Gram-Positive Cocci

Chapter 9

Gram-Positive Cocci

Staphylococcus Species

• Three staphylococcal species commonly cause human disease: S. aureus (most virulent), S. epidermidis, and S. saprophyticus.

Shared staphylococcal properties

Coagulase-positive staphylococci (S. aureus)

• S. aureus is normal flora of skin and the anterior nares.

1. Pathogenesis

• Table 9-1 summarizes the major contributors to the virulence of S. aureus.

2. Diseases caused by S. aureus (Box 9-1)

BOX 9-1   Staphylococcus Aureus Diseases: Quick Cases

Toxin-Mediated Diseases

Food poisoning due to enterotoxin (A-E): Individual with severe nausea, vomiting, and diarrhea developing 4 hours after eating potato salad and ham sandwiches at a picnic in July. Complete recovery after bed rest for 2 days and drinking plenty of fluids

Scalded skin syndrome due to exfoliative toxin: Young child with blister-like lesions widely disseminated over the body. Large areas of desquamated epithelium but no scarring

Toxic shock syndrome due to TSST-1 (superantigen): Young woman with rapid onset of fever, diarrhea, desquamating rash, multisystem organ involvement, kidney failure, shock with generalized flushing of the skin and mucous membranes. Examination shows a tampon lodged in her vagina.

Toxic shock syndrome due to TSST-1: Child or adolescent develops shock with multiorgan failure and generalized flushing of the skin and mucous membranes within days of sustaining a deep wound.

Suppurative Infections

CA-MRSA: A high school football player with necrotizing fasciitis is treated with clindamycin. Other members of the team also have skin infections (boils and abscesses).

Carbuncle: Diabetic patient whose blood glucose is under poor control with a large swollen area of redness on one leg

Endocarditis: Recent onset of fever, petechial lesions, and detection of a new heart murmur in a patient with an intravascular catheter

Impetigo: Child with honey-colored or clear crusts over ruptured pustules (usually bullous lesions) on the face; intense itching

Osteomyelitis and septic arthritis: Child with fever and localized pain and swelling below the right knee following orthopedic surgery; positive blood culture for S. aureus

Pneumonia: Individual suddenly develops fever, difficulty breathing, and empyema (intrapleural abscesses) soon after recovering from influenza.

Wound infection: Elderly man with fever and redness and swelling at the site of recent surgery

• Toxin-mediated diseases

• Community-acquired methicillin resistant S. aureus (CA-MRSA) carries resistance plasmids that also have the Panton valentine leukocidin gene and may be more virulent.

• Inflammatory diseases mediated by pyogenic and necrotic activities of S. aureus

3. Transmission of S. aureus

4. Treatment

Coagulase-negative staphylococci (S. epidermidis and S. saprophyticus)

II Streptococcus Species

• Most common streptococcal pathogens in humans are S. pyogenes, S. agalactiae, viridans group, and S. pneumoniae.

Shared streptococcal properties

1. Gram-positive spherical or football-shaped cocci in pairs or chains

2. Catalase negative

3. Species-dependent hemolysis in blood agar (Table 9-2)


Streptococcal Hemolysis

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Jun 18, 2016 | Posted by in IMMUNOLOGY | Comments Off on Gram-Positive Cocci

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Hemolytic reaction Organism Differentiating property
α (Incomplete) hemolysis