lecture 7.pptx.pdf

Warith Al-Anbiyaa University Lecture 7:Gram-Positive Cocci College of Medicine Department Microbiology- Streptococci Bacteriology  Characteristics: Spherical or oval gram-positive cocci arranged in chains or pairs. They are facultative anaer...

Warith Al-Anbiyaa University Lecture 7:Gram-Positive Cocci College of Medicine Department Microbiology- Streptococci Bacteriology  Characteristics: Spherical or oval gram-positive cocci arranged in chains or pairs. They are facultative anaerobes Grow well on ordinary solid media enriched with blood, serum or glucose as discoid colonies. Capsular streptococcal strains give rise to mucoid colonies They are aerobic bacteria in which growth is enhanced with 10% carbondioxide. They are catalase-negative. They are heterogeneous group of bacteria. Fig. 1 Streptococci The currently used classification is based on : (1) Colony morphology and hemolysis in blood agar (2) Serologic specificity of the cell wall group-specific substances or capsular antigens. (3) Biochemical reactions and resistance to physical and chemical factors. (4) Molecular genetics.  α Hemolysis and β -hemolysis on blood agar Short arrow points to an α -hemolytic colony, probably a viridans group streptococcus Long arrow points to a β -hemolytic colony, probably Streptococcus pyogenes. The specimen was a throat swab taken from a person with a sore throat.  Diseases Str. pyogenes (group A streptococcus) pharyngitis and cellulitis impetigo, necrotizing fasciitis, streptococcal toxic shock syndrome also the inciting factor of two important immunologic diseases, namely, rheumatic fever and acute  Antigenic structure There are two important antigens of β -hemolytic streptococci: 1. Group-specific cell wall antigen Carbohydrate is the basis for serologic grouping of streptococci (Lancefield groups). 2. M protein is the most important virulence factor They are found in hair-like projections protrudes from the outer surface of the streptococcal and it is antiphagocytic. Antibody to M protein provides type-specific immunity. Acid and heat resistance. It’s a major virulent factor for group A beta hemolytic streptococci. Approximately 80 serotypes based on the M protein Strains of Str. pyogenes that produce certain M protein types are rheumatogenic, i.e., cause primarily rheumatic fever, whereas other  Antigenic structure 3. Polysaccharide capsule that plays a role in retarding phagocytosis 1. Protein F( fibronectin-binding proteins) Transmission 2. Most streptococci are part of the normal flora of the human throat, skin, and intestines but produce disease when they gain access to tissues or blood. 3. Viridans streptococci and Str. pneumoniae are found chiefly in the oropharynx; 4. Str. pyogenes is found on the skin and in the oropharynx in small numbers; 5. Str. agalactiae occurs in the vagina and colon. (Group A β-hemolytic streptococci) Streptococcus pyogenes The most pathogenic member of the genus Cause pharyngitis and a very common cause of skin infections. They adhere to pharyngeal epithelium via pili covered with lipoteichoic acid and M protein. Many strains have a hyaluronic acid capsule that is antiphagocytic. The growth of Str. pyogenes on agar plates in the laboratory is inhibited by the antibiotic bacitracin, an important diagnostic criterion. It produces different types of enzymes and exotoxins. Bacitracin test—right : zone of inhibition of growth of group A streptococci (Streptococcus pyogenes) caused by bacitracin that has diffused from the disk labeled left : group B streptococci (Streptococcus agalactiae) and there is no zone of inhibition around the bacitracin disk.  Pathogenesis Group A streptococci (Str. pyogenes) cause disease by three mechanisms: 1. Pyogenic inflammation. 2. Exotoxin production. 3. Immunologic, Group A streptococci produce three important inflammation- related enzymes: 4. Hyaluronidase degrades hyaluronic acid, Hyaluronidase is known as spreading factor because it facilitates the rapid spread of Str. pyogenes in skin infections (cellulitis). 5. Streptokinase (fibrinolysin) activates plasminogen to form plasmin, which dissolves fibrin in clots, thrombi, and emboli.  Group A streptococci produce five important toxins and hemolysins: 1. Erythrogenic toxin (Pyrogenic toxin) causes the rash of scarlet fever. Its mechanism of action is similar to that of the toxic shock syndrome toxin (TSST) of Sta. aureus, i.e., it acts as a superantigen. It is produced by certain strains of Str. pyogenes lysogenized by a bacteriophage carrying the gene for the toxin. The injection of a skin test dose of erythrogenic toxin (Dick test) gives a positive result in persons lacking antitoxin (i.e., susceptible persons). 2. Streptolysin O is a hemolysin that is inactivated by oxidation (oxygen-labile). It causes β-hemolysis only when colonies grow under the surface of a blood agar plate. It is antigenic, and antibody to it (ASO) develops after group A streptococcal infections. The titer of ASO antibody can be important in the diagnosis of rheumatic fever.  1. Streptolysin S is a hemolysin that is not inactivated by oxygen (oxygen- stable). It is not antigenic but is responsible forβ -hemolysis when colonies grow on the surface of a blood agar plate. 2. Pyogenic exotoxin A is the toxin responsible for most cases of streptococcal toxic shock syndrome. It has the same mode of action as does staphylococcal TSST, i.e., it is a superantigen that causes the release of large amounts of cytokines from helper T cells and macrophages. 3. Exotoxin B is a protease that rapidly destroys tissue and is produced in large amounts by the strains of Str. pyogenes, the so-called "flesh-eating" streptococci that cause necrotizing fasciitis. Clinical Findings Str. pyogenes causes three types of diseases: (1) pyogenic diseases such as pharyngitis and cellulitis, (2) toxigenic diseases Sore Throat Streptococcal Infection  Necrotizing Fasciitis Impetigo Scarlet fever Erythema marginatum is a major criterion in the diagnosis of rheumatic fever but it is not always present.  Group B streptococci Str. agalactiae Colonize the genital tract of some women. They are usually bacitracin-resistant. They hydrolyze (break down) hippurate, an important diagnostic criterion. Clinical features Neonatal sepsis, pneumonia, and meningitis The main predisposing factor is prolonged (longer than 18 hours) rupture of the membranes in women who are colonized with the organism and lack antibody to group B streptococci IgGs. Meningitis during the first month of life This organism also causes pneumonia, endocarditis, arthritis, and osteomyelitis in adults. Postpartum endometritis also occurs. Diabetes is the main predisposing factor for adult group B streptococcal infections