Staphylococcus aureus

Diseases: abscesses of many organs, endocarditis, gastroenteritis (food poisoning), toxic shock syndrome, surgical wound infections.

• #1 isolated organism in bacterial tracheitis (membranous croup)
• #1 cause of bacterial pericarditis (strep viridans is a common cause of bacterial endocarditis, but not bacterial pericarditis)

Characteristics: gram positive cocci in clusters, coagulase positive, catalase positive

Habitat and Transmission: Habitat is in the human skin and nose. Transmission is via the hands.

Pathogenesis:

• many enzyme and toxins are made; the two most important are coagulase and enterotoxins
• coagulase is the best correlate of pathogenicity
• enterotoxins cause food poisoning
• Toxic Shock Syndrome Toxin (TSST) is a superantigen that causes toxic shock syndrome by stimulating many helper T cells to relase large amounts of lymphokines, especially IL-2
• Protein A is an important virulence factor because it binds to the heavy chain of IgG and prevents the activation of complement
• predisposing factors are breaks in the skin, foreign bodies such as sutures, neutrophil levels below 500 microliters, IV drug abuse, tampon use.

Laboratory diagnosis

• gram stained smear and culture
• yellow or gold colonies on blood agar
• S. aureus is coagulase positive, S. epidermidis is coagulase negative

Resistance

• Plasmid mediated B-lactamase mediates most resistance
• 85% resistant to penicillin G
• Resistance to nafcillin may be due to changes in binding proteins
• rare vancomycin resistant strains have emerged

Prevention

• Cefazolin is used to prevent surgical wound infections
• No vaccine is available
• Handwashing reduces spread.

Treatment

• Penicillin G for sensitive isolates
• B-lactamase-resistant penicillins (ie nafcillin) for resistant isolates
• vancomycin for methicillin-resistant isolates
• Bactrim and clindamycin are effective for community acquired MRSA skin infections

References

Levinson and Jawetz. Medical Microbiology and Immunology. 6th edition