Streptococcal Infections (S. pyogenes — Group A streptococci)

Reading Assignments: (1) Text Chapters 12, (2) HANDOUT: Cunha, B. 1999. Patients 35, 36, 37, In: Infectious Disease Pearls, Hanley and Belfus, Inc., Philadelphia, PA., (3) Gilligan, P.H., M.L. Smiley, and D.S. Shapiro. 1997. Cases in Medical Microbiology and Infectious Diseases (2nd ed.), pp. 292-295. American Society for Microbiology, Washington, D.C., (4) Low, D.E., B. Schwartz, and A. McGeer. 1998. The Reemergence of Severe Group A Streptococcal Disease: an Evolutionary Perspective, pp. 106-112. In: Scheld, W.M, D. Armstrong, and J.M. Hughes (eds.) Emerging Infections 1. ASM Press, Washington, D.C.

  1. INTRODUCTION

  2. Streptococci are Gram positive spherical or ovoid cells, arranged in chains or pairs. The genus includes 37 species — all of which are obligate parasites of mucosal membranes (and for some, the tooth surfaces of humans and animals). It includes both commensal microflora on the mucosal membranes of the upper respiratory, intestinal and genital tracts of humans (many of which are opportunistic pathogens) and true pathogens. The type species is S. pyogenes.
  3. CLASSIFICATION OF STREPTOCOCCI (three ways):
  1. Hemolysis Pattern (on blood agar)

    1. beta hemolysis

    2. alpha hemolysis

    3. gamma hemolysis

  2. Lancefield group — Serologically grouped according to their major cell- wall carbohydrate antigen
  3. Species — grouped based on metabolic reactions in culture media
  1. GROUP A STREPTOCOCCUS (S. pyogenes)
    1. Group A Strep Virulence Factors
      1. Cell Surface Structures
        1. Hyaluronic Acid Capsule
          1. antiphagocytic
          2. nonimmunogenic "a self-antigen"
        3. The M protein —the major virulence factor
          1. The ability of S. pyogenes to persist in infected tissues is due to the cell surface-exposed M protein, a molecule which allows it to resist phagocytosis by neutrophils (in the absence of specific antibodies)
          2. It is required for virulence. Bacteria lacking the M protein are readily opsonized by complement (via the alternative pathway), are phagocytosed, and destroyed. Those with the M protein cannot be opsonized by complement (generated by the alternative pathway).
          3. The host must produce antibodies specific for the M protein in order to opsonize and destroy this pathogen
          4. Recent studies indicate the M protein is not an adhesin
          5. There are 100 different serotypes of M proteins, and individuals may suffer recurrent infections with different serotypes of the M protein. Certain M types area associated with invasive disease, rheumatic fever, and glomerulonephritis. (Anti-M antibody is type specific.)
          6. Structure -an alpha helical fibrillar molecule made up of 500 amino acids with its carboxy terminal rooted in the peptidoglycan of the cell wall and the amino terminal region extending toward the surface. Two fibrils of M protein wind together to make up the molecule on the surface of the bacterium. Fibrils protrude up to 200 nm from the cell surface.
          7. Individual M protein molecules are composed of several sequence repeat regions designated by capital letters (A, B, C). There is variability in the sequence of the N terminal half of the molecule and diversity in this area of the molecule is generated by recombination and horizontal gene transfer involving related genes. (The tip of the molecule is a region of hypervariablilty and antibodies against this region effectively opsonize group A streptococci for phagocytosis.) The C-terminal half of the molecule, which includes the wall-spanning region, is more conserved.
          8. Different areas of the M protein molecule help this pathogen escape phagocytosis by avoiding opsonization with complement :
            1. Distal portion of the molecule - binds fibrinogen shielding the pathogen from complement deposition. (As your book points out, luckily, the tips of the M protein molecule protrude past the fibrinogen coat, so that antibody can oponize the pathogen)
            2. Proximal portion of the molecule — binds factor H, a protein that inhibits the C3 convertase from cleaving C3 into C3a and C3b. With high concentrations of factor H bound to the bacterial cell surface any C3 convertase that forms on the cell surface is quickly destroyed.
          10. Some M proteins have receptors for other serum proteins including human fibrinogen, B2 microglobulin, the Fc portion of IgG, and serum albumin. Presumed function — immune cloaking.
          11. M proteins share sequence homology with a number of mammalian proteins. Ex. M5, M6, M12, and M24 proteins share 30% sequence homology with myosin proteins. This cross reactivity may explain the pathogenesis of rheumatic fever (see below).
        5. M-like Surface Proteins
          1. Structural and sequence similarities to M proteins. Like M proteins they are embedded in the bacterial cell wall via the carboxy terminal end with their amino terminal ends exposed on the bacterial sruface.
          2. Many bind the Fc portions of IgG and IgA (can bind nonspecifically to antibodies not directed against their own surfaces). This coats the bacteria with a layer of host proteins and "cloaks them" (makes them less visible) to complement and the immune system.
          3. Like M proteins, some M-like proteins bind other host proteins including fibrinogen, fibronectin, human serum albumin, alpha 2 macroglobulin, or factor H. Function — hiding from the immune system.
          4. The distinction between M proteins and M-like proteins is not always clear.
        7. Protein F (Fibronectin binding protein) - Mediates adhesion to fibronectin-coated epithelial cells.
        8. Lipoteichoic acid — also mediates adhesion
      3. Extracellular Toxins of Group A streptococci (includes Toxins, Hemolysins, and Extracellular Enzymes)
        1. Streptococcal Pyrogenic Exotoxins (Spe)
          1. Members of a family of toxins that are closely related — including staphylococcal enterotoxin serotypes A-F, staphylococcal pyrogenic exotoxins A and B, and staphylococcal toxic shock syndrome (TSS) toxin. Properties of this family of toxins include pyrogenicity and effects on the immune system.
          2. Invasive Group A strep strains produce one or more than one of three different toxins — SPE A, SPEB, SPB C. Many different combinations of these exotoxins are recovered from invasive strains suggesting that no single toxin is responsible for toxic- shock like syndrome (TSLS).
            1. SPE A (erythrogenic toxin, scarlet fever toxin) is produced by strains lysogenized with a temperate bacteriophage. Acts as a superantigen.
            2. SPE B — cysteine protease that cleaves a number of host proteins including fibronectin and vitronectin (2 abundant extracellular matrix proteins). This proteolytic activity is directly responsible for the extensive tissue damage seen in patients with severe invasive and toxic shock-like syndrome., This enzyme also cleaves human interleukin-1 B precursor to generate biologically active IL-1B, suggesting an important role in inflammation and shock. Evidence for its importance in pathogenesis:
              1. An inhibitor of cysteine protease can cure mice of an otherwise lethal dose of S. pyogenes.
              2. Immunization of mice with cysteine protease protects mice against intraperitoneal challenge.
              3. In patients with recent invasive infection there is a significant association between lack of antibody to SPE B and more severe disease course.
            4. SPE C — resembles SPE A in several ways. Associated with lysogeny; biological effects similar to SPE B.
        3. Streptolysins S and O (hemolysins responsible for beta hemolysis on blood agar plates)
          1. Streptolysin O (oxygen labile)
            1. Similar structure and mechanism of action to pneumolysin (S. pneumoniae), tetanolysin (C.tetani), theta toxin (C. perfngens), cereolysin (B. cereus), and lysteriolysin (L. monocytogenes)
            2. Binds to cholesterol on the cell surface — pore forming toxin — binds to the cell membrane causing lysis of leukocytes, platelets, and erythrocytes
            3. Intravenous injection into mice, rabbits and guinea pigs causes death within seconds! Death is the result of an acute toxic action on the heart. Does this toxin play a role in rheumatic fever?
            4. Immunogenic — Serum antibodies to streptolysin O are made after streptococcal infections. The more severe the infection, the higher the titers. Highest titers in rheumatic fever patients. (See diagnosis below)
          3. Streptolysin S (oxygen stable) — responsible for hemolysis seen around colonies of group A streptococci on BAP’s incubated aerobically. Nonimmunogenic. Significance in vivo?
        5. Streptokinase (Fibrinolysin, streptococcal spreading factor), lyses blood clots and may facilitate spread of bacteria in tissues
        6. DNAse
          1. All strains of S. pyogenes form at least one DNAse
          2. Depolymerizes cell-free DNA in purulent material
        8. C5a peptidase
          1. a surface-associated serine protease produced by all strains of S. pyogenes
          2. Cleaves human C5a, one of the main chemoattractants of PMN’s during inflammation
        10. Hyaluronidase
          1. Produced by all strains of S. pyogenes
          2. Believed to facilitate invasion and spread of bacteria
    3. A General Scheme for Group A Streptococcal Pathogenesis (Review Falkow’s

    4. Definition of a Pathogen and Mim’s Obligatory Steps for Infectious Microbes and the Introductory Lectures given in this course (1-12))

      Falkow’s definition of a pathogen (transparency)

      MIM’s - Obligatory Steps for Infectious Microorganisms (transparency)

      Since Group A streptococci cause several different types of diseases — from superficial infections such as impetigo (and pharyngitis) to life threatening systemic disease (such as necrotizing fasciitis and toxic shock), the answer to these questions must be tailored to fit the clinical presentation.

    5. EpidemiologyHow does a person acquire Group A streptococci? What is the reservoir for these organisms?
      1. Group A streptococci transiently colonize the oropharynx and the skin. Carriage rates in children estimated at 15-20% varying with the season.
      2. Disease is usually caused by a recently acquired strain that can establish an infection of the pharynx or skin before specific antibodies (to the M protein) are produced or competitive oranisms can proliferate.
      3. Transmission
        1. Respiratory infections — person to person spread via aerosols (droplets)
        2. Skin infections — through breaks in the skin after direct contact with an infected person, a fomite, or an arthropod vector
        3. Predisposing factors?
    7. Clinical Syndromes
      1. Pharyngitis (remember viruses are the most common cause of acute pharyngitis)
        1. Characterized by pain, redness and swelling of the posterior pharynx, accompanied by a greyish white tonsillar exudate, tenderness of the cervical lymph nodes, fever and general malaise (see Case 1 in text p. 144)
        2. A common infection in school-aged children.
        3. Usually self-limiting although complications can occur:
          1. peritonsillar abscess — quinsy
          2. Otitis media, sinusitis, mastoiditis, caused by local spread of S. pyogenes.
          3. Scarlet fever (see #2, below)
          4. Sequelae - Rheumatic fever (RF) and acute glomerulonerphitis (AGN) (discussed below)
        5. Diagnosis — throat swab and culture on BAP with susceptibility to bacitracin; rapid diagnostic tests based on extraction of the group antigen from throat swabs.
        6. Treatment - Penicillin is given to prevent RF and AGN
      3. Scarlet Fever (See Handout: Patient 37 - Infectious Disease Pearls)
        1. Certain strains of Group A strep produce a pyrogenic exotoxin (SPE A, scarlet fever toxin, erythrogenic toxin) coded for by a lysogenic bacteriophage.
        2. Signs and Symptoms- A few days after the onset of pharyngitis, a diffuse erythematous rash (sandpaper texture) appears on the upper chest and then spreads to the extremities. Pathogenesis? A yellowish white coating initially covers the tongue and is later shed, revealing a red, raw surface beneath —"strawberry tongue". The rash disappears over the next 5 to 7 days and is followed by superficial desquamation of the skin. Rarely seen — a severe form of scarlet fever associated with high fever and systemic toxicity.
        3. Diagnosis made by positive throat culture and the compatible clinical features. Treatment is the same as for streptcoccal pharyngitis — Penicillin
      5. Skin and Soft Tissue Infections (Range from mild and self-limiting to life threatening. As the text points out, pharyngeal infections may be locally very severe (with inflammation, abscess formation, and even necrosis necrosis in and around the tonsils), and there may be occasional spread to the bloodstream —however, spread to adjacent tissues is unusual. This contrasts with skin and soft tissue infections. With the exception of impetigo, which is a localized infection, the other infections of skin and soft tissues are highly invasive infections which spread very rapidly (virulence factors?) We will not review the anatomy we talked about previously — see Lecture 24)
        1. Impetigo- superficial infection of the dermis (caused by Strep and Staph) that resolves spontaneously in 1-2 weeks. Begins as a pinpoint papule, progresses to vesicles, which become pustular and crust over. Vesicles may rupture and ulcerate.
          1. Epidemiology, Who gets impetigo?
            1. associated with warm, humid climates
            2. can occur in Northern climates — pre-school children, prison inmates, residents of mental institutions, and workers handling raw meat.
            3. predisposing factor — poor hygiene
          3. pathogenesis
            1. The infection has been studied experimentally. Can develop within 10 days of skin colonization; transfer to the nose and throat can occur within 14 and 20 days respectively following initial skin colonization.
            2. Lesions thought to arise when colonizing organisms are introduced into broken skin via minor trauma or insect bites.
            3. Associated with restricted M types (different than those that normally colonize the pharyx)- display a greater variety of M types than throat colonizing strains.
            4. The danger of impetigo — can be followed by acute glomerulonephitis (not rheumatic fever).
        3. Erysipelas- an acute spreading inflammation of the dermis. Lesion is erythematous, swollen, has a well defined edge, and may be accompanied by symptoms of fever and chills.
          1. Pathogenesis? Questions you might ask to understand the pathogenesis of this clinical presentation? What is known:- Norrby (1992) studied strains isolated from erysipelas
            1. Predominance of M1 serotype
            2. Low level production of erythrogenic toxin (Spe A) but high level production of Spe B and Spe C.
            3. Higher frequency of polymorphism in the genes for the M protein.
            4. may be preceded by URI or infection of broken skin, or many times etiology is unknown.
            5. lesions usually resolve in 2-14 days; not uncommon for patients to have repeated attacks in the same area of skin.
        5. Cellulitis — infection of the subcutanous tissue caused when streptococci gain access to broken skin.
          1. Develops rapidly progressing from minor trauma to severe septicemia in 24-48h.
          2. Rapidly spreading inflammation
            1. localized symptoms of pain, swelling and redness of the skin (not raised or clearly demarcated like erysipelas), may have local desquamation of the skin.
            2. accompanied by fever, chills, lymphangitis (inflammation of a lymphatic vessel or node) ( indicative of spreading!!), and progressing to bacteremia (blood cultures +).
            3. May be accompanied by toxic shock
            4. Case studies about cellulitis (Case 9 and Patient 35)
            5. Who gets these infections?
              1. Most are previously healthy adults
              2. Predisposing factors
                1. children with chickenpox
                2. women — radical mascetomy — recurrent arm cellulitis
                3. coronary bypass patients — recurrent leg cellulitis
        7. Necrotizing fasciitis- "the flesh eating infection" — deeper infection of the soft tissue characterized by extensive destruction of fat and muscle that spreads along planes of fascia.
          1. Symptoms — introduction of streptococci into the skin — 24 h later, the infection starts out like cellulitis (redness and swelling) although the pain experienced seems out of proportion with the findings in the tissue.
          2. Within 24-48 h the erythema darkens to a purplish than blue hue and bullae appear. Gangrene and systemic symptoms then develop. Unlike cellulitis, which can be treated with antibiotics alone, necrotizing fasciitis must be treated aggressively with the surgical debridement of nonviable tissue.
        9. Severe streptococcal infection accompanied by Stretococcal Toxic Shock Syndrome (STSS). (See Case study 64 on the Web and in the Science Library)
          1. Necrotizing fasciitis and STSS may occur together and are the most serious manifestations of invasive GAS infection. Common additional symptoms are fever, tachycardia and hypotension. Shock, DIC, renal failure and acute respiratory distress syndrome (multiorgan system failure) follows. In spite of appropriate treatment - fatality rate 30-50%.
          2. If you compare TSS caused by S. aureus, with STSS caused by group A strep, S. aureus is localized at the site of infection (in a tampon, or an infected wound), while S. pyogenes is invasive, moving from the original site of infection into the bloodstream producing a systemic infection (bacteremia, septicemia, + blood cultures). Even though these two organisms produce similar toxins, death rates are 10 fold higher for STSS compared with TSS.
          3. Epidemiology
            1. Increasing in frequency around the world. Outbreaks of disease, including Austin (why?).
            2. Occurs most often in previously healthy adults.
            3. Predisposing conditions (those with increased risk)?
              1. 40% of children have concurrent varicella infections.
              2. Adults — skin disorders, chronic heart and lung disease, diabetes, and malignancy (most common underlying conditions).
          5. Pathogenesis? (You give me the step by step pathogenesis) How can you explain the recent increase in severe STSS? Strains of S, pyogenes with increased virulence? (Virulent clones associated with certain M types cause outbreaks? This was not the case in Austin)
          6. Host factors?
            1. Predisposing factors?
            2. A susceptible host population (without antibodies against the M protein or the Spe toxins?). Host antibodies to M protein will abort infections, but antibodies to Spe will allow for invasive infections but with less severe disease.
          8. Bacterial virulence factors? These rapid and devastating infections are associated with certain M types (1,3, 18) and with the Spe toxins ( Spe A and SpeB). How do these toxins produce shock and death?
      7. Other Suppurative Diseases
        1. Puerperal or childbed fever — Text Case 2, p. 144. A historical case from 1846 — Group A strep infections were the cause of postpartum endomyometritis and sepsis- spread by the hands of physicians who had performed autopsies and then came and delivered babies without washing their hands. One of the first examples of infection control when Ignaz Semmelweiss required doctors to disinfect their hands with chloride of lime.
        2. Bacteremia
        3. Pneumonia
      9. Postreptococcal (Nonsuppurative) Diseases — Disorders in which local infection with Group A strep is followed 1-4 weeks later by inflammation in an organ that was not infected with streptococci
        1. Rheumatic Fever- occurs when preceded by pharyngitis
          1. Symptoms include fever, malaise, polyarthritis, and evidence of inflammation of all parts of the heart. Leads to thickened and deformed heart valves and granulomas in the myocardium which are finally replaced by scar tissue. Patients may later develop subacute bacterial endocarditis.
          2. Epidemiology — increasing since the mid 1980’s, clusters seen in school children and military camps. Associated with certain M types ( the return of strains able to cause the condition?).
          3. Pathogenesis- due not to bacterial infection of the heart but to some direct or indirect effect of circulating bacterial products. Hypotheses:
            1. Autoimmune explanation-M protein serotypes found on rheumatogenic strains have epitopes that that cross react with epitopes on cardiac mysoin. T cells or antibodies that recognize these epitopes could attack heart tissue and cause an inflammatory response that damages heart valves.
            2. Direct damage to heart tissue by toxins.
        3. Glomerulonephritis- occurs when preceded by pharyngitis or a skin infection
          1. Symptoms: renal glomerular damage, hypertension, hypertension, edema, and proteinuria and hematuria
          2. Associated with nephritogenic strains;
          3. Pathogenesis: Hypotheses
            1. Evidence supports that renal damage is the result of immune complex deposition on the glomerular basement membrane and complement activation that generates a massive inflammatory response.
            2. Direct damage to kidney by toxins.
    9. Laboratory Diagnosis
      1. Microbiologic
        1. Specimens- throat swab, pus, or blood, CSF, etc. is obtained for culture. Serum is obtained for antibody determination.
        2. Direct smears- not helpful for throat swabs because hemolytic streptococci are always present and look the same as Group A strep on stained smears. Are helpful for other specimens.
        3. Antigen Detection Kits- for rapid detection of group A strep from throat swabs. Kits use enzymatic or chemical methods to extract antigens from the swab, then use ELISA or agglutination tests to demonstrate the presence of the antigen. (60-90% sensitive and 98-99% specific when compared to culture.)
        4. Culture- 2 BAPs incubated in 10% CO2 and anaerobically. Look for hemolysis and appearance of colonies. Presumptive I.D.- inhibition of growth by bacitracin. Definitive I.D. with direct FA test, or by rapid tests specific for the group A carbohydrate antigen.
      3. Serologic: A rise in titer of anti-Streptolysin O antibodies (ASO).
    11. Treatment: All group A strep are sensitive to penicillin G and most are sensitive to erythromycin. Antimicrobial drugs have no effect on established rheumatic fever or glomerulonephritis. (This is why its important to rapidly eradicate streptococci from the patient with an acute streptococcal infection (before day 8). Group A strep infections are self- limiting but patients are treated to prevent these late sequelae.)
    12. Immunity Resistance against Group A strep disease is correlated with antibody type specific antibody production against the M protein. Immunity to scarlet fever is against the specific exotoxin that caused the rash. Immunity to skin and puerperal sepsis disease does not occur. Reoccurences of RF are frequent; any of the other M serotypes can initiate the preceding pharyngitis. Reocurrences of AGN are rare due to the limited number of nephritogenic strains.

    13.  

       

    14. Prevention and Control
      1. No available vaccine- problems include multiple M protein types, and the potential for cross reaction with shared epitopes in the host.
      2. Control - Use penicillin to eliminate cases and carriers during community outbreaks. Chemopropylaxis for prevention of recurrent rheumatic fever prior to dental or other surgical procedures.

    SEE CHAPTER 12 IN THE TEXT FOR DISCUSSION OF THE STREPTOCOCCI OUTLINED BELOW:

  3. GROUP B STREPTOCOCCUS (S. agalactiae)
    1. Epidemiology :Inhabitants of the lower GI tract and female genital tracts (vaginal colonization). common).
    2. The etiological agent of:
      1. Postpartum and neonatal infections: Neonatal sepsis and meningitis (the leading cause). Transmission occurs from mother to child at birth; isolated from the mucous membranes and skin of newborns.
      2. Cellulitis, arthritis and meningitis in adults- usually seen in older adults with predisposing conditions such as diabetes mellitius, cancer and HIV infection.
    4. Virulence is associated with a polysaccharide capsule (Serotype III associated with neonatal disease)
  5. GROUP D STREPTOCOCCUS (Enterococci (S. faecalis is now Enterococcus faecalis) and Nonenterococci (S. bovis)
    1. Part of the normal flora of the gastrointestinal tract.
    2. Text: "Enterococci - the world’s toughest pathogen"; resistant to penicillin and many antibiotics including vancomycin (VRE = vancomycin resistant enterococci —"the nosocomial pathogen of the 90’s"). Transfer of the gene for vancomycin resistance to S. aureus demonstrated in the laboratory.
    3. Cause urinary, biliary and cardiovascular infections (much lower virulence than other streptococci).
  7. VIRIDANS STREPTOCOCCI (alpha hemolytic)
    1. Inhabitants of the normal oropharynx (make up 30-60% of the normal flora)
    2. Etiological agents of :
      1. Dental caries — S. mutans
      2. Infective Endocarditis — the heart is seeded by bacteria during transient bacteremia; infection involves the endothelial lining of the heart, usually the heart valves. (See Text chapter 64 and Case 36 — Infectious Disease Pearls)
      3. Infection of native heart valves
        1. Acute bacterial endocarditis — 60% S. aureus
        2. Subacute bacterial endocarditis
          1. associated with viridans streptococci (S. sanguis, S. mutans, and S. mitior).
          2. Pathogenesis - streptococci enter the bloodstream during a dental procedure (teeth cleaning) or flossing, and adhere to aggregates of platelets and fibrin on damaged heart valves, mutiply and attach further fibrin and platelet deposition. The bacteria are protected from host defenses and can grow to several centimeters in size. A slow process — taking approx. 5 weeks before symptoms appear.
          3. Symptoms - fever, a heart murmur, and nonspecific malaise.
          4. Blood culture — most important laboratory test.
          5. Endocarditis in Intravenous Drug users — S. aureus
          6. Prosthetic valve endocarditis — S. epidermidis, S. aureus, Gram negative rods, and fungi
          7. THE MORTALITY OF INFECTIVE ENDOCARDITIS IS 20- 30% DESPITE ANTIBIOTIC TREATMENT. Complete eradication can take several weeks because
            1. Organisms are relatively inaccessible within the vegetations both to the antibiotics and the host defenses
            2. Organisims grow to a high population density, and multiply relatively slowly.