Study of Skin Infections by Streptococcus Pyogenes PDF

Study of Skin Infections - Streptococcus pyogenes  Introduction Streptococcus pyogenes (group A Streptococcus) is one of the most important bacterial causes of skin and soft tissue infections (SSTIs) worldwide. Specifically, this organism causes infections in...

Study of Skin Infections - Streptococcus pyogenes  Introduction Streptococcus pyogenes (group A Streptococcus) is one of the most important bacterial causes of skin and soft tissue infections (SSTIs) worldwide. Specifically, this organism causes infections in the superficial keratin layer (impetigo), the superficial epidermis (erysipelas), the subcutaneous tissue (cellulitis), the fascia (necrotizing fasciitis), or muscle (myositis and myonecrosis). It is also the etiologic agent of scarlet fever and Streptococcal Toxic Shock Syndrome (StrepTSS).  Streptococcus pyogenes Morphology - Streptococci are spherical or ovoid and are arranged in chains. - Gram positive - Nom-motile, nonsporulating - Shows presence of Hyaluronic acid Capsule - The S pyogenes cell wall contains proteins (M, T, R antigens), carbohydrates (group specific), and peptidoglycans.  Cultural characterictics Streptococci are generally grown on agar media supplemented with blood.(Blood agar) at 37 for 48hrs Shows beta hemolysis Growth and hemolysis are aided by incubation in 10% CO2 S pyogenes strains, giving rise to either matte or glossy colonies. facultative anaerobes and grow under aerobic and anaerobic conditions.  Virulent Factors -Antigenic Structure 1.M protein - This substance is a major virulence factor of S pyogenes. - M protein is a filamentous structure anchored to the cell membrane that penetrates and projects from the streptococcal cell wall. - resist phagocytosis - there are more than 150 types of M protein, a person can have repeated infections with S pyogenes of different M types.  Virulent Factors -Enzymes and Toxins 1.Streptokinase(Fibrinolysin) - It activates and transforms the plasminogen of human plasma into plasmin, an active proteolytic enzyme that digests fibrin and other proteins. - This allows the bacteria to escape from blood clots.  Virulent Factors -Enzymes and Toxins 2.Deoxyribonucleases - Streptococcal deoxyribonucleases degrade DNA (DNases) and similar to streptokinase facilitate the spread of streptococci in tissue by liquefying pus. - This enzymatic activity can be measured by the decrease in the viscosity ok known DNA solutions.  Virulent Factors -Enzymes and Toxins 3.Hyaluronidase - Hyaluronidase splits hyaluronic acid, an important component of the ground substance of connective tissue - hyaluronidase aids in spreading infecting microorganisms (spreading factor). - Hyaluronidases are antigenic and specific for each bacterial or tissue source. After infection with hyaluronidase-producing organisms, specific antibodies are found in the serum.  Virulent Factors -Enzymes and Toxins 4.Pyrogenic exotoxins (Erythrogenic toxins) - The streptococcal pyrogenic exotoxins have been associated with streptococcal toxic shock syndrome and scarlet fever. - There are three antigenically distinct streptococcal pyrogenic exotoxins (Spe): A, B, and C. - The pyrogenic exotoxins act as superantigens, which stimulate T cells by binding to the class II major histocompatibility complex in the Vβ region of the T-cell receptor. - The activated T cells release cytokines that mediate shock and tissue injury.  Virulent Factors -Enzymes and Toxins 5.Hemolysins - The β-hemolytic group A S pyogenes elaborates two hemolysins (streptolysins) that not only lyse the membranes of erythrocytes but also damage a variety of other cell types 1.Streptolysin O is a protein that is hemolytically active in the reduced state (available– SH groups) but rapidly inactivated in the presence of oxygen. It combines quantitatively with antistreptolysin O (ASO), an antibody that appears in humans after infection with any streptococci that produce streptolysin O. An ASO serum titer in excess of 160–200 units is considered abnormally high and suggests either recent infection with S pyogenes or persistently high antibody levels caused by an exaggerated immune response to an earlier exposure in a hypersensitive person  Virulent Factors -Enzymes and Toxins 5.Hemolysins 2.Streptolysin S is the agent responsible for the hemolytic zones around streptococcal colonies growing on the surface of blood agar plates. -It is elaborated in the presence of serum—hence the name streptolysin S. It is not antigenic. Most isolates of S pyogenes produce both of these hemolysins. Up to 10% produce only one  Pathogenesis and Clinical Findings There are variety of different types of skin and soft tissue infections caused by Strep. pyogenes 1)Impetigo Superficial small rash that merge together. Initially a vascular infection that rapidly evolves into pustules and ruptures later. Honey coloured crusty appearance.  Clinical Findings 2)Ecthyma Impetigo turns into ecthyma by ulceration. Highly contagious. 3)Folliculitis Inflammation at the hair follicule that causes erythematous papules and pustules surrounding individual hair 4)Pustules Begin as vesicles that rupture creating circular erythematous ledions with adherent crust. Impetigo Ecthyma Folliculitis  Clinical Findings 5)Furuncle Deep seated inflammatory nodules with a pustule centre that develops around hair(painful localized abscess)which later starts becoming hard. 6)Macule Just discolouration not a raised area other than normal skin 7)Papule Discolouration and raised portion. 8)Vesicle Transparent boil with water in it. Papule Macule  Clinical Findings 9)Nodule Hard to touch. 10)Carbuncles Involvement of several adjacent follicles with pus discharging from multiple follicule orifices. 11)Cutaneous abscesses Painful ,bright fluctuant, red, tender swelling on which rest a pustule. Cutaneous abscess Cellulitis  Cellulitis Erysipelas  Clinical Findings 12)Erysipelas Erythema and swelling of cutaneous surfaces involves the superficial dermis(redness due to inflamed walls of lymphatic vessels) raised leisions. 13)Cellulitis Erythematous, hot swollen skin with irregular edge. Effects deeper epidermis and subcutaneous fat.(Caused by Streptococci or Staphylococci) 14)Acne Infection of sebaceous follicle with plunge of keratinin blocking the sebaceous canal resulting in black heads.  Clinical Findings 15)Necrotizing fasciitis (streptococcal gangrene) Rapidly spreading cellulitis with necrosis(skin and deeper fascia and can involve muscele). The group A streptococci that cause necrotizing fasciitis have sometimes been termed flesh-eating bacteria. Amputation- part infected need to be cut. Begin with fever, systemic toxicity, severe pain in the development of painful ,red swelling that rapidly progress to necrosis of subcutaneous tissue and overlaying skin.  Clinical Findings 16) Bacteremia or sepsis —Infection of traumatic or surgical wounds with streptococci results in bacteremia, which can rapidly be fatal. S pyogenes bacteremia can also occur with skin infections, such as cellulitis.  Lab Diagnosis 1)Specimen Collection A skin swab and pus sample is collected. Serum is obtained for antibody determinations. 2)Smear preparation Smears from pus samples are prepared and Gram stained. Typical cocci observed arranged in chains. Sometimes single cocci or pairs are observed.  Lab Diagnosis 3)Culture -Specimens suspected of containing streptococci are cultured on blood agar plates. Sheep blood infused blood Agar is used(SIBA). -Incubation in 10% CO2 often speeds hemolysis. -group A may be presumptively identified by inhibition of growth by bacitracin 4)Catalase test -This test is used to detect the presence of cytochrome oxidase enzymes. -A drop of 3% hydrogen peroxide solution is placed on a slide, and a small amount of the bacterial growth is placed in the solution. -No reaction indicates a negative test result.  Lab Diagnosis 3)Culture -Specimens suspected of containing streptococci are cultured on blood agar plates. Sheep blood infused blood Agar is used(SIBA). -Incubation in 10% CO2 often speeds hemolysis. -group A may be presumptively identified by inhibition of growth by bacitracin 4)Catalase test -This test is used to detect the presence of cytochrome oxidase enzymes. -A drop of 3% hydrogen peroxide solution is placed on a slide, and a small amount of the bacterial growth is placed in the solution. -No reaction indicates a negative test result.  Lab Diagnosis 4) Antigen Detection Tests -Several commercial kits are available for rapid detection of group A streptococcal antigen -These kits use enzymatic or chemical methods to extract the antigen from the swab, then use enzyme immunoassay (EIA) or agglutination tests to demonstrate the presence of the antigen.  Treatment -All S pyogenes are susceptible to penicillin G. Macrolides, such as erythromycin and clindamycin -Mild cellulitis can be treated with oral antibiotics, including penicillin, cephalosporins (e.g., cephalexin), dicloxacillin, or clindamycin. If signs of systemic infection are present, then intravenous antibiotics can be considered, such as penicillin, ceftriaxone, cefazolin, or clindamycin. THANK YOU!