Lec 5: Agents of Skin Infection I (O'Brien) Flashcards
Alpha hemolysis
Partial blood lysis; hemoglobin leaks out, which leads to a green color on blood-agar plates
Beta hemolysis
Complete blood lysis and heme degradation; produces clear zone around bacterial colony on blood-agar plates
Gamma hemolysis
No hemolysis; aka nonhemolysis
Group A Streptococci
Species: S. pyogenes Hemolysis: Beta Gram (+) and forms in chains Diseases: Pharyngitis, soft tissue infection, toxic shock syndrome, necrotizing fasciitis, acute rheumatic fever, etc Carbohydrate Group A (Lancefield)
M Protein
Major virulence factor for GAS
In Group A Streptococci; anti-phagocytic, adhesin to keratinocytes, attached to cell wall of GAS (Group A Strep), outermost regions are variable in structure
Some M proteins appear similar to host proteins, so cannot use these as vaccine targets and an adaptive immune response to some M proteins may lead to autoimmune disorders
Impetigo
Superficial bacterial infection of the skin, leaves crusty, honey-colored lesions; epidermis alone involved
Commonly seen during warm, moist weather
Associated with s. Aureus and s. Pyogenes
Hyaluronic Acid Capsule
Disguise for the bacteria since HA is common in humans, antiphagocytic
Scarlet Fever
Rash on body coming from pharyngeal infections, caused by bacterial toxins: Streptococcal pyrogenic exotoxins A, B, and C. A and C are superantigens capable of inducing strep toxic shock syndrome
Signs: Circumoral redness, strawberry tongue, desquamation post-rash
Sequelae
Secondary effects, like a disease caused by a preceding disease (ex: acute rheumatic fever)
Acute Rheumatic Fever
Non-suppurative sequela to strep pharyngitis, cause of valvular heart disease, appears 1 to 4 weeks after pharyngitis
Manifestations: Carditis, polyarthritis, chorea (uncontrolled dancing movements), etc. Thought to be an autoimmune disease
Requires long term treatment with Penicillin to prevent future GAS infections
Suppurative
Pus-forming
Acute Poststreptococcal Glomerulonephritis
Occurs 1-4 weeks post strep infection; Caused by only a few M-protein Types, pathogenesis suspected to be immune complex formation at glomeruli, leading to complement activation and degradation.
Manifestations: Blood and elevated protein in urine
Erysipelas
Skin infection causing a raised, bright red patch with sharp edges, dermal lymph tissue involved, commonly on the face
May spread quickly and is dangerous if not treated with Penicillin
Myositis/Necrotizing Fasciitis
Potentially lethal infections of muscle (deep), requires aggressive IV therapy
Rapid Strep Test
Very specific, NOT sensitive, strep ruled out when negative test appears. Detects Group A carbohydrate antigen from bacteria
Group B Strep
Species: S. agalactiae
Hemolysis: Beta and Gamma
Diseases: Neonatal meningitis and sepsis
Neonates at high risk, pregnant women screen for GBS before birth
Uses polysaccharide capsule for evasion of opsonization and phagocytosis
Gram (+) and forms in chains
Carbohydrate Group B Antigen
Soft Tissue Infections:
Exogenous
Endogenous
Toxin-Mediated
Exo - direct invasion of microbe from external environment
Endo - invasion of microbe from internal environment
Toxin - manifestations at sites away from infected area
Cellulitis
Subcutaneous fat layer infection
All Streptococci are:
Gram Positive
Catalase Negative
Hyalouronidase
AKA Spreading Factor, helps bacteria spread by breaking down Hyaluronic Acid in the ECM (made by S. Aureus and S. pyogenes)
Abscess Formations (the 3 types)
Folliculitis (hair follicles), Furuncles (boils), Carbuncles (large boil/series of boils)
Necrotizing Infections
Fasciitis and gas gangrene (caused by Clostridium genus)
Diseases of Staphylococci
Folliculitis, Carbuncles, Furuncles, Impetigo, Cellulitis
Diseases of Streptococci
Impetigo, Erysipelas. Lymphangitis, Cellulitis, Synergistic Cellulitis, Necrotizing Fasciitis
Superantigen Toxins
A toxin released by bacteria which cross-links the T-Cell Receptor and MHC Class II molecules, causing massive cytokine release from T Helper Cells and Macrophages
PANDAS
Pediatric Autoimmune Neuropsychiatric Disorder Associated with group A Streptococci
Five Criteria
1) OCD or tic disorder
2) Pediatric Onset
3) Abrupt onset, episodic course of symptoms
4) Association with GAS infections
5) Association with neurologic abnormalities (tics, motoric hyperactivity (excessive movement), choreiform movements (jerky involuntary movements across whole body)
Penicillin G
Drug of choice for GAS infections
Resident vs. Transient Skin Flora
Resident - long-term bacteria on host, typically not pathogenic
Transient - short-lived bacteria on host, can be pathogenic (S. aureus and S. pyogenes)
PYR Test
Pyrrolidonyl Arylamidase Test
If you already know your culture is streptococcus, you may use this test to determine if it is GAS or not.
(+) = GAS (colony turns red)
