STREPTOCOCCAL AND ENTEROCOCCAL INFECTIONS
Streptococci and enterococci are gram-positive cocci that form chains when grown in liquid media.
Culture on blood agar reveals three hemolytic patterns:
– α-Hemolysis results in partial hemolysis that imparts a greenish appearance to agar. This pattern is seen with S. pneumoniae and viridans streptococci.
– β-Hemolysis results in complete hemolysis around a colony. This pattern is seen with streptococci of Lancefield groups A, B, C, and G, which all form large (≥0.5-mm) colonies. Lancefield grouping is based on cell-wall carbohydrate antigens.
– γ-Hemolysis describes the absence of hemolytic ability. This pattern is typical of enterococci, nonenterococcal group D streptococci, and anaerobic streptococci.
Streptococci and enterococci colonize the respiratory, GI, and genitourinary tracts as part of the normal flora. Several of these species are also important causes of human diseases.
GROUP A STREPTOCOCCUS (GAS)
Epidemiology and Pathogenesis
GAS (S. pyogenes) causes suppurative infections and is associated with postinfectious syndromes such as acute rheumatic fever (ARF) and poststreptococcal glomerulonephritis (PSGN).
Up to 20% of people may have asymptomatic pharyngeal colonization with GAS.
The incidence of all GAS infections is ∼10-fold higher in low-income than in high-income countries. Worldwide, GAS contributes to ∼500,000 deaths per year.
The major surface protein (M protein) and the hyaluronic acid polysaccharide capsule protect GAS against phagocytic ingestion and killing.
GAS makes a large number of extracellular products that may contribute to local and systemic toxicity; these include streptolysins S and O, streptokinase, DNases, and the pyrogenic exotoxins that cause the rash of scarlet fever and contribute to the pathogenesis of toxic shock syndrome (TSS) and necrotizing fasciitis.
Respiratory droplets provide the usual route of transmission, although other mechanisms have been described.
After an incubation period of 1–4 days, pts develop sore throat, fever, chills, malaise, and GI manifestations.
Examination may reveal an erythematous and swollen pharyngeal mucosa, purulent exudates over the posterior pharynx and tonsillar pillars, and tender anterior cervical adenopathy.
Viral pharyngitis is the more likely diagnosis when pts have cough, coryza, hoarseness, conjunctivitis, or mucosal ulcers.
Throat culture is the gold standard for diagnosis.
– Latex agglutination or enzyme immunoassay is highly specific (>95%) and can be relied on for a rapid, definitive diagnosis.
– Given a variable sensitivity of 55–90%, a negative rapid-assay result should be confirmed with a throat culture.
TREATMENT Gas Pharyngitis