The Streptococci
The Streptococci are gram –positive, spherical bacteria that characteristically form pairs or chains during growth. They are widely distributed in nature. Some are members of the normal human flora (especially mouth and throat); others are associated with important human diseases attributed in part to infection by streptococci (either opportunistic infection, or as primary infection), in part to sensitization to them. Streptococci elaborate a variety of extracellular substance and enzymes.
*most common Streptococcal pathogens in humans are; Streptococcus pyogens, Str.agalactiae, viridance group, Str.faecalis, and Str.pneumonia.
Classification of Streptococci
1- According to hemolysis on blood agar;
A- Beta-hemolysis: is associated with complete lysis (clear zone) of red blood cells surrounding the colony due to production of two hemolysins: streptolysin O, which is antigenic and oxygen labile (inactivated by oxygen), and streptolysin S, which is non-antigenic, but oxygen stable.
B- Alpha- hemolysis: some strains that do not produce soluble hemolysin, cause partial clearing, and often a green coloration.
C- Gamma hemolytic: actually they are not hemolytic, and give no obvious change around colonies on blood agar.
Hemolysis is affected by the species and age of red blood cells as well as by other properties of base medium. Group A Streptococcus Beta-hemolytic; group B could manifest alpha, beta, or gamma Beta-hemolysis. Most strains of Str.pneumonia are alpha hemolytic. Most of the oral streptococci and enterococci are non-hemolytic (gamma hemolytic). The property of hemolysis is not very reliable for the absolute identification of Streptococci, but is widely used in rapid screens for identification of Stre.pyogens and Str.pneumonia.
2- Lance field classification: groups based on serological identification of group-specific C-carbohydrates on cell wall. Her scheme includes 20 sero groups designated sequentially with the letters A-H and K-V. Group B Streptococcus are a leading cause of neonatal sepsis and meningitis.
The beta-hemolytic strains of group A causes many important human infections and named Streptococcal pyogens .The important characteristic of them are; Gram-positive spherical occurs in chains or pairs of cells, 0.6-1.0micrometer in diameter, it is facultative anaerobes, optimum temp. Of growth 37?C. grow best on enriched medium contains blood or serum in which exhibit beta (clear) hemolysis. Group A streptococci typically have a capsule composed of hyaluronic acid it is. Catalase negative, and insoluble in bile.
Identification of group A Streptococcal pyogen in the lab. The growth of group A Streptococcal pyogen can be inhibited by a low concentration of (0.02 to 0.04 units) of bacitracin in paper disks on blood agar medium in which the zone of inhibition is more than 10mm, while in other Streptococci needs more concentration of) of bacitracin also called ?A disk?
Streptococcal pyogens- Lance field group A is found in 5-15% of normal individual harbors in the respiratory tract as a normal flora, but when the host defense mechanism is compromised , or when the organism is able to exert its virulence, or when it is introduced into vulnerable tissues or hosts, an acute infection will occur. E.g. of suppurative infection;* erysipelas=well-demarcated swollen, erythematous area on the face or legs, associated with fever, headache and swollen lymph nodes.
Impetigo=honey –crust on erythematous base regional lymphadenopathy. *
*cellulites,* pharyngitis *necrotizing fascitis=extensive tissue distuction+fever+systemic toxicity.
Toxin –mediated disease; *scarlet fever: child with strawberry toung and diffuse erythematous rash over the body within 24-48hours after onset of streptococcal pharangitis, rash fades in 5-7days with desquamation.
*streptococcal toxic shock syndrome
Autoimmune sequalae; *acute glomerulonephritis *rheumatic fever.*erythema nodosum.
Virulence factors:-
Extracellular products: invasins and exotoxins
Colonization of the upper respiratory tract and acute pharyngitis may spread to other portions of the upper or lower respiratory tracts resulting in infections of the middle ear (otitis media), sinuses (sinusitis), or lungs (pneumonia). In addition, meningitis can occur by direct extension of infection from the middle ear or sinuses to the meninges or by way of bloodstream invasion from the pulmonary focus. Bacteremia can also result in infection of bones (osteomyelitis) or joints (arthritis). During these aspects of acute disease the streptococci bring into play a variety of secretory proteins that mediate their invasion.
For the most part, streptococcal invasins and protein toxins interact with mammalian blood and tissue components in ways that kill host cells and provoke a damaging inflammatory response. The soluble extracellular growth products and toxins of Streptococcus pyogenes have been studied intensely. Streptolysin S is an oxygen-stable leukocidin; Streptolysin O is an oxygen-labile leukocidin. NADase is also leukotoxic. Hyaluronidase (the original
spreading factor ) can digest host connective tissue hyaluronic acid as well as the organism s own capsule. Streptokinases participate in fibrin lysis. Streptodornases A-D possesses deoxyribonuclease activity; Streptodornases B and D possess ribonuclease activity as well. Protease activity similar to that in Staphylococcus aureus has been shown in strains causing soft tissue necrosis or toxic shock syndrome. This large repertoire of products is important in the pathogenesis of S. pyogenes infections. Even so, antibodies to these products are relatively insignificant in protection of the host.
Three pyrogenic exotoxins (formerly known as Erythrogenic toxin) of S. pyogenes (SPEs) are recognized: types A, B, C. these toxins act as superantigens by a mechanism similar to those described for staphylococci. As antigens, they do not requiring processing by antigen presenting cells. Rather, they stimulate T cells by binding class II MHC molecules directly and nonspecifically. With superantigens about 20% of T cells may be stimulated (vs 1/10,000 T cells stimulated by conventional antigens) resulting in massive detrimental cytokine release. SPEs A and C are encoded by lysogenic phages; the gene for SPE B is located on the bacterial chromosome. Re-emergence in the late 1980 s of these exotoxin-producing strains has been associated with a toxic shock-like syndrome similar in pathogenesis and manifestation to staphylococcal toxic shock syndrome and other forms of invasive disease associated with severe tissue destruction.
Pathogenesis
The cell surface of Streptococcus pyogenes accounts for many of the bacterium’s determinants of virulence, especially those concerned with colonization and evasion of phagocytosis and host immune responses. Cell surface components include capsular polysaccharide, peptidoglycan and lipoteichoic acids, and certain membrane proteins, in addition to several structural proteins (Figure ).
In Group A streptococci, the R and T proteins are used as epidemiologic markers and have no known role in virulence. The M proteins are clearly virulence factors associated with both colonization and resistance to phagocytosis. More than 50 types of S. pyogenes M proteins have been identified on the basis of antigenic specificity, and it is the M protein that is the major cause of antigenic shift and antigenic drift in the Group A streptococci. The streptococcal M protein, peptidoglycan, N-acetylglucosamine, and group-specific carbohydrate portions of the cell surface all have antigenic epitopes that mimic those of mammalian muscle and connective tissue. The cell surface of recently emerging (³flesh-eating²) strains of streptococci is distinctly mucoid (indicating that they are highly encapsulated) and rich in M protein. Protein F, thought involved in attachment to fibronectin, is presumably a nonfimbrial adhesin located on the bacterial cell surface.
The capsule of S. pyogenes is non antigenic since it is composed of hyaluronic acid, which is chemically similar to that of host connective tissue. This allows the bacterium to hide its own antigens and to go unrecognized as antigenic by its host. The cytoplasmic membrane of S. pyogenes contains some antigens similar to those of human cardiac, skeletal, and smooth muscle, heart valve, fibroblasts, and neuronal tissues, resulting in molecular mimicry and a tolerant or suppressed immune response by the host.
Colonization of tissues by S. pyogenes is thought to result from a failure in the innate defenses (normal flora and other nonspecific defense mechanisms) which allows establishment of the bacterium at a portal of entry (often the upper respiratory tract or the skin) where the organism multiplies and causes an inflammatory purulent lesion. Some strains of streptococci show a predilection for the respiratory tract; others, for the skin. Generally, streptococcal isolates from the pharynx and respiratory tract do not cause skin infections.
There is abundant evidence that Streptococcus pyogenes utilizes lipoteichoic acids in the cell wall as adhesins. The lipoteichoic acid (LTA) is anchored to proteins on the bacterial surface, including the M protein. Both the M proteins and lipoteichoic acid are supported externally to the cell wall on fimbriae and appear to mediate bacterial adherence to host epithelial cells. It was been proposed that both LTA and the M protein are needed for attachment to mucosal surfaces and that this explains the role of the M protein as a determinant of virulence (Nonetheless, the M protein is a proven determinant of virulence since it inhibits phagocytic ingestion of non-opsonized streptococci.). A nonfimbrial protein (Protein F) has also been shown to mediate streptococcal adherence to the amino terminus of fibronectin on mucosal surfaces.
Diagnosis: 1-direct smear for gram stain 2-culture on blood agar plates 3-arise in the titer of antibodies to many group A streptococcal antigens can be estimated. Such as antibodies include antistreptolysinO (ASO), particularly in respiratory disease; anti-DNase and antihyaluronidase, particularly in skin infections; antistreptokinase, anti M type-specific antibodies; and others. Of these, the anti-ASO titer is most widely used.
Treatment: All S.pyogens are susceptible to penicillin G, and most are susceptible to erythromycin. Some are resistant to tetracyclines. Antimicrobial drugs have no effects on established glomerulonephritis and rheumatic fever. In acute streptococcal infections, however, every effort must be made to rapidly eradicate streptococci from the patient, eliminate the antigenic stimulus (before day 8), and thus prevent post streptococcal diseases. Dose penicillin or erythromycin that results in effective tissue levels for 10 days usually accomplishes this. Antimicrobial drugs are also very useful in preventing reinfection with B-hemolytic group A streptococci in rheumatic fever patients.