Which organism is characterized as an obligate parasite requiring living cells for growth?

Rickettsia is characterized as an obligate parasite because it specifically requires living host cells to grow and reproduce. Unlike many other bacteria that can thrive in a variety of environments, including inanimate surfaces, Rickettsia cannot survive or multiply outside of a host. This dependence on living cells is a defining feature of obligate parasites, which often undergo intricate life cycles within their hosts, utilizing the host's cellular mechanisms for their own reproduction and survival. Consequently, Rickettsia is known to cause various diseases, often transmitted through vectors like ticks or lice, whereby it invades host cells to reproduce and spread. This unique relationship underscores its classification as an obligate parasite compared to the other organisms listed, which can survive and replicate in more varied environments.