Which of the following is an obligate parasite?: (A) Mycoplasma, (B) Chlamydia trachomatis, (C) Gram -ve bacilli, (D) Gram +ve cocci
Answer is B. Chlamydiae is an obligate intracellular bacterial parasite.
They can be isolated only in cell culture. 
McCoy and HeLa cell lines are MC cell lines used to culture chlamydia.