Streptococci Flashcards
Streptococcus and Enterococcus
Streptococci are catalase-negative, gram-positive, spherical, ovoid, or lancet-shaped cocci, often seen in pairs or chains.
Facultatively anaerobic
some strains require added CO2 for their initial isolation but may lose this requirement in subcultures.
broadly classified according to the hemolytic reaction on blood agar:
B-hemolytic completely hemolyze red cells around their colonies, and are further characterized by their Lancefield groups based on serologically reactive carbohydrates.
A-hemolytic are gram-positive cocci in chains the produce partial hemolysis causing “greening” of the agar
Gamma-hemolytic to not hemolyze blood cells
B-hemolytic Streptococci
Streptococcus pyogenes (group A)
Streptococcus agalactiae (group B)
A-hemolytic Streptococci
S. pneumoniae
Gamma-hemolytic
collectively called Viridans streptococci
S. mutans, S. sanguis, S mitis, S. salivarius, S. angionosus
Enterococcus
previously designated as group D streptococci because their cell walls reacted with group D antisera, are sufficiently different from other members of the genus Streptococcus to be considered a separate genus
gram-positive cocci that occur singly, in pairs, or short chains
facultatively anaerobic
most are alpha or gamma hemolytic, but some may exhibit Beta hemolysis
most common are Enterococcus faecium and Enterococcus faecalis
Group A Strep manifestations
Group A - pharyngitis.
may be accompanied by scarlet fever, a punctate exanthem overlying diffuse erythema that usually first appears on the neck or upper chest, becomes generalized, and then desquamates
skin infections of group A include cellulitis, erysipelas, and pyoderma
acute rheumatic fever, characterized by carditis, polyarthritis, erythema marginatum, chorea, and subcutaneous nodules may occur 1-5 weeks ager pharyngitis
acute glomerulonephritis may develop 10 days to 3 weeks ager pharyngitis OR pyoderma
recently serious syndromes including necrotizing fasciitis, myositis, malignant scarlet fever, bacteremia and toxic shock-like syndrome increased in frequency with high mortality rates of up to 30% or more. probably due to greater virulence potential
Group A Streptococcus pathogenesis
S. pyogenes
antiphagocytic cell wall M protein
60 protein types exist, so antibodies not effective against other types
lipoteichoic acid is cell wall component that permits bacterial adherence to respiratory epithelium
elaborates about 20 extracellular products, including enzymes (streptolysins, hyaluronidase, streptokinase, DNases, and NADase, and erythrogenic toxins
Streptolysin O is an antigenic, exygen-labile enzyme that produces subsurface hemolysis on blood agar plates
streptolysin S, a nonantigenic, oxygen-stable enzyme. produces surface hemolysis.
neither streptomycin has a proven role in pathogenesis
Streptokinase promotes fibrinolytic activity be converting plaminogen to plasmin
Hyaluronidase may enhance the spread of the organism through connective tissue
Pyrogenic toxins (A,B,C) are produced by S. progenies infected with a specific temperate bacteriophage. Their pyrogenicity is caused by a direct action on the hypothalamus.
Superantigens with high mitogenic capabilities cause more sever infection such as necrotizing fasciitis and or toxic shock syndrome
pathogenesis of rheumatic fever not fully understood
presence of complexes of immunoglobulin and C3 component of complement along the scrolemmal sheaths of cardiac myofibers suggest myocarditis results from from the production of antibodies directed against streptococcal cell wall M protein that cross reacts with myocardial tissue.
renal damage caused by deposits of circulating streptococcal-antistreptococcal immune complexes in the glomeruli and subsequent activation of complement
some isolates have been linked to toxic shock syndrome
Group B Strep manifestations
neonatal sepsis, pneumonia, and meningitis
colonization of the maternal genital tract is associated with colonization of infants and risk of neonatal dz occurring within the first few days or after 1 week
CDC publishes specific guidelines to identify and treat women colonized, and id and treatment
all pregnant woment at 35-37 weeks of gestations should have vaginal/rectal specimens collected and processed for group B. results of the test should be available during delivery so that prophylaxis can be given to the the mother before delivery to prevent infection to the newborn
isolation of group B from the urine can also be used as a marker for prophylaxis
if urine is positive, screen of vaginal/rectal cultures may not be necessary
In adults: postpartum endometritis, UTI, bacteremia, skin and soft tissue infections, pneumonia, endocarditis, meningitis, arthritis, and osteomyelitis
Group B and G Strep manifestations
wide range of infections including bacteremia, endocarditis, meningitis, arthritis, and respiratory and skin infections
pharyngeal infection is similar to group A, except that the nonsuppurative sequalae of rheumatic fever do not occur
infections of S. pneumonia include pneumonia, meningitis (especially in infants and elderly), spontaneous bacteremia (in persons who do not have a spleen), otitis, sinusitis, and spontaneous peritonitis
normal flora in 25%-50% of preschool children, 36% of primary school children, and nearly 20% of adults (carriers)
spread is enhanced by URI and crowding
most cases are endogenous from aspiration
person-to-person during epidemics through droplet aerosols
Streptococcus pneumoniae virulence
antiphagocytic polysaccharide capsule, and some strains have thick, mucoid capsule and are especially virulent
there are vaccines against infection against may of the predominant capsular polysaccharide types
a new vaccine is available for use in infants and children to prevent invasive disease
Strep Viridans
bacterial endocarditis is the most common infection
also abscesses in the brain or liver, bacteremia, and dental carries
the miller streptococci complex (S. constellatus, S. intermedius, and S. angionosus) are the most common viridans responsible for liver, spleen, and brain abscesses, and often more susceptible to antibiotics and other vridans strains. Penicillin resistance is increasing
S. bovis bacteremia is associated with malignancies of the GI tract.
Streptococci Laboratory Diagnosis
grow well on blood and chocolate agar. blood agar is preferred because the hemolytic properties can be assessed
when culturing vaginal/rectal swabs from pregnant women specifically for group B, specimens should first be inoculated on selective broth, such as Lim or carrot broth, or selective agar like Granada agar to enrich for this organism
more than 99% of group A are susceptible to bacitracin, but a very small percentage of isolates of group B and 10-20% of group C and G are also susceptible. Therefore bacitracin susceptibility provides a presumptive id.
an isolate may be called group A presumptively based on hydrolysis of PYRase test.
All group A and 99% of Enterococcus are PYRase-positive
ID of group A is confirmed by serotyping using latex agglutination or nucleic acid probe
A nucleic acid probe (Gen-Probe) is available for direct detection of group A on throat swab
Group A may be rapidly detected directly in throat swab specimens with commercial kits, which are highly specific but low sensitivity, therefore a negative test should be followed by culture or probe
Serologic tests to detect antibodies in acute and convalescent serum samples to streptomycin O and DNase B are used primarily to diagnose acute rheumatic fever and acute glomerulonephritis following group A infection
cultures found positive for colonies that are B-hemolytic and hippurate hydrolysis positive and/or that have a positive CAMP test reaction presumptively can be called group B strep.
if presumed group B is from sterile site, should be identified by serotyping using latex agglutination or coagglutination test or by chemiluminescent DNA problem growing on Lim broth or other selective broth cultures. It is not sensitive enough to use directly on clinical specimens.
for screening pregnant women between weeks 35 and 37, use broth enrichment with or as a replacement for agar-based media
chromogenic broth, including carrot, can be used as enrichment; colonies of B-hemolytic group B will convert the color from clear to yellow or orange. non-hemolytic will not change the color.
B-hemolytic groups C,D,F,and G are identified by serotyping with latest agglutination
commerical PCR kits can be used to detect group B directly in vaginal-rectal specimens and in culture on Lim or carrot broth.
there are other nucleic acid amplifications assays for the detection of group B in vaginal/rectal samples that are rapidly becoming FDA cleared
latex agglutinations assays are available for direct detection of group B (as well as S. pneumonia, some N. meningitides, E. coli, and H influenza type b) in CSF, serum, and urine. theses have sensitivities equivalent or lower than a Gram stain and may give false positive results so are not used in clinical laboratory
test used to presumptively id alpha and game hemolytic strep and enterococci: a-Hemolytic colonies that are mucoid or flattened with depressed center are suggestive of S. pneumonia; they should be test for susceptibility to ethyhyroxycupreine hydrochloride (P disk), and are positive for bile solubility.
S. pneumoniae is suceptible to both; other a-hemolytic strep are resistant to optochin and are variable in response to bile.
A urinary antigen assay for detection of S. pneumoniae has been shown in some studies to be the non culture diagnostic method of choice for severe pneumococcal infection, for diagnosis of pneumococcal exacerbation in COPD, and as a tool for diagnosis of otitis media
as with any antigen assay, results may reflect prior infection
alpha hemolytic that are not S. pneumonia and gamma hemolytic colonies are tested for PYRase hydrolysis; enterococci are PYRase positive, and viridans are negative
all enterococci grow in 6.5 NaCl, but viridans do not
Enterococci hydrolyze esculin in the presence of bile causing visible growth and blackening of the agar, but up to 10% of viridans also positive
identification of individual species of viridans is usually not necessary, but possible through commercial kits
members of viridans belonging to the miller can be recognized by their characteristic “caramel” odor can be reported because of their propensity for abscess formation and uniform susceptibility to penicillin
There are no vancomycin-resistant streptococci
Antimicrobial susceptibility
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