In a pregnant patient with fever and flu-like symptoms, a blood culture on SBA yields small, translucent beta-hemolytic colonies. Which organism is most likely the etiologic agent?

• A. Listeria monocytogenes
• B. Propionibacterium acnes
• C. Streptococcus agalactiae
• D. Streptococcus pyogenes

Answer: (A)

Listeria monocytogenes is a classic cause of febrile illness in pregnant women because it can cross the placenta and infect the fetus, leading to fetal loss or neonatal sepsis. In culture, this organism tends to form small, translucent (often beta-hemolytic) colonies on blood agar, a clue that helps distinguish it from other common postpartum pathogens. Listeria is a facultative intracellular rod that can survive and replicate inside host cells, and it grows well at refrigeration temperatures, which also mirrors its association with foodborne transmission from dairy or deli meats.

Propionibacterium acnes is a skin-associated, slow-growing anaerobe and is not a typical cause of acute bacteremia in pregnancy. Streptococcus agalactiae (Group B strep) can cause neonatal sepsis and meningitis and is beta-hemolytic, but its colonies are usually not described as the small, translucent type characteristic of Listeria; the clinical context of a pregnant patient with flu-like symptoms and the specific colony appearance point toward Listeria monocytogenes. Streptococcus pyogenes can cause invasive disease but is less classically linked to pregnancy-associated febrile illness.