A 6-hour-old boy is evaluated in the newborn nursery due to temperature instability, grunting, and lethargy. The patient was born at full term to a 21-year-old woman who came to the emergency department in active labor with ruptured membranes; she had not received adequate prenatal care. On physical examination, the boy is hypothermic, tachycardic, and tachypneic, and he has mottled skin. Blood samples are obtained, and empiric antibiotics are initiated for the treatment of sepsis. Cultures on blood agar grow bacterial colonies with a narrow zone of clear hemolysis. When these bacteria are grown in the presence of Staphylococcus aureus, the zone of hemolysis is enhanced around the staphylococcal colonies. Which of the following is the most likely pathogen infecting this patient?
Show Explanatory Sources
Streptococcus agalactiae (group B Streptococcus [GBS]) is often a normal commensal of the genitourinary and gastrointestinal tracts of pregnant women. Those who are colonized and do not receive intrapartum antibiotic prophylaxis can transmit the bacteria to the fetus during passage through the vaginal canal, leading to neonatal sepsis (eg, irritability, lethargy, grunting, tachypnea, temperature instability, hypotension) within hours of birth.
GBS is a leading cause of early neonatal sepsis and can be identified by the following laboratory features:
Narrow zone of beta-hemolysis (clear) when plated on blood agar.
Production of CAMP factor, a phospholipase that enhances the activity of beta-hemolysins secreted by certain strains of Staphylococcus aureus. Testing for CAMP factor (CAMP test) can confirm the presence of GBS; this test plates suspected GBS colonies perpendicular to an appropriate S aureus strain and evaluates for enhanced areas of hemolysis (arrow-shaped zones in the image above).
GBS is also catalase-negative (as are all streptococci), contains the Lancefield group B antigen in its cell wall, and hydrolyzes hippurate.
(Choice A) Escherichia coli, a gram-negative rod, is a common cause of early neonatal sepsis. Although E coli generates beta-hemolysins, it does not produce CAMP factor; therefore, enhancement of S aureus hemolysis does not occur.
(Choice B) Enterococcus faecalis, a gram-positive cocci, is a common cause of sepsis in preterm neonates but is rare in those born at term. In addition, it does not hemolyze red blood cells (gamma hemolytic) or generate camp factor.
(Choice C) Staphylococcus epidermidis, a gram-positive coccus that grows in clusters, is a common skin commensal that does not cause hemolysis when plated on blood agar.
(Choice E) Streptococcus pneumoniae, a lancet-shaped, gram-positive bacterium, is alpha-hemolytic (partial, green hemolysis) in aerobic conditions and beta-hemolytic in anaerobic conditions. It does not generate CAMP factor.
Educational objective:
Streptococcus agalactiae (group B Streptococcus), one of the most common causes of early neonatal sepsis, is typically identified by the presence of a narrow zone of beta-hemolysis when plated on blood agar. It is also identified by the production of CAMP factor, which enhances beta-hemolysis by particular strains of Staphylococcus aureus.