A 34-year-old pregnant woman at 28 weeks of gestation presents to the antenatal clinic with a 3-day history of fever (38.5°C), myalgia, and malaise. She denies vaginal bleeding or abdominal pain. Blood cultures are drawn and subsequently grow a gram-positive rod that is catalase-positive, oxidase-negative, and displays characteristic tumbling motility at room temperature. The organism is resistant to cephalosporins. What is the most likely causative organism and the most appropriate antibiotic for maternal-fetal infection prevention?

Explanation

## Clinical Presentation and Organism Identification **Key Point:** Listeria monocytogenes is a gram-positive rod that causes bacteremia and meningitis in pregnant women, neonates, and immunocompromised hosts. The organism is catalase-positive, oxidase-negative, and displays characteristic **tumbling motility** at room temperature (20–25°C) due to peritrichous flagella. **High-Yield:** Listeria is notably **resistant to cephalosporins** (including ceftriaxone) because it lacks a D-alanyl-D-alanine carboxypeptidase target. This resistance pattern is a critical diagnostic clue and has major therapeutic implications. ## Pathogenesis in Pregnancy Listeria crosses the placental barrier and causes intrauterine infection, leading to: - Maternal bacteremia (often with nonspecific flu-like symptoms) - Transplacental transmission to the fetus - Neonatal sepsis or meningitis (if delivery occurs during infection) - Spontaneous abortion or preterm labor ## Antibiotic Management | Feature | Ampicillin/Penicillin G | Cephalosporins | Gentamicin | |---------|-------------------------|-----------------|------------| | **Activity vs. Listeria** | Excellent (bactericidal) | None (resistant) | Synergistic only | | **Placental penetration** | Adequate | Good but ineffective | Poor | | **First-line status** | **Yes** | Contraindicated | Adjunct only | **Clinical Pearl:** In pregnant women with meningitis or bacteremia of unknown etiology, ampicillin MUST be added to the empiric regimen (alongside ceftriaxone) until Listeria is excluded, because cephalosporins alone will fail. **Mnemonic:** **LIMP** = **L**isteria, **I**mmunocompromised, **M**eningitis, **P**regnancy — the four risk groups for Listeria infection. ## Why Ampicillin/Penicillin G? 1. **Bactericidal activity** against Listeria (inhibits cell wall synthesis via penicillin-binding proteins). 2. **Adequate placental crossing** to prevent fetal infection. 3. **Standard of care** in pregnant women with fever and bacteremia pending culture results. [cite:Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases Ch 200]