Staphylococci Lecture -3- Dr. Raz Nawzad

There are two medically important genera of gram-positive cocci: Staphylococcus and Streptococcus. Staphylococci and streptococci are nonmotile, do not form spores & gram-positive cocci, but they are distinguished by two main criteria: Microscopically, staphylococci appear in grapelike clusters, whereas streptococci are in chains. Biochemically, staphylococci produce catalase (i.e., they degrade hydrogen peroxide), whereas streptococci do not. 10/29/2017

Staphylococci Group Characteristics Staphylococci are spherical gram-positive cocci arranged in grapelike clusters or in chains. Some single cells, pairs, and short chains are also seen Staphylococci have a typical Gram-positive cell wall structure They are non-flagellate, non-motile, and non–spore forming. Staphylococci grow best aerobically but are facultatively anaerobic staphylococci produce catalase. 10/29/2017

Three species of staphylococci are human pathogens: S. aureus, S. epidermidis, and S. saprophyticus 10/29/2017

Some strains of S. aureus produces a golden colored carotenoid pigment Some strains of S. aureus produces a golden colored carotenoid pigment. This pigment enhances the pathogenicity of the organism by inactivating the microbicidal effect of superoxides and other reactive oxygen species which the host immune system uses to kill pathogenss. S. epidermidis does not synthesize this pigment and produces white colonies. The virulence of S. epidermidis is significantly less than that of S. aureus. Two other characteristics further distinguish these species, namely, S. aureus usually ferments mannitol and hemolyzes red blood cells, whereas the others do not. 10/29/2017

Staphylococcus aureus Morphology and Structure: In growing cultures, the cells of S aureus are uniformly Gram-positive and regular in size, fitting together in clusters. The cell wall of S aureus consists of a typical Gram-positive peptidoglycan interspersed with considerable amounts of teichoic acid. The peptidoglycan of the cell wall is commonly overlain with polysaccharide and surface proteins. 10/29/2017

Staphylococcus aureus S. aureus has several important cell wall components and antigens. Polysaccharide capsule is also an important virulence factor. Most strains of S. aureus are coated with a small amount of polysaccharide capsule (microcapsule) that is antiphagocytic Teichoic acids are polymers of ribitol phosphate. They mediate adherence of the staphylococci to mucosal cells and play a role in the induction of septic shock. Protein A is the major protein in the cell wall. 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. The coagulase-negative staphylococci do not produce protein A. Surface proteins such as clumping factor (Clf), which binds to fibrinogen, and fibronectin-binding proteins (FnBP) likely play a role in the early stages of infection 10/29/2017

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Staphylococcus aureus More than 90% of S. aureus strains contain plasmids that encode beta-lactamase, the enzyme that degrades many, but not all, penicillins. Some strains of S. aureus are resistant to the beta-lactamase-resistant penicillins, such as methicillin and nafcillin, by virtue of changes in the penicillin-binding protein in their cell membrane. These strains are commonly known as methicillin-resistant S. aureus (MRSA) or nafcillin-resistant S. aureus (NRSA). Rare strains called vancomycin-intermediate S. aureus (VISA), with reduced sensitivity to vancomycin, have emerged. 10/29/2017

Toxins alpha-Toxin(pore forming toxin): S aureus produces a number of named cytolytic toxins of which alpha-toxin is the most important. alpha-Toxin is a protein secreted by almost all strains of S. aureus, but not by coagulase-negative staphylococci. It is a pore-forming cytotoxin that lyses the cytoplasmic membranes of a wide variety of host cell types by direct insertion into the lipid bilayer to form transmembrane pores that cause to cell death. 10/29/2017

Toxins 2.Exfoliatin responsible for the staphylococcal scalded skin syndrome Exfoliatin produced by S. aureus binds to a specific cell membrane ganglioside found only in the stratum granulosum of the keratinized epidermis of young children and rare adults. There are two antigenically distinct forms of the toxin, ETA and ETB. The toxins have esterase and protease activity and apparently target a protein which is involved in maintaining the integrity of the epidermis. 10/29/2017

Toxins 3.Staphylococcal Superantigen Toxins There are now more than 15 described staphylococcal superantigen toxins (StaphSAgs), the most important of which in human disease are staphylococcal enterotoxins and toxic shock syndrome toxin (TSST-1). Superantigens stimulate T cells non-specifically without normal antigenic recognition also Superantigens bind directly to class II major histocompatibility complexes of antigen-presenting cells Cytokines are released in large amounts, causing the symptoms of TSS. 10/29/2017

There are six immunologic types of enterotoxin, types A–F. Enterotoxin causes food poisoning characterized by prominent vomiting and watery, nonbloody diarrhea. It acts as a superantigen within the gastrointestinal tract to stimulate the release of large amounts of interleukin-1 (IL-1) and interleukin-2 (IL-2) from macrophages and helper T cells, respectively. The prominent vomiting appears to be caused by cytokines released from the lymphoid cells, which stimulate the enteric nervous system to activate the vomiting center in the brain. Enterotoxin is fairly heat-resistant and is therefore usually not inactivated by brief cooking. It is resistant to stomach acid and to enzymes in the stomach and jejunum. There are six immunologic types of enterotoxin, types A–F. 10/29/2017

Toxic shock syndrome toxin (TSST) causes toxic shock, especially in tampon-using menstruating women or in individuals with wound infections. Toxic shock also occurs in patients with nasal packing used to stop bleeding from the nose. TSST is produced locally by S. aureus in the vagina, nose, or other infected site. The toxin enters the bloodstream, causing a toxemia. TSST is a superantigen and causes toxic shock by stimulating the release of large amounts of IL-1, IL-2, and tumor necrosis factor (TNF) 10/29/2017

Epidemiology S. aureus is the "all-time champion" of microbial pathogens. Humans are the reservoir for staphylococci. The nose is the main site of colonization of S. aureus and approximately 30% of people are colonized at any one time. The skin, especially of hospital personnel and patients, is also a common site of S. aureus colonization. Hand contact is an important mode of transmission and hand washing decreases transmission. S. aureus is also found in the vagina of approximately 5% of women, which predisposes them to toxic shock syndrome. 10/29/2017

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Transmission Most S. aureus infections acquired in the community are autoi-nfections with strains that the subject has been carrying in the anterior nares, on the skin, or both. Community outbreaks are usually associated with poor hygiene and fomite transmission from individual to individual. Hospital spread is on the hands of medical personnel . 10/29/2017

Staphylococcal Infections 1. Furuncle and Carbuncle The furuncle or boil is a superficial skin infection develops in a hair follicle, sebaceous gland, or sweat gland (Fig 3). Furunculosis is often a complication of acne vulgaris The course of the infection is usually benign, and the infection resolves upon spontaneous drainage of pus. No surgical or antimicrobic treatment is needed. 10/29/2017

Staphylococcal Disease Figure 3: Furuncle (boil) 10/29/2017

Staphylococcal Disease Furuncle and Carbuncle Infection can spread from a furuncle with the development of one or more abscesses in adjacent subcutaneous tissues. This lesion, known as a carbuncle Occurs most often on the back of the neck, but it may involve other skin sites (fig 4). Carbuncles are serious lesions that may result in bloodstream invasion (bacteremia). 10/29/2017

Figure 4: Staphylococcal carbuncle. Multiple abscesses have coalesced 10/29/2017

Staphylococcal Disease Chronic Furunculosis Some individuals are subject to chronic furunculosis, in which repeated attacks of boils are caused by the same strain of S aureus. Chronic staphylococcal disease may be associated with factors that depress host immunity, such as diabetes or congenital defects of polymorphonuclear leukocyte function. 10/29/2017

Staphylococcal Disease 2. Impetigo Strains of S aureus that produce exfoliatin cause a characteristic form called bullous impetigo, characterized by large blisters containing many staphylococci in the superficial layers of the skin. Bullous impetigo is a localized form of staphylococcal scalded skin syndrome 10/29/2017

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Staphylococcal Disease 3. Deep Lesions These include infections of bones, joints, deep organs, and soft tissues, including surgical wounds. Infections of deep tissues by bacteremic spread from the skin lesions. More than 90% of the cases of acute osteomyelitis in children are caused by S aureus Staphylococcal pneumonia may also develop. 10/29/2017

Staphylococcal Disease Bacteremia and endocarditis can develop. Predisposing factors such as, diabetes, leukocyte defects, or general reduction of host defenses by alcoholism, malignancy, old age, or steroid or cytotoxic therapy. Severe S aureus infections, including endocarditis, are particularly common in drug abusers using injection methods. 10/29/2017

Is most common in neonates and children less than 5 years of age. 4. Scalded Skin Syndrome Is most common in neonates and children less than 5 years of age. Staphylococcal scalded skin syndrome results from the production of exfoliatin in a staphylococcal lesion (fig5) The face, axilla, and groin tend to be affected first Can spread to all parts of the body The disease occasionally occurs in adults, particularly those who are immunocompromised Scalded skin syndrome is characterized by fever, large bullae, and an erythematous macular rash. Large areas of skin slough, serous fluid exudes, and electrolyte imbalance can occur. Hair and nails can be lost. Recovery usually occurs within 7–10 days. 10/29/2017

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5. Toxic Shock Syndrome Toxic shock syndrome (TSS) was first described in children, but came to public attention during the early 1980s when hundreds of cases were reported in young women using intravaginal tampons The disease is characterized by high fever, vomiting, diarrhea, sore throat, and muscle pain. Within 48 hours, it may progress to severe shock with evidence of renal and hepatic damage. 10/29/2017

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6. Staphylococcal Food Poisoning (gastroenteritis) Ingestion of staphylococcal enterotoxin–contaminated food results in acute vomiting and diarrhea within 1 to 5 hours. There is prostration, but usually no fever. Recovery is rapid, except sometimes in the elderly and in those with another disease. 10/29/2017

Diagnosis of Staphylococcal Infections 1. In Most acute, untreated lesions, Smears from staphylococcal lesions reveal gram-positive cocci in grapelike clusters. Cultures of S. aureus typically yield golden-yellow colonies that are usually beta-hemolytic. S. aureus is coagulase-positive.  Mannitol-salt agar is a commonly used screening device for S. aureus. Antibiotic susceptibility tests are indicated because of the emerging resistance to multiple antimicrobics, particularly methicillin-resistant S aureus (MRSA) 10/29/2017

Diagnosis of Staphylococcal Infections 2. Deep staphylococcal infections such as: osteomyelitis and deep abscesses present special diagnostic problems when the lesion cannot be directly aspirated or surgically sampled. Blood cultures are usually positive in conditions such as acute staphylococcal arthritis, osteomyelitis, and endocarditis. 10/29/2017

Treatment of Staphylococcal Infections Superficial lesions resolve spontaneously. Those that are more extensive, deeper, or in vital organs require a combination of surgical drainage and antimicrobics for optimal outcome: Penicillins and cephalosporins are active against S aureus cell wall peptidoglycan. the penicillinase-resistant penicillins (methicillin, nafcillin, oxacillin) and first-generation cephalosporins are more commonly used because of the high frequency of penicillin resistance (more than 80%). 10/29/2017

Treatment of Staphylococcal Infections For MRSA strains resistant to these agents or in patients with β-lactam hypersensitivity, the alternatives are vancomycin, clindamycin, or erythromycin Synergy between cell wall–active antibiotics and the aminoglycosides is present when the staphylococcus is sensitive to both types of agents. For strains resistant to both methicillin and vancomycin daptomycin is a new alternative 10/29/2017

Prevention Clothes and bedding that may cause re-infection should be dry-cleaned or washed at a sufficiently high temperature. Anti-staphylococcal soaps block infection. Chemoprophylaxis during high-risk surgery is effective 10/29/2017