Streptococcus viridans


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Streptococcus viridans


Introduction to bacteria

Bacterial structure and functions

Gram positive bacteria

Staphylococcus epidermidis

Staphylococcus aureus

Staphylococcus saprophyticus

Streptococcus viridans

Streptococcus pneumoniae

Streptococcus pyogenes (Group A Strep)

Streptococcus agalactiae (Group B Strep)


Clostridium perfringens

Clostridium botulinum (Botulism)

Clostridium difficile (Pseudomembranous colitis)

Clostridium tetani (Tetanus)

Bacillus cereus (Food poisoning)

Listeria monocytogenes

Corynebacterium diphtheriae (Diphtheria)

Bacillus anthracis (Anthrax)


Actinomyces israelii

Gram negative bacteria

Escherichia coli

Salmonella (non-typhoidal)

Salmonella typhi (typhoid fever)

Pseudomonas aeruginosa


Klebsiella pneumoniae


Proteus mirabilis

Yersinia enterocolitica

Legionella pneumophila (Legionnaires disease and Pontiac fever)

Serratia marcescens

Bacteroides fragilis

Yersinia pestis (Plague)

Vibrio cholerae (Cholera)

Helicobacter pylori

Campylobacter jejuni

Neisseria meningitidis

Neisseria gonorrhoeae

Moraxella catarrhalis

Francisella tularensis (Tularemia)

Bordetella pertussis (Pertussis/Whooping cough)


Haemophilus influenzae

Haemophilus ducreyi (Chancroid)

Pasteurella multocida


Mycobacterium tuberculosis (Tuberculosis)

Mycobacterium leprae

Mycobacterium avium complex (NORD)

Other bacteria

Mycoplasma pneumoniae

Chlamydia pneumoniae

Chlamydia trachomatis

Borrelia burgdorferi (Lyme disease)

Borrelia species (Relapsing fever)


Treponema pallidum (Syphilis)

Rickettsia rickettsii (Rocky Mountain spotted fever) and other Rickettsia species

Coxiella burnetii (Q fever)

Ehrlichia and Anaplasma

Gardnerella vaginalis (Bacterial vaginosis)


Streptococcus viridans


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USMLE® Step 1 questions

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High Yield Notes

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Streptococcus viridans

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USMLE® Step 1 style questions USMLE

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A study is done to identify the normal bacterial flora of the mouth. One bacterial species that was found to colonize the mouth is a gram-positive coccus that grows in chains and is catalase-negative. When it is grown on blood agar, alpha-hemolysis is noted. Additionally, this bacterium was tested for optochin sensitivity and was optochin-resistant. Which of the following is an additional feature of this bacterium?  

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Dental plaque

viridans streptococci p. 127


viridans streptococci p. 126

Viridans streptococci p. 134

α-hemolysis p. 133

bacterial endocarditis p. 320

biofilm production p. 127

brain abscesses p. 177

Gram-positive algorithm p. 132

normal flora p. 175

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Content Reviewers

Viviana Popa, MD


The term “streptococcus viridans”, refers to five groups of Streptococci that include Streptococcus anginosus group, mitis group, sanguinis group, salivarius group, and finally the mutans group.

And each of these groups has approximately 30 species that can cause disease in humans.

So the more correct term would be “viridans streptococci”, to reflect that there’s more than one of them.

Generally speaking, viridans streptococci can’t be assigned to a Lancefield group based on their cell wall antigens.

However, in the Streptococcus anginosus group, some species can be assigned to Lancefield groups A, C, F or G.

Now, viridans streptococci have a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so they’re gram-positive bacteria.

They’re also catalase negative - which means they don’t produce an enzyme called catalase. Ok, now, when cultivated on a medium called blood agar, viridans streptococci colonies cause alpha hemolysis, also called green hemolysis, because they produce hydrogen peroxide, which partially oxidizes initially red hemoglobin in the blood agar to green methemoglobin.

One way to remember this is to think that 'viridans’ means ‘green’ in Latin.

Some species in the Streptococcus anginosus group, however, can also be beta-hemolytic or nonhemolytic.

These guys are special little snowflakes.

Now, other Streptococcus species, like Streptococcus pneumoniae, are also alpha hemolytic.

So, an optochin test is done to distinguish them.

That’s where an optochin-containing paper disc is placed on a plate of blood agar that has been inoculated with the bacteria.

With Strep pneumoniae, the growth of the bacteria is inhibited in the zone that surrounds the optochin disc, meaning that Strep pneumoniae is optochin sensitive.

On the flip side, viridans streptococci are optochin resistant - meaning there’s no inhibition of the growth around the optochin disc.

Another test that can be used to distinguish Strep viridans from Strep pneumoniae is the bile solubility test, which can be done in a test tube, or more commonly, by dripping drops of bile on bacterial colonies.


Streptococcus viridans is a group of gram-positive, alpha-hemolytic streptococci that is commonly found in the oropharynx, epithelial surfaces of the oral cavity, teeth, skin, the gastrointestinal and genitourinary tract of humans. The term streptococcus viridans �, refers to five groups of Streptococci that include Streptococcus anginosus group, the mitis group, the sanguinis group, the salivarius group, and the mutans group.

S. viridans is part of the normal human microbiota and usually does not cause disease, however, it has been implicated in causing several infections, especially in immunocompromised individuals. S. viridans dental caries, periodontal disease, brain and liver abscesses, and bacterial endocarditis.

Some strains of S. viridans are capable of forming biofilms on medical implants such as catheters and prosthetic heart valves, which can offer them some antibiotic resistance. Treatment of S. viridans infections typically involves antibiotics such as penicillin, vancomycin, or erythromycin.


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