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Bacterial structure and functions
Streptococcus pyogenes (Group A Strep)
Streptococcus agalactiae (Group B Strep)
Clostridium botulinum (Botulism)
Clostridium difficile (Pseudomembranous colitis)
Clostridium tetani (Tetanus)
Bacillus cereus (Food poisoning)
Corynebacterium diphtheriae (Diphtheria)
Bacillus anthracis (Anthrax)
Salmonella typhi (typhoid fever)
Legionella pneumophila (Legionnaires disease and Pontiac fever)
Yersinia pestis (Plague)
Vibrio cholerae (Cholera)
Francisella tularensis (Tularemia)
Bordetella pertussis (Pertussis/Whooping cough)
Haemophilus ducreyi (Chancroid)
Mycobacterium tuberculosis (Tuberculosis)
Mycobacterium avium complex (NORD)
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)
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viridans streptococci p. 127
viridans streptococci p. 126
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
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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