Biology: Staphylococci Characteristics and Pathogenicity

Characteristics of Staphylococci

• Staphylococci are typical Gram-positive bacteria that occur as irregular clusters of cocci.
• They are widespread in nature, but mainly found on the skin, skin glands, and mucous membranes of mammals and birds.

Pathogenicity of Staphylococci

• S. aureus is more pathogenic than S. epidermidis and S. saprophyticus.
• S. epidermidis can cause hospital-acquired infections, such as prosthetic or indwelling devices.
• S. saprophyticus is mainly associated with urinary tract infections in young, sexually active females.

Virulence Factors of S. aureus

• Surface proteins that promote colonization of host tissues
• Leukocidin, kinases, and hyaluronidase that promote bacterial spread in tissues
• Capsule and Protein A that inhibit phagocytic engulfment
• Carotenoids and catalase that enhance staphylococcal survival in phagocytes
• Protein A and coagulase that act as immunological disguises
• Hemolysins, leukotoxin, and leukocidin that are membrane-damaging toxins
• TSST (toxic shock syndrome toxin) and ET (exfoliative toxin) that damage host tissues or provoke symptoms of disease
• Inherent and acquired resistance to antimicrobial agents

Common Infections Caused by Staphylococci

• S. aureus causes:
• Superficial skin lesions (boils) and localized abscesses
• Deep-seated infections, such as osteomyelitis and endocarditis
• Hospital-acquired infections of surgical wounds
• Food poisoning by releasing enterotoxins into food
• Toxic shock syndrome by releasing superantigens into the bloodstream
• S. saprophyticus causes urinary tract infections, especially in girls.

Diagnosis of Staphylococcal Infections

• Diagnosis begins with attempting to culture the bacteria from an infected site.
• Any area with pus, crusty drainage, or blisters should be cultured.
• Blood from patients with sepsis, toxic shock syndrome, or pneumonia should be cultured.

Laboratory Identification of Staphylococci

• S. aureus colonies on agar are round, convex, and 1-4 mm in diameter with a sharp border.
• S. aureus usually displays a light to golden yellow pigment on blood agar.
• S. aureus is usually beta-hemolytic on blood agar.
• Mannitol salt agar (MSA) is a selective and differential media used for the isolation of Staphylococci.
• MSA consists of 7.5% NaCl, which is selective for species of Staphylococcus.
• Mannitol fermentation produces acid end products that turn the medium yellow.