As a consequence, no C3b is produced, and the opsonization and phagocytosis of the organisms are greatly reduced. It is an important virulence factor because it binds to the Fc portion of IgG at the complement-binding site, thereby preventing the activation of complement. (1) Protein A is the major protein in the cell wall. aureus has several important cell wall components and antigens: These genes are located in a transposon on a plasmid and encode the enzymes that substitute D-lactate for D-alanine in the peptidoglycan. aureus is the same as the cassette that provides vancomycin resistance in enterococci. The cassette of genes that encodes vancomycin resistance in S. aureus with intermediate resistance to vancomycin (VISA) and with full resistance to vancomycin (VRSA) have also been detected. The most common strain of MRSA in the United States is the “USA300” strain. aureus strains isolated from hospital patients in the United States. MRSA currently accounts for more than 50% of S. These strains are commonly known as methicillin-resistant S. Genes on the bacterial chromosome called mecA genes encode these altered PBPs. aureus are resistant to the β-lactamase–resistant penicillins, such as methicillin and nafcillin, by virtue of changes in the penicillin-binding protein (PBP) in their cell membrane. aureus strains contain plasmids that encode β-lactamase, the enzyme that degrades many, but not all, penicillins. The iron in hemoglobin is recovered by the bacteria and utilized in the synthesis of cytochrome enzymes used to produce energy. aureus is the source of iron required for growth of the organism. Hemolysis of red cells by hemolysins produced by S. aureus usually ferments mannitol and hemolyzes red blood cells, whereas S. Two other characteristics further distinguish these species, namely, S. epidermidis is significantly less than that of S. epidermidis does not synthesize this pigment and produces white colonies. This pigment enhances the pathogenicity of the organism by inactivating the microbicidal effect of superoxides and other reactive oxygen species within neutrophils. aureus produces a carotenoid pigment called staphyloxanthin, which imparts a golden color to its colonies. ( Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.) Arrow points to clotted plasma formed by coagulase produced by S. It is the most common cause of bacterial conjunctivitis.įIGURE 15–6 Coagulase test-Upper tube inoculated with Staphylococcus aureus lower tube inoculated with Staphylococcus epidermidis. It is an important cause of skin infections, such as folliculitis ( Figure 15–3), cellulitis, and impetigo ( Figure 15–4). It is one of the most common causes of hospital-acquired pneumonia, septicemia, and surgical-wound infections. Staphylococcus aureus causes abscesses ( Figure 15–1), various pyogenic infections (e.g., endocarditis, septic arthritis, and osteomyelitis), food poisoning, scalded skin syndrome ( Figure 15–2), and toxic shock syndrome. (2) Biochemically, staphylococci produce catalase (i.e., they degrade hydrogen peroxide), whereas streptococci do not. (1) Microscopically, staphylococci appear in grapelike clusters, whereas streptococci are in chains. Staphylococci and streptococci are nonmotile and do not form spores.īoth staphylococci and streptococci are gram-positive cocci, but they are distinguished by two main criteria: Two of the most important human pathogens, Staphylococcus aureus and Streptococcus pyogenes, are described in this chapter. There are two medically important genera of gram-positive cocci: Staphylococcus and Streptococcus.
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