Gram-Positive Cocci

Chapter 9


Gram-Positive Cocci




I Staphylococcus Species



Trigger words:


S. aureus: catalase, coagulase, grape-like clusters, MRSA, pus, 3-hour gastroenteritis, toxins, TSST


S. epidermidis: catalase, catheters, coagulase negative, shunts



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



A Shared staphylococcal properties



B Coagulase-positive staphylococci (S. aureus)



S. aureus: major cause of nosocomial and wound infections


S. aureus ferment mannose, other staph don’t.



• 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.



CA-MRSA also carries Panton valentine leukocidin gene.


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



3. Transmission of S. aureus



4. Treatment



Antibiotics and staphylococci: most staphylococci are resistant to penicillin (produce a β-lactamase). Resistance to methicillin means resistance to all β-lactams.


Staphylococci = catalase (positive); streptococci = catalase (negative).



C 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.



Trigger words:


S. pyogenes: Bacitracin (A disk-group A) sensitivity, β hemolysis, gram-positive cocci in chains, necrotizing fasciitis, pus, streptolysin O and S


A 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)



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