Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (477 page)

   
Meningitis
:
Streptococcus pneumoniae
is one of the most common causes of bacterial meningitis in all age groups. Hematogenous dissemination is the most common route of infection, but direct invasion from infected sinuses is also well described; basilar skull fracture may cause recurrent
S. pneumoniae
meningitis.

   Laboratory Findings

Gram stain
: The typical Gram stain of sputum from patients with pneumococcal pneumonia shows many PMNs and many lancet-shaped GPCs in pairs (diplococci).

Culture
:
Streptococcus pneumoniae
grows on routine media after overnight incubation but may lose viability after or during transport or storage. Culture of sputum for isolation of
S. pneumoniae
has a sensitivity of approximately 45% of patients with community-acquired pneumonia. Collection of blood cultures may improve detection in critically ill patients with pneumonia; blood cultures are positive in approximately 25% of untreated patients. Pleural effusions yield organisms in approximately 15% of patients.
Streptococcus pneumoniae
is a welldocumented cause of spontaneous bacterial peritonitis in patients with alcoholic cirrhosis; bedside inoculation of peritoneal fluid directly into blood culture media improves isolation compared to culture onto solid media in the laboratory.

Susceptibility testing
: Testing of
S. pneumoniae
isolates must be performed for significant clinical isolates.

Urine antigen detection
: Antigen testing for direct detection of
S. pneumoniae
is available as an adjunct for diagnosis of
S. pneumoniae
respiratory infections. See
Streptococcus pneumoniae
Urine Antigen Test in Chapter
17
, Infectious Disease Assays for information.

STREPTOCOCCUS PYOGENES
(GROUP A) INFECTION

   Definition

Streptococcus pyogenes
(GAS) isolates are nonfastidious GPCs that grow on routine media under aerobic or anaerobic conditions. Staining shows GPCs that form moderate-length chains. GAS colonizes the upper respiratory tract and skin, and infections at these sites are the most common manifestations of GAS disease. Invasive pyogenic infections are commonly caused by GAS; infections in all organ systems have been described. In addition to primary GAS infections, GAS may cause clinically significant superinfections (e.g., GAS pneumonia complicating influenza, GAS cellulitis complicating chicken pox). GAS infections may result in suppurative complications, immune-mediated nonsuppurative sequelae, and toxinmediated disease.

Diseases caused by GAS include the following:

   
Pharyngitis
: see Chapter
13
, Respiratory, Metabolic, and Acid–Base Disorders

   
Cellulitis and soft tissue infections
: Impetigo describes a superficial vesicular rash, usually presenting in children. The vesicles evolve into pustules, which break down and scab over the following week. Erysipelas is a soft tissue infection that most often affects adults, who present with fever and erythematous, edematous areas of inflammation with well-demarcated edges, usually on the face. GAS may also cause cellulitis in tissue surrounding infected wounds or trauma.

   
Acute rheumatic fever
: This disorder is a nonsuppurative complication following prior GAS pharyngitis (2–5 weeks). Common manifestations include carditis, chorea, erythema marginatum, polyarthritis, and subcutaneous nodules.

   
Acute poststreptococcal GN (PSGN)
: Acute GN is a nonsuppurative complication following GAS pharyngitis (>10 days) or GAS skin infections (3–6 weeks). Clinical symptoms include headache, malaise, fatigue, edema, hypertension, and encephalopathy.

   
Group A streptococcal toxic shock
–
like syndrome
: This disorder may develop in patients infected with GAS strains capable of elaborating streptococcal pyrogenic exotoxins. The syndrome is often preceded by nonspecific symptoms (fever, chills, malaise). There may be prominent symptoms at the site of primary infections. Disease progresses to shock and multiorgan failure.

   Laboratory Findings

Culture
: GAS isolates grow well on routine media incubated in aerobic or anaerobic conditions; selective cultures improve detection from specimens likely to be contaminated with endogenous flora. Most strains demonstrate β-hemolysis on SBA. Gram stain shows gram-positive cocci that form chains of moderate length.

Susceptibility testing
: Group A
Streptococcus
isolates are predictably susceptible to penicillins and related antibiotics, the drugs of choice for these infections. For penicillin-allergic patients, susceptibility testing of GAS must be performed for other antibiotics.

Serology
: Not recommended for diagnosis of acute GAS infection but may be used for diagnosis of infection in the recent past in patients with symptoms of GN or RF. Several specific assays are most useful for detection of GAS antibodies. See: Streptozyme, Antistreptococcal Antibodies, Antistreptolysin O [ASO], Anti-DNase-B [ADB] in Chapter
17
, Infectious Disease Assays.