Rickettsiae. Rickettsiae are rod‐shaped and coccoid bacteria belonging to the order Rickettsiales. These bacteria cannot be seen with the light microscope, and therefore the Gram stain is not used for identification. However, their walls have the characteristics of Gram‐negative cell walls. Rickettsiae are obligate intracellular parasites that infect humans as well as arthropods such as ticks, mites, and lice. They are cultivated only with great difficulty in the laboratory and generally do not grow on cell‐free media. Tissue cultures and fertilized eggs are used instead.
Rickettsiae are very important as human pathogens. Various species cause Rocky Mountain spotted fever, epidemic typhus, endemic typhus, scrub typhus, Q fever, and ehrlichiosis.
Chlamydiae. Chlamydiae are extremely tiny bacteria, below the resolving power of the light microscope. Although the Gram stain is not used for identification, the bacteria have cell walls resembling those in Gram‐negative bacteria.
Chlamydiae display a growth cycle that takes place within host cells. The bacteria invade the cells and differentiate into dense bodies called reticulate bodies. The reticulate bodies reproduce and eventually form new chlamydiae in the host cell called elementary bodies. Chlamydiae cause several diseases in humans, such as psittacosis, a disease of the lung tissues; trachoma, a disease of the eye; and chlamydia, an infection of the reproductive tract.
Mycoplasmas. Mycoplasmas are extremely small bacteria, below the resolving power of the light microscope. They lack cell walls and are surrounded by only an outer plasma membrane. Without the rigid cell wall, the mycoplasmas vary in shape and are said to be pleomorphic. Certain species cause a type of mild pneumonia in humans as well as respiratory tract and urinary tract diseases.