In this section, we will focus on three major groups of pathogenic or potentially pathogenic bacteria that are atypical of the ones studied in the previous sections. These include the mycoplasmas, the rickettsias, and the chlamydias. In addition, we have also included information on bacteria of the family Spirochaetaceae, since two of the pathogens profiled belong to this family.

• Mycoplasmas

  • Mycoplasmas are smaller than ordinary bacteria, typically being 0.15 to 0.30 µm (micrometres) in size. They are the smallest microorganisms that can independently grow on a cell-free medium.

  • Mycoplasmas are the only prokaryotes that lack a cell wall and contain sterols in their cytoplasmic membrane. They are surrounded only by a cytoplasmic membrane and are therefore highly pleomorphic, 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, 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.

• Rickettsias

• Rickettsias are small and typically 0.3 to 1.0 µm in size. They appear as pleomorphic bacillary or coccobacillary forms.

• Most are obligate intracellular parasites unable to make their own ATP but rather getting it from their host cell. The infectious form or elementary body utilizes adhesins to adhere to the surface of the host cell. It then uses invasins to enter the cell. The invasins trick the host cell into engulfing the bacteria (endocytosis) and placing it in an endocytic vacuole where it develops into a larger reticulate body. The reticulate body then divides successively by binary fission and condenses to form more elementary bodies that are then released from the host cell.

• With rare exceptions, mammals become infected with rickettsia only through the bites of infected arthropods.

• In humans, most rickettsias replicate within endothelial cells of the small blood vessels, thus causing a characteristic rash.

• Their structure and replication are similar to gram-negative bacteria.

Rickettsia rickettsii 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 causes epidemic typhus fever and is transmitted by infected human body lice. Infected fleas transmit Rickettsia typhi, the causative agent of endemic typhus fever.

• Chlamydias

• Chlamydias are coccoid bacteria that are also quite small, typically being 0.2 to 0.7 µm in size. Chlamydias also lack peptidoglycan in their cell wall

• They are classified as a type of rickettsia that do not require arthropods for transmission to humans.

• Chlamydias are obligate intracellular parasites 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 is about 0.3 µm in diameter. The bacterium utilizes adhesins to adhere to the surface of the host cell and then uses invasins to enter the cell. The invasins trick the host cell into engulfing the bacteria (endocytosis) and placing it in an endocytic vacuole where it develops into a larger reticulate body. The reticulate body then divides successively by binary fission and condenses to form more elementary bodies that are then released from the host cell.

Family: Spirochaetaceae

• Borrelia

• Borrelia is a genus of bacteria of the spirochete phylum. It causes borreliosis, a zoonotic, vector-borne disease transmitted primarily by ticks and some by lice, depending on the species. There are 36 known species of Borrelia.

• Treponema

• Any of several spirochetes of the genus Treponema, certain species of which cause diseases in warm-blooded animals, as the syphilis spirochete in humans. The major species is Treponema pallidum.

The following list of Gram-positive bacteria have been profiled:

• Borrelia burgdorfei
• Chlamydia pneumoniae
• Mycobacterium leprae
• Mycoplasma pneumoniae
• Rickettsia prowazekii
• Rickettsia rickettsia
• Treponema pallidum