A 30 year old woman with a bad obstetric history presents with fever. The blood culture from the patient grows gram—positive small to medium coccobacilli that are pleomorphic, occurring in short chains. Direct wet mount from the culture shows tumbling motility. The most likely organism is:
A. Listera monocytogenes
B. Corynebacterium sp.
C. Enterococcus sp.
D. Erysipelothrix rhusiopathiae
Correct answer: A. Listera monocytogenes
Listeria are small gram positive coccobacilli with characteristic tumbling motility. They are non motile at 38 degree Celsius as the flagellae are produced optimally at 20-30 degree Celsius.