Which one of the following microorganisms cannot be cultivated on artificial cell-free media?

• A. Chlamydia trachomatis
• B. Mycoplasma hominis
• C. Mycoplasma pneumoniae
• D. Ureaplasma urealyticum

Answer: (A)

The concept here is that certain bacteria must live inside host cells to replicate. Chlamydia trachomatis is an obligate intracellular parasite, meaning it can only multiply within living host cells. It cycles between infectious elementary bodies and metabolically active reticulate bodies inside a cell, so a cell-free artificial medium cannot support its growth. In contrast, the other organisms listed are Mycoplasmas and Ureaplasma, which are cell-wall–deficient bacteria that can be grown on specialized artificial media (enriched with cholesterol and other nutrients) without host cells. Therefore, Chlamydia trachomatis cannot be cultivated on artificial cell-free media, while the others can.