Lesson 2: Public Health Microbiology

Section II: Bacteria

2-11

[Previous] [Next]

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 pathogens.

b. Rickettsia.

(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 0.2µ to lµ. 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).

Table 2-3. Common rickettsial diseases

d. Mycoplasma. Mycoplasmas are the smallest organisms known that are capable of growth and reproduction outside of living host cells. Because of their variations of shape, the actual size of the individual cells is variable. It is generally agreed that they range from about 0.12µ to 0.25µ in diameter. The major difference between the mycoplasmas and typical bacterial cells is that mycoplasmas completely lack a cell wall; therefore, they assume a coccoid shape. Cell reproduction is more complex than typical bacteria. They vary in their growth requirements, but all can be grown on special artificial media. Mycoplasmas cause a number of infections (Table 2-3).

[Previous] [Next]