II. THE PROKARYOTIC CELL: BACTERIA
C. ATYPICAL PATHOGENIC BACTERIA
The overall purpose of this Learning Object is to introduce several medically important mycoplasmas, rickettsias, and chlamydias.
In this section on the prokaryotic cell we will look at three groups of pathogenic or potentially pathogenic bacteria that are atypical of the ones studied in the previous section. These include the mycoplasmas, the rickettsias, and the chlamydias.
1. Mycoplasmas
a. Mycoplasmas (def) are smaller than ordinary bacteria, typically being 0.15 - 0.30 micrometers (µm) in size. They are the smallest microorganisms that can independently grow on a cell-free medium.
b. Mycoplasmas are the only prokaryotes that lack a cell wall and contain sterols (def) in their cytoplasmic membrane. They are surrounded only by a cytoplasmic membrane and are therefore highly pleomorphic (def), that is, their shape varies. The sterols in the cytoplasmic membrane may provide added strength. In addition, mycoplasmas are able to maintain a nearly even pressure between the outer environment and the cytoplasm by actively pumping out sodium ions.
The most important mycoplasma in terms of human infections is Mycoplasma pneumoniae. This bacterium is a common cause of both upper and lower respiratory infections, including tracheobronchitis and primary atypical pneumonia. Ureaplasma urealyticum causes non-gonococcal urethritis (def) (NGU), pneumonia in newborns, and is associated with chronic lung disease in premature infants. Mycoplasma hominis is commonly found in the vagina but may cause opportunistic infections of the uterine tubes.
2. Rickettsias
a. Rickettsias are small and typically 0.3-1.0 µm in size. They appear as pleomorphic bacillary or coccobacillary forms.
b. Most are obligate intracellular parasites (def) unable to make their own ATP (def) but rather getting it from their host cell. The infectious form or elementary body (def) utilizes adhesins (def) to adhere to the surface of the host cell. It then uses invasins (def) to enter the cell. The invasins trick the host cell into engulfing the bacteria (endocytosis) and placing it in an endocytic vacuole (see Fig. 1 and Fig. 2) where it develops into a larger reticulate body (def). The reticulate body then divides successively by binary fission and condenses to form more elementary bodies that are then released from the host cell. See the Chlamydia life cycle summarized in Fig. 3-8.
- Electron micrograph of Ehrlichia infecting a host cell by endocytosis.
- Electron micrograph of Ehrlichia appearing as reticulate cells and as dense-core cells.
- Electron micrograph of Ehrlichia being released from an infected host cell.
c. With rare exceptions, mammals become infected with rickettsia only through the bites of infected arthropods (def).
d. In humans, most rickettsias replicate within endothelial cells of the small blood vessels thus causing a characteristic rash.
e. Their structure and replication are similar to gram-negative bacteria.
Rickettsia rickettsii (inf) causes Rocky Mountain Spotted Fever and is transmitted to humans by infected wood ticks in the Western U.S. and by dog ticks in the East. Rickettsia prowazekii (inf) causes epidemic typhus fever and is transmitted by infected human body lice. Infected fleas transmit Rickettsia typhi (inf), the causative agent of endemic typhus fever. The vector for Ehrlichia (inf), the rickettsia causing human granulocytic erlichiosis, is the deer tick.
3. Chlamydias
a. Chlamydias are coccoid bacteria that are also quite small, typically being 0.2-0.7 µm in size. Chlamydias also lack peptidoglycan (def) in their cell wall.
b. They are classified as a type of rickettsia that do not require arthropods for transmission to humans.
c. Chlamydias are obligate intracellular parasites (def) of vertebrates unable to make their own ATP but rather getting it from their host cell. Like the other Rickettsias mentioned above, the Chlamydias have a unique intracellular pattern of replication. The infectious form or elementary body (def) is about 0.3 µm in diameter. The bacterium utilizes adhesins (def) to adhere to the surface of the host cell and then uses invasins (def) to enter the cell. The invasins trick the host cell into engulfing the bacteria (endocytosis) and placing it in an endocytic vacuole (see Fig. 3 and 4) where it develops into a larger reticulate body (def) . The reticulate body (see Fig. 5) then divides successively by binary fission (see Fig. 6) and condenses to form more elementary bodies (see Fig. 7) that are then released from the host cell (see Fig. 8). The life cycle is summarized in Fig. 3-8.
One species, Chlamydia trachomatis, causes trachoma (def) and lymphogranuloma venereum (LGV) (def), mostly in underdeveloped countries. In the United States, it is the primary cause of nongonococcal urethritis (NGU) (def) as well as being a frequent cause of epididymitis (def), pelvic inflammatory disease (PID) (def), and neonatal (def) respiratory and eye infections. C. trachomatis is the leading cause of female reproductive problems in the U.S. Chlamydia psittaci causes psittacosis (ornithosis) (def).
Highlighted Bacterium: Chlamydia trachomatisClick on this link, read the description of Chlamydia trachomatis, and be able to match the bacterium with its description on an exam.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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