Lippincotts_Illustrated_Reviews_Microbiology_3rd_Edition_by_Richard_A _Harvey_Cynthia_Nau_Cornelissen_Ph D
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rash (not purpuric) and was caused by overproduction of toxic schock syndrome toxin (TSST)-1 by coloniz- ing S. aureus triggered by something in hyperab- sorbent tampons. Many signs and symptoms are the results of the superantigen activity of TSST, which activates a whole subclass of T cells, caus- ing overproduction of cytokines. S. saprophyticus is a frequent cause of cystitis in women, but is not associated with toxic shock syndrome. Correct answer = B. The patient is probably suffer- ing from bacterial endocarditis caused by S. epider- midis infection of the prosthetic heart valve. S. epidermidis is a coagulase-negative organism that is unable to ferment mannitol and is sensitive to novobiocin but usually resistant to penicillin. Patients with congenital heart malformations, acquired valvular defects (for example, rheumatic heart disease), prosthetic valves, and previous bac- terial endocarditis show an increased incidence of bacterial endocarditis. Intravenous drug users also have a high risk for infection. S. pneumoniae and S. agalactiae can be ruled out, because strepto- cocci are catalase negative, which is a feature that distinguishes them from catalase-positive staphyococci. Correct answer = E. Exfoliatin is a virulence factor, produced by some Staphylococcus aureus strains, cleaves desmosomes, resulting in loss of the outer layers of skin. This manifestation is also known as scalded skin syndrome. The toxic shock syndrome toxin is a superantigen produced by some S. aureus strains. This toxin causes systemic effects and has been associated with tampon use. Panton- Valentine Leukocidin is a hemolysin that lyses white blood cells and is produced by many community- acquired MRSA strains. Protein A is a virulence fac- tor that allows S. aureus to evade an immune response by binding the Fc region of IgG, resulting in the inverse orientation of the antibody. Thus, the antibody cannot effectively opsonize the bacterium. The thin microcapsule of S. aureus is also associ- ated with immune evasion. Streptococci 9 Free-living Bacteria Figure 9.1 Classification of streptococci. See pp. 350\u2013351 for summaries of these organisms. Gram-negativeGram-positive RodsCocci Staphylococcus Streptococcus Enterococcus Streptococcus pyogenes Streptococcus agalactiae Enterococcus fecalis Enterococcus faecium Streptococcus pneumoniae Streptococcus bovis Streptococcus mutans S S S S S S 79 I. OVERVIEW Streptococci and staphylococci (see Chapter 8) constitute the main groups of medically important gram-positive cocci. Streptococci are gram- positive, nonmotile, and catalase negative. Clinically important genera include Streptococcus and Enterococcus (Figure 9.1). They are ovoid to spherical in shape and occur as pairs or chains (see Figure 9.16). Most are aerotolerant anaerobes because they grow fermentatively even in the presence of oxygen. Because of their complex nutritional requirements, blood enriched medium is generally used for their isolation. Diseases caused by this group of organisms include acute infections of the throat and skin caused by group A streptococci (Streptococcus pyogenes); female genital tract colonization, resulting in neonatal sepsis caused by group B streptococci (Streptococcus agalactiae); pneumonia, otitis media, and meningitis caused by Streptococcus pneumoniae; and endocarditis caused by the viridans group of streptococci. II. CLASSIFICATION OF STREPTOCOCCI Streptococci can be classified by several schemes, for example, by the hemolytic properties of the organisms, and according to the presence of specific surface antigens determined by immunologic assays. A. Hemolytic properties on blood agar \u3b1-Hemolytic streptococci cause a chemical change in the hemo - globin of red cells in blood agar, resulting in the appearance of a green pigment that forms a ring around the colony (see Figure 9.16). \u3b2-Hemolytic streptococci cause gross lysis of red blood cells, result- ing in a clear ring around the colony (see Figure 9.16). \u3b3-Hemolytic is a term applied to streptococci that cause no color change or lysis of red blood cells. The traditional division of streptococci based on the ability of the bacterial colony to hemolyze erythrocytes in the blood agar medium is still considered the first step in the classification of streptococci. B. Serologic (Lancefield) groupings Many species of streptococci have a polysaccharide in their cell walls known as C-substance, which is antigenic and easily extractable with dilute acid. The Lancefield scheme classifies primarily \u3b2-hemolytic streptococci into groups A through U on the basis of their C-sub- 80 9. Streptococci A Streptococci B C D . . . T U or or or Key: Figure 9.2 Classification schemes for streptococci. Lancefield groups Streptococcus pyogenes Streptococcus agalactiae Streptococcus pneumoniae Streptococcus bovis Streptococcus mutans \u3b1 \u3b1 \u3b1 \u3b2 \u3b2 \u3b2 \u3b1 \u3b3 \u3b3 \u3b3 \u3b1 = \u3b1-Hemolytic = \u3b1 or \u3b3 hemolytic= \u3b2-Hemolytic = \u3b3-Hemolytic Others \u3b3 Figure 9.3 Schematic representation of the streptococcal M protein. M Protein Fimbria Cell membrane Cell wall N-Terminus C-Terminus stance. The clinically most important groups of \u3b2-hemolytic strepto- cocci are Types A and B (Figure 9.2). Commercial kits in which group- specific antisera are coupled to latex beads are now widely used for identification of \u3b2-hemolytic streptococci. III. GROUP A \u3b2-HEMOLYTIC STREPTOCOCCI S. pyogenes, the most clinically important member of this group of gram- positive cocci, is one of the most frequently encountered bacterial pathogens of humans worldwide. It can invade apparently intact skin or mucous membranes, causing some of the most rapidly progressive infections known. A low inoculum suffices for infection. Some strains of S. pyogenes cause postinfectious sequelae, including rheumatic fever and acute glomerulonephritis. Nasopharyngeal carriage is common especially in colder months and particularly among children. Unlike staphylococcal species, S. pyogenes does not survive well in the envi- ronment. Instead, its habitat is infected patients and also normal human carriers in whom the organism resides on skin and mucous mem- branes. S. pyogenes is usually spread person to person by skin contact and via the respiratory tract. A. Structure and physiology S. pyogenes cells usually form long chains when recovered from liq- uid culture (see Figure 9.16), but may appear as individual cocci, pairs, or clusters of cells in Gram stains of samples from infected tis- sue. Structural features involved in the pathology or identification of group A streptococci include: 1. Capsule: Hyaluronic acid, identical to that found in human connec- tive tissue, forms the outermost layer of the cell. This capsule is not recognized as foreign by the body and, therefore, is nonim- munogenic. The capsule is also antiphagocytic. 2. Cell wall: The cell wall contains a number of clinically important components. Beginning with the outer layer of the cell wall, these components include the following (Figure 9.3): a. M protein: S. pyogenes is not infectious in the absence of M protein. M proteins extend from an anchor in the cell mem- brane, through the cell wall and then the capsule, with the N- terminal end of the protein exposed on the surface of the bacterium. M proteins are highly variable, especially the N-ter- minal regions, resulting in over 80 different antigenic types. Thus, individuals may have many S. pyogenes infections throughout their lives as they encounter new M protein types for which they have no antibodies. M proteins are antiphagocytic and they form a coat that interferes with complement binding. b. Group A-specific C-substance: This component is composed of rhamnose and N-acetylglucosamine. [Note: All group A streptococci, by definition, contain this antigen.] c. Protein F (fibronectin-binding protein) mediates attachment to fibronectin in the pharyngeal epithelium. M proteins and lipo - teichoic acids also bind to fibronectin.