Without question, S. aureus is the most virulent species of staphylococci encountered. A wide spectrum of factors, not all of which are completely understood, contribute to this organism’s ability to cause infections and disease.

S. aureus and S. epidermidis produce a polysaccharide capsule that inhibits phagocytosis. The capsule, which is produced in various amounts by individual clinical isolates, may appear as a slime layer or biofilm, allowing the organisms to adhere to inorganic surfaces and circum venting the actions of antibiotics. The gram-positive cell wall chemical composition is also implicated in the mediation of pathogenesis. The peptidoglycan resembles the endotoxin effect of gram negatives by activating complement, interleukin 1 (IL-1), and acting as a chemotactic factor for the recruitment of PMNs. This cascade of events causes swelling and may lead to the exacerbation of tissue damage because of the additional virulent factors produced by the organisms. S. aureus produces a surface protein, known as protein A. This protein is bound to the cytoplasmic membrane of the organism and has a high affinity for the Fc receptor on IgG molecules as well as complement. This provides a mechanism for the organisms to bind the immune active molecules, decreasing the ability for clearance of the organism from the site of infection. Several toxins and enzymes mediate tissue invasion and survival at the infection site (Table 1). Cytotoxins alpha, beta, delta, and gamma are produced by a variety of species. Most strains of S. aureus produce alpha toxin, which disrupts the smooth muscle in blood vessels and is toxic to erythrocytes, leukocytes, hepatocytes, and platelets. Beta toxin, believed to work in conjunction with the alpha toxin, is a heat-labile sphingomyelinase, which catalyzes the hydrolysis of membrane phospholipids resulting in cell lysis. S. aureus, S. epidermidis, and S. haemolyticus have been identified as capable of producing Delta toxin, which is cytolytic to erythrocytes and demonstrates nonspecific membrane toxicity to other mammalian cells. Gamma toxin is produced by all strains of S. aureus and may actually function in association with the Panton-Valentine leukocidin (PVL). Elaboration of these factors is chiefly responsible for the various skin, wound, and deep tissue infections commonly caused by S. aureus. Many of these infections can rapidly become life threatening if not treated and managed appropriately.

Table1. Pathogenesis and Spectrum of Diseases

Thirty to fifty percent of all S. aureus strains are capable of producing one of eight distinct serologic types of a heat-stable enterotoxin. The enterotoxins are resistant to hydrolysis by the gastric and intestinal enzymes. The toxins, which are often found in milk products, are associated with pseudomembranous enterocolitis and toxic shock syndrome, and they may exacerbate the normal immune response, resulting in further tissue damage and systemic pathology.

Localized skin or soft tissue infections (SSTIs) may involve hair follicles (i.e., folliculitis) and spread into the tissue causing boils (i.e., furuncles). More serious, deeper infections result when the furuncles coalesce to form carbuncles. Impetigo, the S. aureus skin infection involving the epidermis, is typified by the production of vesicles that rupture and crust over. Regardless of the initial site of infection, the invasive nature of this organism always presents a threat for deeper tissue invasion, bacteremia, and spread to one or more internal organs including the respiratory tract. Furthermore, these serious infections have emerged more frequently among the general population and are associated with strains that produce the PVL toxin. PVL is toxic to white blood cells, preventing clearance of the organism by the immune system. These serious soft tissue “community-associated” infections are frequently mediated by methicillin-resistant S. aureus (community-acquired MRSA or CA-MRSA).

S. aureus also produces toxin-mediated diseases, such as scalded skin syndrome and toxic shock syndrome. In these cases, the organisms may remain relatively localized, but production of potent toxins causes systemic or widespread effects. With scalded skin syndrome (Ritter’s disease), which usually afflicts neonates, the exfoliative toxin is a serine protease that splits the intracellular bridges of the epidermidis, resulting in extensive sloughing of epidermis to produce a burnlike effect on the patient. The toxic shock syndrome toxin (TSST-1), also referred to as pyrogenic exotoxin C, has several systemic effects, including fever, desquamation, and hypotension potentially leading to shock and death.

Other coagulase-positive or variable staphylococci are normal flora of a variety of animal species including dogs. These species include S. intermedius, S. pseudointermedius, and S. delphini. These organisms may be associated with skin infections in dogs, as well as invasive infections in immunocompromised humans or a result of a bite or scratch wound.

The coagulase-negative staphylococci, among which S. epidermidis is the most commonly encountered, are substantially less virulent than S. aureus and are opportunistic pathogens. Their prevalence as nosocomial pathogens is as much, if not more, related to medical procedures and practices than to the organism’s capacity to establish an infection. Infections with S. epidermidis and, less commonly, S. haemolyticus and S. lugdunensis usually involve implantation of medical devices (see Table 1). This kind of medical intervention allows invasion by these normally noninvasive organisms. Two organism characteristics that do enhance the likelihood of infection include production of a slime layer or biofilm-facilitating attachment to implanted medical devices and the ability to acquire resistance to most of the antimicrobial agents used in hospital environments. S. lugdunensis infections resemble S. aureus infections.

Although most coagulase-negative staphylococci are primarily associated with nosocomial infections, urinary tract infections caused by S. saprophyticus are clear exceptions. This organism is most frequently associated with community-acquired urinary tract infections in young, sexually active females but is not commonly associated with hospital-acquired infections or any infections at non–urinary tract sites. It is the second most common (following Escherichia coli) as the cause of urinary tract infections in young women.

Because coagulase-negative staphylococci are ubiquitous colonizers, they are frequently found as contaminants in clinical specimens. This fact, coupled with the emergence of these organisms as nosocomial pathogens, complicates laboratory interpretation of their clinical significance. When these organisms are isolated from clinical specimens, every effort should be made to substantiate their clinical relevance in a particular patient.

The Micrococcaceae and Dermacoccaceae are generally normal flora of the skin, some of the genera including Micrococcus, Kocuria, and Kytococcus spp. have been associated with infections such as endocarditis, pneumonia, sepsis, and skin infections in immunocompromised patients. What, if any, virulence factors are produced by the remaining genera within this group is not known. Because these organisms are rarely associated with infections in healthy individuals, they are probably of low virulence.

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ما هي أفضل العصائر لصحة القلب وتقوية المناعة والنوم العميق؟

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وجع الأوتار قد يخفي خطرا صحيا مرتبطا بالقلب

أزمة "السفينة الموبوءة"..الأرجنتين تحقق في مصدر تفشي "هانتا"

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الآخبار التكنلوجية

باحثون يحددون تأثير القهوة على الميكروبيوم والمزاج

روسيا.. ابتكار تقنية لمعالجة سبائك التيتانيوم لحماية الطائرات والسيارات

بكين وواشنطن تدرسان إطلاق محادثات رسمية حول الذكاء الاصطناعي

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مواضيع ذات صلة

Strategies to Control Bacteria at the Environmental Level

Definition and Measurement of Bacterial Death

Growth in Biofilms

Morphologic Changes During Growth

Staining

Endospores

The Acid-Fast Cell Wall

Special Components of Gram-Negative Cell Walls

Special Components of Gram-Positive Cell Walls

The Peptidoglycan Layer

Bacterial Cultivation

Principles of Bacterial Cultivation : Environmental Requirements