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?