Microbiology Practice Exam

Chlamydia is characterized by the presence of elementary bodies, which are the infectious form of the bacteria. These elementary bodies are unique in that they are small, metabolically inactive, and adapted for extracellular survival and transmission. Once they enter a host cell, they differentiate into reticulate bodies, which are larger and metabolically active, allowing for replication within the host.

In the context of the other options, gram-positive refers to a stain classification used to identify bacteria, which doesn't apply to Chlamydia, as it does not fit neatly into the gram stain categories due to its unique cell wall composition. Spore-forming refers to bacteria that can produce spores as a survival mechanism in adverse conditions, which is also not a characteristic of Chlamydia. Capsulated refers to bacteria that possess a protective capsule, which is typically associated with increased virulence and protection against phagocytosis, but Chlamydia does not have a typical capsule structure. Thus, the elementary bodies are the defining characteristic directly linked to the biology of Chlamydia.