General Microbiology - Schlegel, H. 1987

Prokaryotes: A Systematic Survey
Obligate Intracellular Parasites (Group 18)

Earlier (Section 3.14), we noted that certain Bacteria, such as Bdellovibrio bacteriovorus, require intact Cells of other bacteria for their reproduction. Presumably, many bacterial endosymbionts of insects or Ciliates are inextricably linked to their hosts. The cause of such obligate intracellular parasitism is generally to be found in metabolic simplification. Among bacteria, rickettsiae and chlamydiae are the best-studied obligate intracellular parasites. Both groups are pathogens of humans and animals and possess Gram-negative Cell wall structures.

Rickettsiae. Rickettsiae are named after the American researcher H. T. Ricketts, who first described the CAUSATIVE AGENT OF Rocky Mountain spotted fever. Based on the best-studied pathogen (Rickettsia prowazekii) and the disease it causes, this entire group is also referred to as the typhus group. The natural reservoirs of rickettsiae are Arthropods (ticks, chewing lice, fleas, lice), within which these microbes apparently reside as harmless parasites or even symbionts. Upon entering other hosts—animals or humans—via insect bites, Skin scratching, or inhalation, rickettsiae can cause severe pathological conditions.

Although rickettsiae are comparable in size to certain Viruses, they are distinctly different from them. Rickettsial cells contain both DNA and RNA (in a 1:3.5 ratio) and are surrounded by a cell wall containing muramic acid that is sensitive to Lysozyme. Transmission Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF ultrathin sections clearly reveal the nuclear region and The Cell wall.

Most rickettsiae have never been successfully cultured outside of living cells, but they can be propagated in embryonated eggs and animal Tissues; up to 10⁹ cells can be harvested from the yolk sac of a chicken egg. Certain Intermediary METABOLISM Enzymes can be detected in isolated rickettsial cells. During cultivation, the metabolic rate of such cells declines, but The addition of ATP, organic acids, and Amino Acids restimulates their Respiration. Thus, rickettsiae possess their own metabolism; however, presumably due to altered cell-surface permeability, they are unable to regulate the uptake and excretion of metabolites.

The most prominent pathogens among rickettsiae belong to the typhus group. Rickettsia prowazekii is the causative agent of epidemic typhus. Humans serve as the reservoir. The pathogen is transmitted by lice (body and HEAD lice). Infected lice themselves die within a few days. The infection is transmitted via louse feces. R. typhi is the causative agent of endemic, or murine, typhus; clinically, this disease resembles typhus but is less severe. Its pathogen is disseminated by rats, in which it causes no symptoms. It is transmitted from rat to rat, and occasionally to humans, by rat fleas.

The aforementioned rickettsiae are relatively sensitive to high temperatures and desiccation; however, the Q fever agent, Coxiella burnetii, is capable of surviving outside its host's body. Ticks transmit it to sheep, goats, and cattle. Humans become infected not only through tick bites, but also via wool and fur dust, contaminated soil, and milk. Standard milk pasteurization (60°С, 30 min) does not kill Coxiella.

Chlamydiae. Chlamydiae are well known as human pathogens. Chlamydia trachomatis is the causative agent of trachoma (Egyptian ophthalmia), which begins with Conjunctivitis and leads to blindness, and it also causes the sexually transmitted disease lymphogranuloma venereum. In both cases, transmission occurs through contact. Chlamydia psittaci is the causative agent of psittacosis (ornithosis), most notably manifesting as Pneumonia and fever; birds serve as the primary hosts for these chlamydiae.

Biochemical characteristics made it possible to establish that chlamydiae belong to bacteria rather than viruses, as was previously thought. They contain RNA and DNA in ratios typical of bacteria and synthesize substances that cannot be produced by eukaryotic host cells, such as muramic acid, diaminopimelic acid, D-Alanine, and Folic acid. These properties are also consistent with their susceptibility to penicillin and sulfonamides. The chlamydial genome is very small (0.66∙10⁹ Da), meaning it contains four times less Genetic information than The Genome of Escherichia coli.

Chlamydiae develop exclusively within living cells. They can be cultured in chicken embryos or tissue cultures. The dependence of chlamydiae on the host cell's metabolism is apparently due to their lack of an ATP-regenerating system. They are incapable of either phosphorylating or breaking down glucose. On the other hand, chlamydiae are exceptionally permeable to ATP and coenzyme A. Therefore, they can be regarded as "energy parasites."

Obligate intracellular parasitism is evidently the result of regressive evolution. Adaptation to existence within a host cell is accompanied by the loss of the capacity for numerous biosynthetic processes.