2-11. RICKETTSIAE, CHLAMYDIAE, AND MYCOPLASMP
a. General. The rickettsiae, chlamydiae, and mycoplasmas are bacteria that
are different from the typical bacteria discussed above. These organisms have unusual
and more exacting growth requirements. Several of these organisms are significant
(1) Morphology. Rickettsiae are cocco-bacillary to coccal in shape, varying
in size from 0.3 p in length. They are plemorphic--that is, they have many shapes within
the same species--and are found either singly, in pairs, or in chains. They are gram-
negative and difficult to strain, but when properly stained they can be observed under
the ordinary light microscope. The cell structure of the rickettsiae is similar to that of the
typical bacterial cell, frequently exhibiting a capsule.
(2) Physiology. Physiologically, the intracellular rickettsiae resemble typical
bacteria in some respects and viruses in others. Like typical bacteria, they multiply by
binary fission. However, like viruses, they are obligate parasites--that is, they require a
living cell for their existence and propagation. They differ from viruses in that they do
contain some enzymes and demonstrate some independent metabolism.
(3) Pathogenic rickettsiae. The rickettsiae are transmitted from animal,
animal to man, or man to man through the body of an intermediate arthropod host. The
most common arthropod vectors are fleas, lice, ticks, and mites. The rickettsiae are
nonpathogenic to the arthropod vectors and, because of their minute size, are capable
of being transferred from a parent arthropod to the offspring through a process known
as transovarian transmission. An exception is the louse, which dies within 8 to 10 days
after infection with Rickettsia prowazekii. This process is particularly common in ticks
and mites. The infectious organism passes through the reproductive system of the
adult female into the eggs, and is thus transmitted to succeeding generations. The
rickettsial diseases are characteristically manifested by high fevers and skin eruptions.
Table 2-3 lists the most common rickettsial diseases, the causative agents, the
reservoirs of infection, and the arthropod vectors responsible for their transmission.
c. Chlamydia. Chlamydiae are nonmotile, coccoid bacteria ranging in size from
about 0.2 to l. Except for possessing a thicker cell wall, their structure and
composition are like the typical gram-negative bacteria. However, they exhibit a unique
developmental cycle within the higher living cells they infect. They are obligate
intracellular parasites that are associated with several human diseases (Table 2-3).