteria to produce acid which can demineralize the dental enamel.

3. Cariogenic microflora which comprises of various species of streptococci and lactobacilli. Among the streptococci, S. mutans, S. mitior and S. sanguis have been shown to cause dental caries even in experimental animals. Species of Lactobacillus which are generally present in oral cavity and have been incriminated as possible causative agents of dental caries are L. casei, L. fermentum, Bifidobacterium bifidum and L. acidophilus.

Infective Endocarditis: Viridans streptococci account for about 40% of the cases of infective endocarditis. Strep. sanguis and Strep. mitis are most commonly incriminated followed by Strep. mutans and Strep. milleri.

Factors predisposing to infective endocarditis include:  i. Cardiac factors; Rheumatic heart disease, Congenital heart disease, Cardiac surgery, Prosthetic heart valves.  ii. Non-cardiac factors; Dental manipulations, Sepsis.

RELATED;

1.  STREPTOCOCCI

2.  STAPHYLOCOCCI

3.  BACTERIOLOGY

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