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Staphylococci
are gram-positive, aerobic organisms. Staphylococcus
aureus is the most pathogenic; it typically causes
skin infections and sometimes pneumonia, endocarditis, and osteomyelitis.
It commonly leads to abscess formation. Some strains elaborate toxins
that cause gastroenteritis, scalded skin syndrome, and toxic shock
syndrome. Diagnosis is by Gram stain and culture. Treatment is usually with
penicillinase-resistant β-lactams,
but because antibiotic resistance is common, vancomycin may be required.
Some strains are resistant to all but the newest ribosome-targeted antibiotics
(eg, linezolid, quinupristin plus dalfopristin) or daptomycin
(a lipopeptide antibiotic).
The ability to clot blood by producing coagulase determines the virulence of the several species of staphylococci. Coagulase-positive Staphylococcus
aureus is among the most ubiquitous and dangerous human pathogens, both for its virulence and its ability to develop antibiotic resistance. Coagulase-negative species like S. epidermidis are increasingly associated with hospital-acquired infections, whereas S. saprophyticus causes urinary infections.
Pathogenic staphylococci are ubiquitous. They are carried, usually transiently, in the anterior nares of about 30% of healthy adults and on the skin of about 20%. Hospital patients and personnel have higher rates.
Newborns and nursing mothers are predisposed to staphylococcal infections, as are patients with influenza, chronic bronchopulmonary disorders (eg, cystic fibrosis, emphysema), leukemia, tumors, transplants, implanted prostheses or other foreign bodies, burns, chronic skin disorders, surgical incisions, diabetes mellitus, and indwelling intravascular plastic catheters. Patients receiving adrenal steroids, irradiation, immunosuppressants, or antitumor chemotherapy are also at increased risk. Predisposed patients may acquire antibiotic-resistant staphylococci from hospital personnel. Transmission via the hands of personnel is the most common means of spread, but airborne spread also can occur.
Staphylococcal
Diseases
Staphylococci cause disease by direct tissue invasion and sometimes by exotoxin production. S.
aureus bacteremia, which frequently causes metastatic foci of infection, may occur with any localized staphylococcal infection but is particularly common with infection related to intravascular catheters or other foreign bodies. It also may occur without any obvious primary site. S. epidermidis and other coagulase-negative staphylococci increasingly cause hospital-acquired bacteremia associated with catheters and other foreign bodies. They are important causes of morbidity (especially prolongation of hospitalization) and mortality in debilitated patients. The diseases listed below are further discussed elsewhere in The Manual.
Direct invasion:
Skin infections are the most common form of staphylococcal disease. Superficial infections may be diffuse, with vesicular pustules and crusting (impetigo) or sometimes cellulitis, or focal and nodular (furuncles and carbuncles). Deeper cutaneous abscesses are common. Staphylococci are commonly implicated in wound and burn infections, postoperative incision infections, and mastitis or breast abscess in nursing mothers.
Neonatal infections usually appear within 6 wk after birth and include skin lesions with or without exfoliation, bacteremia, meningitis, and pneumonia.
Pneumonia that occurs in the community setting is not common but may develop with influenza, in patients receiving corticosteroids or immunosuppressants, and in those with chronic bronchopulmonary or other high-risk diseases. However, S.
aureus is a common cause of hospital-acquired pneumonia. Staphylococcal pneumonia is occasionally characterized by formation of lung abscesses followed by rapid development of pneumatoceles and empyema.
Endocarditis develops, particularly in IV drug abusers and patients with prosthetic heart valves. It is an acute febrile illness often accompanied by abscesses, embolic phenomena, pericarditis, subungual petechiae, subconjunctival hemorrhage, purpuric lesions, heart murmurs, and valvular heart failure.
Osteomyelitis occurs more commonly in children, causing chills, fever, and pain over the involved bone. Redness and swelling subsequently appear. Periarticular infection frequently results in effusion, suggesting septic arthritis rather than osteomyelitis.
Toxin-mediated
disease:
Staphylococci may produce multiple toxins. Some have local effects; others trigger cytokine release from certain T cells, causing serious systemic effects, including skin lesions, shock, organ failure, and death.
Toxic shock syndrome (see Gram-Positive Cocci: Toxic Shock Syndrome (TSS)) may occur from use of vaginal tampons or as a complication of a seemingly minor postoperative infection.
Staphylococcal scalded skin syndrome (see Bacterial Skin Infections: Staphylococcal Scalded Skin Syndrome), which is caused by several toxins termed exfoliatins, is an exfoliative dermatitis of childhood characterized by large bullae and peeling of the upper layer of the skin. Eventually, exfoliation occurs.
Staphylococcal food poisoning is caused by ingesting a preformed heat-stable staphylococcal enterotoxin. Food can be contaminated by staphylococcal carriers or people with active skin infections. In food that is incompletely cooked or left at room temperature, staphylococci reproduce and elaborate enterotoxin. Many foods can serve as growth media, and despite contamination, they have a normal taste and odor. Severe nausea and vomiting begin 2 to 8 h after ingestion, typically followed by abdominal cramps and diarrhea. The attack is brief, often lasting < 12 h.
Diagnosis
Diagnosis is by Gram stain and culture of infected material. Susceptibility studies should be done because methicillin-resistant organisms are now common and require alternative therapy.
Staphylococcal food poisoning is usually suspected because of case clustering (eg, within a family, attendees of a social gathering, or customers of a restaurant). Confirmation (typically by the health department) entails isolating staphylococci from suspected food and sometimes testing for enterotoxins.
X-ray changes of osteomyelitis may not be apparent for 10 to 14 days, and bone rarefaction and periosteal reaction may not be detected for even longer. Abnormalities in MRIs, CT scans, or radionuclide bone scans often are apparent earlier.
Treatment
Management includes abscess drainage, debridement of necrotic tissue, removal of foreign bodies (including vascular catheters), and administration of antibiotics. Initial choice and dosage of antibiotics depend on infection site, illness severity, and probability that resistant strains are involved. Thus, it is essential to know local resistance patterns for initial therapy (and ultimately, actual drug sensitivity).
Treatment of staphylococcal intoxications, the most serious of which is toxic shock syndrome, involves decontamination of the producing area (exploration of surgical wounds, irrigation, debridement), intensive support (including vasopressors and respiratory assistance), electrolyte balancing, and antimicrobials. In vitro evidence supports a preference for protein synthesis inhibitors (eg, clindamycin 900 mg IV q 8 h) over other classes of antibiotics. IV immune globulin has benefited severe cases.
Antibiotic resistance is common in staphylococci. Staphylococci often produce penicillinase, an enzyme that inactivates several β-lactam antibiotics. Most staphylococci are resistant to penicillin G , ampicillin , and antipseudomonal penicillins. Most community-acquired strains are susceptible to penicillinase-resistant penicillins (methicillin, oxacillin , nafcillin , cloxacillin , dicloxacillin ), cephalosporins, carbapenems (imipenem, meropenem , ertapenem ), macrolides, gentamicin , vancomycin , and teicoplanin.
Methicillin-resistant S. aureus (MRSA) isolates have become common, especially in hospitals. In addition, community-acquired methicillin-resistant S. aureus (CA-MRSA) has emerged over the past several years. CA-MRSA tends to be less resistant to multiple drugs than hospital-acquired isolates. These strains are usually susceptible to trimethoprim-sulfamethoxazole (TMP-SMX), doxycycline , or minocycline and are often susceptible to clindamycin , but there is the potential for emergence of resistance by strains inducibly resistant to erythromycin . Vancomycin is effective for most hospital-acquired MRSA, sometimes with the addition of rifampin and an aminoglycoside for serious infections. However, vancomycin -resistant strains have appeared in the US. Table 1: Gram-Positive Cocci: Antibiotic Treatment of Staphylococcal Infections in Adults summarizes treatment options.
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Table 1
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Antibiotic Treatment of
Staphylococcal Infections in Adults
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Infection
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Drugs
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Community-acquired cutaneous infections (non-MRSA)
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Dicloxacillin or cephalexin 250–500 mg po q 6 h for 7–10 days
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Penicillin-allergic patients
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Erythromycin 250–500 mg po q 6 h; clarithromycin 500 mg po q 12 h; azithromycin 500 mg po on the 1st day then 250 mg po q 24 h or clindamycin 300 mg po q 8 h
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Serious infections that are unlikely to be MRSA
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Nafcillin or oxacillin 1–2 g IV q 4–6 h or cefazolin 1 g IV q 8 h
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Penicillin-allergic patients
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Clindamycin 600 mg IV q 8 h or vancomycin 15 mg/kg q 12 h
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Serious infection with high likelihood of being MRSA
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Vancomycin 15 mg/kg IV q 12 h or linezolid 600 mg IV q 12 h
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Documented MRSA
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By reported sensitivities
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Vancomycin -resistant staphylococci*
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Linezolid 600 mg IV q 12 h; quinupristin plus dalfopristin 7.5 mg/kg q 8 h; daptomycin 4 mg/kg q 24 h
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MRSA = methicillin-resistant Staphylococcus aureus.
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*No clinical data, but listed drugs appear to be active in vitro (doses not established).
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Prevention
Aseptic precautions (eg, thoroughly washing hands between patient examinations and sterilizing shared equipment) help decrease spread in institutions. Strict isolation procedures should be used for patients harboring resistant microbes until their infections have been cured. An asymptomatic nasal carrier need not be isolated unless the strain is MRSA or is the suspected source of an outbreak. Cloxacillin , dicloxacillin , TMP-SMX, ciprofloxacin (each of these often combined with rifampin ), and topical mupirocin have been useful in treating MRSA in carriers, but the organism recurs in up to 50% and frequently becomes resistant.
Staphylococcal food poisoning can be prevented by proper food preparation. Patients with staphylococcal skin infections should not handle food, and food should be consumed immediately or refrigerated and not kept at room temperature.
Last full review/revision November 2005
Content last modified November 2005
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