Bacteria are microorganisms that have circular double-stranded DNA and (except for mycoplasmas) cell walls. Most bacteria live extracellularly. Some bacteria (eg, Salmonella typhi; Neisseria gonorrhoeae; Legionella, Mycobacteria, Rickettsia, Chlamydia, and Chlamydophila species) preferentially reside and replicate intracellularly. Some bacteria such as chlamydiae, Chlamydophila species, and rickettsiae are obligate intracellular pathogens (ie, able to grow, reproduce, and cause disease only within the cells of the host). Others (eg, Salmonella typhi, Brucella species, Francisella tularensis, N. gonorrhoeae, Neisseria meningitidis, Legionella and Listeria species, Mycobacterium tuberculosis) are facultative intracellular pathogens.