Page 166 - Concise Pathology for Exam Preparation ( PDFDrive )

7 Infections 151

Staphylococcus
aureus
Clusters
(staphylococci)

Streptococcus
pyogenes

Chains
(streptococci)
FIGURE 7.1. Bacterial arrangements.

• S. aureus is a pyogenic, nonmotile, Gram-positive bacterium that forms grape-like clus-
ters (Fig. 7.1). It is mainly found in the nasal passages, but may also inhabit the skin,
oral cavity and gastrointestinal tract. It is considered the most virulent of the more than
30 known pathogenic staphylococcal species. The remaining species of staphylococci
are collectively labelled coagulase-negative staphylococci and are important pathogens in
infections associated with implants and prosthetic devices. S. epidermidis is a skin com-
mensal associated with opportunistic infections. S. saprophyticus is a common cause of
urinary tract infections.
• S. aureus produces membrane damaging or haemolytic toxins including a-toxin (inter-
calates into plasma membrane to form pores); b-toxin (a sphingomyelinase); g-toxin
(lyses RBCs) and d-toxin (detergent-like peptide). It also produces exfoliative toxins A
and B which are serine proteases that cleave desmoglein-1 (aprotein that holds the
keratinocytes together) to detach keratinocytes from one another and from the base-
ment membrane (responsible for impetigo and staphylococcal scalded skin syndrome
[SSSS]).
Clinical Manifestations
• S. aureus is a common cause of wound infections, respiratory tract infections, lung
abscess, empyema (pus in the pleural cavity), sinusitis, otitis media, breast abscess,
umbilical sepsis osteomyelitis, endocarditis, pericarditis and bacteraemia, ocular
infections including conjunctivitis and endophthalmitis, infection of the nail bed
(paronychia) and most hospital-acquired infections.
• It is a major cause of invasive infections of the skin such as folliculitis (infection of
hair follicles), formation of furuncles (boils in the hairy, moist regions of the body) and
carbuncles (suppurative collection in the lower neck reaching up to the subcutaneous
tissue), abscesses, impetigo (superficial infection of the skin), cellulitis (infection of
deeper layers of skin and subcutaneous tissue), lymphadenitis and hidradenitis
suppurativa (chronic abscess formation in apocrine gland regions, most frequently
axillae).
• Toxic shock syndrome (TSS), food poisoning, SSSS and necrotizing pneumonia are
the other manifestations of S. aureus infection. TSS, which is due to superantigens of S.
aureus, is usually seen in tampon-wearing menstruating women and patients with in-
fected surgical wounds. Its clinical features include high fever, mental confusion, diar-
rhoea, hypotension, pharyngitis and an erythematous rash that occur during or soon
after menses. The rash occurs predominantly on the hands and feet, and resolves with
desquamation in 7–10 days. SSSS (also called Ritter disease) is attributed to the
staphylococcal exotoxins A and B, which lead to an exfoliative dermatitis that most
frequently follows nasopharyngeal and skin infections in children.

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