Microbiology Chapter 12

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Micro Chapter 12

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Streptococci and Enterococci

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alpha hemolysis

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partial hemolysis of blood agar (appears green)

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beta hemolysis

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complete hemolysis of blood agar (appears clear)

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gamma hemolysis

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no hemolysis observed

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most important streptococcal pathogen to humans

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GAS (S. pyogenes)

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primary presenttion of GAS

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acute pharyngitis

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pyoderma

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GAS infection of skin and soft tissues

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delayed sequellae of GAS infections

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acute glomerulonephritis and acute rheumatic fever

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how are streptococcal pyoderma infections acquired

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through break in skin/trauma

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M protein

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adhesin found in GAS that allows it to bind to keratinocytes

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HA capsule

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found in GAS and may facilitate binding to host receptor CD44 seen on pharyngeal epithelium as well as skin keratinocytes

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impetigo

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crusty honey colored lesions of mastoid air cells or middle ear indicitive of benign GAS infection

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erysiplelas and cellulitis

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caused by deep layers of skin becoming infected with GAS

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consequence of GAS reaching fascial planes b/w skin and muscle

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necrotizing fasciatis and myositis

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enzymes secreted by GAS which resut in necrotizing fasciitis

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proteases, hyaluronidase, Dnase, streptokinase

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streptolysins S and O

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lyse membranes of various host cells such as RBCs

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describe the binding regions of the protein and what they do

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proximally the C and D regions bind compliment control protein factor H which prevents opsinozation while distally B region binds fibrinogen which also interferes with compliment formation

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why do prior exposures to GAS not protect from reinfection

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although we can effectively form antibodies to the M protein, there are over 200 distinct M types meaning immuntiy to one will likely not prevent reinfection

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how is the HA capsule protective of GAS

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human CT has a lot of HA so a thick HA capsule camoflogues the GAS from immune detection

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septicemia vs toxemia

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septicemia is presece of bacteria in blood while toxemia is just the presense of toxins in the blood. Both can be deadly if untreated

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scarlet fever

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GAS associated toxemia

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toxins responsible for scarlet fever

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streptococcal pyrogenic exotoxins A,B, and C

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superantigens seen in scarlet fever

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SPE A and C

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superantigens

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produce large nonspecific T cell response resulting in massive cytokine release which can lead to shock

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STSS (streptococcal toxic shock syndrome)

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result of superantigens SPE A/C resulting in a widespread toxemia

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only type of streptococcal infection that can lead to acute rheumatic fever

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pharyngitis

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jones criteria

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symptoms appearing 1-4 weeks after strep throat that are indicitave of acute rheumatic fever. Include carditis, polyarthritis, chorea, subcutaneous nodules, and erythema marginatum

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most common manifestations of acute rheumatic fever

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carditis and polyarthritis

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rheumatic heart disease

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consequence of inflammation caused by rheumatic fever, results in mitral and aortic valvular scarring and stenosis

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best way to prevent ARF

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treat strep with antibiotics prior to large immune response

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rheumatogenic strains of GAS

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more commonly associated with rheumatic fever. May be due to cross reactivity between M proteins and tissues of the heart/brain/synovium etc…

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nonsuppurative sequela of GAS that may follow pharyngitis or pyodermal infection and is unaffected by antibiotic tx

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acute poststreptococcal glomerulonephritis

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possible pathogenesis of acute poststreptococcal GN

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immune complexes of strep proteins and antibodies attach to the glomerular basement membrane and leads to complement fixation resulting in kidney injury

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erysiplelas

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raised bright red patch of skin with sharply demarcated but rapidly advancing margin

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rapid strep test detects

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only group A antigen (GAS)

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you cannot rule out strep throat until what

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a neative throat culture is returned

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how to distinguish asymptomatic GAS carrage in throat with viral pharyngitis from true GAS infection

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you cant

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for diagnosis of ARF and APSGN what evidence do you need, how do you get it

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evidence of preceeding GAS infection. Have to do an antistreptolysin O titer, anti DNase titer, or streptozyme screen

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resistance to penacillin in GAS

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has not yet been seen

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benzathine penacillin

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longeracting penacillin that lowers risk of ARF but is administered in a painful injection

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tx of GAS in penacillin allergic pt

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erythromycin or other macrolide antibiotics

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Group B strep

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beta hemolytic, don’t cause significant disease in healthy people

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GBS normally colonize

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lower GI and female Genital tracts

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GBS are problematic in

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newborns

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when are pregnant women tested for GBS, why?

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week 35-37, if GBS is present during vaginal birth it can endanger the life of the child

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GBS is a leading cause of what in newborns

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sepsis and meningitis

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how do GBS evade immune response

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polysacchride capsule (type 3 most common in neonatal infections)

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streptococci species that are similar to GAS but cannot cause rheumatic fever

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Group C and G (S. dysgalactiae)

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S. pneumoniae has what hemolysis pattern

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alpha

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virdans streptocicci

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primarily alpha hemolytic, grow in oral cavity, are responsible for dental plaques and caries (S mutans and S sobrinus) can seed bloodstream and cause endocarditis

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nutritionally variant streptococci

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difficult to grow in lab, can cause complicated endocarditis

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E faecalis and E faecium

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medically important enterococci that cause UTI, wound infection, endocarditis, abdominal abscess , and bacteremia

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how are enterococci differentiated from streptococci

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resistance to high salt concentrations and bile found in GI

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why are enterococci becoming more of a problem

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hospital acquired oppurtunisitc infections of enterococcus are becoming resistant to the last line antibiotic for Gram + organisms vancomycin (VRE)

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effect of penicillins on enterococci

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bacteriostatic

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synergistic antibiotic regemin to combact enterococci

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penacillin and aminoglycosides

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linezolid

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bacterial protein synthesis inhibitor being used to treat VRE

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daptomycin

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lipopeptide that causes membrane leak, being used to treat VRE

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tigecycline

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tetracycline derivative used for VRE

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dalfopristin-quinuupristin

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antibiotic for VRE with limited usefullness due to emerging resistance and toxicity

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S agalactiae

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hallmark group B strep

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Enterococci were also known as

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Group D strep

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group C and D srep mainly cause

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respiratory and deep tissue infections