This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
Movement of Listeria monocytogenes inside of host cells is caused by: 
Select and explain. 
A. Inducing host cell actin polymerization, B. The formation of pili (fimbriae) on the listeriae surface, C. Pseudopod formation, D. The motion of listeriae flagella
A: Inducing host cell actin polymerization
Answer: A: Inducing host cell actin polymerization 
Explanation: Ans. A. Inducing host cell actin polymerizationa. Act, a surface protein, induces host cell actin polymerization, which propels them to the cell membrane. Pushing against the host cell membrane, they cause formation of elongated protrusions called filopods.b. These filopods are ingested by adjacent epithelial cells, macrophages, and hepatocytes, the listeriae are released, and the cycle begins again.c. L monocytogenes can move from cell to cell without being exposed to antibodies, complement, or PMN's. Shigella flexneri and Rickettsiae also usurp the host cells' actin and contractile system to spread their infections.