Why is treponema pallidum hard to culture?

Obligate parasite.: Treponema pallidum is the pathogen that causes syphilis. It is very difficult to grow in culture because unlike most bacteria, it cannot survive outside of mammalian cells (i.e. On a petri dish), thus it is considered an obligate parasite. It has limited metabolic capacity, and cannot synthesize many biomolecules, which it requires from a living host. This has posed a hurdle for researchers.