Staphylococcal Infections

Staphylococci are ubiquitous, catalase-positive, gram positive cocci that form grape-like clusters. They are normal colonizers of the skin and mucous membranes. There are over 30 species of Staphylococci, however, only several members of the genus cause disease in humans. Staphylococcus species are divided into coagulase positive and coagulase negative groups.

S. aureus, the most important human pathogen within the genus, is the only coagulase positive species. S. auerus has many virulence factors that contribute to its abiltity to produce very severe disease. Coagulase negative species that may cause human infection include S. epidermidis, S. haemolyticus, S. saprophyticus, S. schleiferi, and S. lugdunensis. Coagulase negative staphylococci are generally less pathogenic unless a foreign body such as a central line is in place.

Between 20 and 50 percent of children are colonized by S. aureus with the skin and mucous membranes as the most common location. While the vast majority of those colonized never develop clinical disease, S aureus can cause a variety of pyogenic and toxin mediated syndromes. The bacterium is generally transmitted by direct contact and is only very rarely spread through airborne droplets.

Diseases caused by S. aureus include infections of the skin, bone, lungs, joints, central nervous system and heart. Toxin mediated diseases caused by this organism include food poisoning, scarlet fever, toxic shock syndrome, and scalded skin syndrome. Medical risk factors for S. aureus infection include diabetes, chronic liver disease, nutritional deficiencies, burns, and congenital and acquired immune deficiencies. S.aureus is a frequent cause of hospital acquired infection and risk factors for nosocomial infection include admission to an intensive care ward or burn unit, prolonged hospital stay, long term intravenous [Page 1607]or central lines, and foreign bodies such as artificial joints and heart valves.

In the absence of established risk factors, positive coagulase negative staphylococcal blood cultures generally represent a contaminant rather than true infection. However, in a high risk patient, such as an infant in a neonatal intensive care unit with clinical signs of infection, a culture positive for coagulase negative staphylococci should be considered a true infection and treatment initiated.

There are no currently licensed vaccines providing protection against Staphylococcal infections and judicious hand washing and sterile procedures in health-care settings and antibiotics remain the mainstays of prevention and therapy. While minor staphylococcal infections may be treated with oral antibiotics, more severe infections necessitate use of intravenous medications. Most strains of S aureus produce β-lactamase enzymes and are thus resistant to β-lactam class antibiotics such as penicillin and ampicillin. First line therapy is therefore oxacillin or a first or second generation cephalosporin antibiotic until sensitivities are known.

A rapidly worsening problem is the spread of methicillin resistant S. aureus (MRSA). In many hospitals in the United States, half or more of all S. aureus isolates are resistant to methicillin. Traditional risk factors for hospital-acquired MRSA infection have included long admissions, recent antibiotic use, and multiple surgical procedures. However, recently an increasing number of community-acquired MRSA in previously healthy adults and children without traditional risk factors is being seen. MRSA infection is treated with intravenous vancomycin.

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