Staphylococci Lecture Notes PDF

Dr.zainab alzubaidy Staphylococci Th e staphylococci are gram-positive spherical cells, usually arranged in grapelike irregular clusters. Th ey grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that varyfrom white to deep yellow. Some...

Dr.zainab alzubaidy Staphylococci Th e staphylococci are gram-positive spherical cells, usually arranged in grapelike irregular clusters. Th ey grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that varyfrom white to deep yellow. Some are members of the normal microbiota of the skin and mucous membranes of humans; others cause suppuration, abscess formation, a variety of pyogenic infections, and even fatal septicemia. Th e pathogenicstaphylococci oft en hemolyze blood, coagulate plasma, and produce a variety of extracellular enzymes and toxins. Th e most common type of food poisoning is caused by a heatstable staphylococcal enterotoxin. Staphylococci rapidly develop resistance to many antimicrobial agents, which consequently presents diffi cult therapeutic problems. Th e genus Staphylococcus has at least 40 species. Th e four most frequently encountered species of clinical importanceare Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus lugdunensis , and Staphylococcus saprophyticus. S aureus is coagulase positive , which diff erentiates it from the other species. S aureus is a major pathogen for humans. Almost every person will have some type of S aureus infection Th e coagulase-negative staphylococci are normal human microbiota and sometimes cause infection, oft en associated with implanted devices, such as joint prostheses, shunts, and intravascular catheters, especially in very young, old, and immunocompromised patients. Approximately 75% of these infections caused by coagulase-negative staphylococci are caused by S epidermidis;. S saprophyticus is a relatively common cause of urinary tract infections in young women, although it rarely causes infections in hospitalized patients. Pathogenesis Staphylococci, particularly S epidermidis, are members of the normal microbiota of the human skin and respiratory and gastrointestinal tracts. Nasal carriage of S aureus occurs in 20–50% of humans. Staphylococci are also found regularly on clothing, bed linens, and other fomites in human Environments The pathogenic capacity of a given strain of S aureus is the combined effect of extracellular factors and toxins together with the invasive properties of the strain. At one end of the disease spectrum is staphylococcal food poisoning, attributablesolely to the ingestion of preformed enterotoxin; at the other end are staphylococcal bacteremia and disseminated abscesses in all organs. Pathogenic, invasive S aureus produces coagulase and tends to produce a yellow pigment and to be hemolytic. Nonpathogenic, noninvasive staphylococci such as S epidermidis are coagulase negative and tend to be nonhemolytic.. S saprophyticus is typically nonpigmented, novobiocin resistant, and nonhemolytic; it causes urinary tract infections in young women. Enzymes and Toxins Catalase Staphylococci produce catalase, which converts hydrogen peroxide into water and oxygen. The catalase test differentiates the staphylococci, which are positive, from the streptococci, which are negative. Coagulase and Clumping Factor S aureus produces coagulase, an enzyme-like protein that clots oxalated or citrated plasma. Coagulase binds to prothrombin; together they become.enzymatically active and initiate fibrin polymerization Clumping factor that is responsible for adherence of the organismsto fibrinogen and fibrin. When mixed with plasma, S aureus forms clumps. Clumping factor is distinct from coagulase. Because clumping factor induces a strong immunogenic response in the host, it has been the focus of vaccine efforts. However, no human vaccines against this factor are available Hemolysins