Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (476 page)

Streptococcus agalactiae
(GBS) isolates are nonfastidious GPCs that grow on routine media under aerobic or anaerobic conditions. Staining shows GPCs that form moderate length chains. GBS is a component of the GI and vaginal flora of healthy adults, which serve as the primary reservoir for infection. Intermittent rectovaginal carriage is seen in approximately 25% of pregnant women. Infant prophylaxis, based on results of screening for maternal carriage at 35–37 weeks of gestation, has resulted in a significant decline in the rate of neonatal GBS infections. Adult disease is playing an increasing role in the spectrum of GBS disease.

   Who Should Be Suspected?

   Adult disease: UTI and bacteremia are the most common manifestation of GBS infection in adults, but any organ system may be affected. Pregnancy, advanced age, and significant underlying medical conditions (e.g., cirrhosis, DM, malignancy) are risk factors for acquisition of GBS disease in adults.

   Neonatal and perinatal disease: Vaginal colonization at the end of gestation may lead to neonatal infection, either by ascending intrauterine infection after rupture of membranes or by exposure during passage through the birth canal. Risk factors include prolonged rupture of membranes, amnionitis, and maternal bacteremia.

   Laboratory Findings

Culture
: GBS isolates grow well on routine media incubated in aerobic 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.

The CDC and American College of Obstetrics and Gynecology now recommend that decisions regarding prophylactic antimicrobial treatment for the prevention of neonatal GBS infection be based on cultures to detect maternal GBS carriage. See Group B Streptococcus Vaginal–Rectal Culture Screen in Chapter
17
, Infectious Disease Assays for recommendations for GBS carrier detection in pregnant women.

Susceptibility testing
: GBS isolates are predictably susceptible to penicillin and related antibiotics, the drugs of choice for these infections. Susceptibility testing of GBS must be performed for other antibiotics, as for use in penicillin-allergic patients.

Antigen detection
: Commercially available agglutination tests for GBS are available for direct detection of GBS and other CNS pathogens using CSF, serum, and urine specimens. Reported sensitivity of assays has ranged from poor to good, and false-positive reactions are well documented. One clinical study showed that the clinical management of patients was not affected by the results of these antigen tests. Bacterial antigen testing for preliminary detection of CNS pathogens is not recommended.

Molecular diagnostics
: FDA-approved PCR testing is available for detection of GBS in rectovaginal specimens or enrichment culture.

STREPTOCOCCUS PNEUMONIAE
INFECTION

   Definition

Streptococcus pneumoniae
isolates are nonfastidious GPCs that grow on routine media under aerobic or anaerobic conditions. Staining shows GPCs in pairs and short chains.
Streptococcus pneumoniae
is a common component of the endogenous upper respiratory flora of healthy humans (approximately 10%) that serves as the source for most infections. Carriage may be transient. The disease may be of endogenous or exogenous origin.

   Who Should Be Suspected?

   Most serious infections occur in children and the elderly. Underlying conditions, like DM, AIDS, alcoholism, and chronic lung disease, increase the risk of infection. Current or recent respiratory viral infection also predisposes to
S. pneumoniae
infection.

   The upper respiratory tract is the most common source of infecting organisms.
Streptococcus pneumoniae
may cause infection in any organ system, usually as a result of bacteremic spread from a primary site of infection. Common infections include the following:

   
Respiratory tract infections
, including pneumonia (community acquired), otitis media, and sinusitis: Abrupt onset of fever and shaking chills with cough with purulent sputum production. Severe disease may lead to respiratory failure, sepsis, and death.

   
Bacteremia
:
Streptococcus pneumoniae
is a significant pathogen in the etiology of bacteremia and sepsis. Bacteremia may occur secondary to a primary site of infection (e.g., otitis media in children, pneumonia in adults) or may be the primary infection.