Which organism is described as a facultative intracellular coccobacillus?

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The concept here is that some bacteria can live inside host cells and also survive outside them, giving them a facultative intracellular lifestyle. The shape described is coccobacillary, meaning the organism is intermediate between a coccus and a bacillus.

Coxiella burnetii fits this description well. It is a small Gram-negative coccobacillus that can replicate within the phagolysosomes of macrophages, a hallmark of intracellular pathogens. At the same time, it can persist outside host cells in the environment in hardy, spore-like forms, allowing infection to occur via aerosols. This combination—intracellular replication with environmental persistence—embodies a facultative intracellular lifestyle.

The other organisms listed are also intracellular pathogens, but they are not as characteristically described as facultative intracellular coccobacilli in typical clinical microbiology contexts, and one of them is not primarily associated with intracellular lifestyle. Escherichia coli is mainly extracellular or extracellularly invasive and is not chiefly noted for intracellular replication as a defining trait, while Yersinia pestis and Brucella melitensis are intracellular but not the classic exemplar of a facultative intracellular coccobacillus in standard teaching.