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Unlike facultative intracellular bacteria that can grow within or outside of a host's body, obligate bacteria cannot survive without host cells. These bacteria cannot reproduce outside of the host cell because they lack the metabolic processes and enzymes needed to reproduce, which the host cell gives them. [ 3 ]
Being obligate intracellular bacteria, rickettsias depend on entry, growth, and replication within the cytoplasm of living eukaryotic host cells (typically endothelial cells). [9] Accordingly, Rickettsia species cannot grow in artificial nutrient culture; they must be grown either in tissue or embryo cultures.
R. rickettsii is an obligate intracellular alpha proteobacterium that belongs to the Rickettsiaceae family. [6] Within the Rickettsia species, these bacteria are divided into four clades. The clades include ancestral group, spotted fever group (SFG), typhus group, and transitional group.
Obligate intracellular parasites cannot reproduce outside their host cell, meaning that the parasite's reproduction is entirely reliant on intracellular resources. All viruses are obligate intracellular parasites. Bacterial examples (that affect humans) include: Chlamydia, and closely related species. [14] Rickettsia; Coxiella
They are dependent on replication inside the host cells; thus, some species are termed obligate intracellular pathogens and others are symbionts of ubiquitous protozoa. Most intracellular Chlamydiota are located in an inclusion body or vacuole. Outside cells, they survive only as an extracellular infectious form.
They are obligate intracellular parasites, and some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans, and Ehrlichia, which causes diseases in livestock. Another genus of well-known Rickettsiales is the Wolbachia , which infect about two-thirds of all arthropods and nearly all filarial nematodes. [ 2 ]
A. phagocytophilum is a small, obligate, intracellular bacterium with a Gram-negative cell wall. It is 0.2–1.0 μm and lacks a lipopolysaccharide biosynthetic machinery. The bacterium first resides in an early endosome, where it acquires nutrients for binary fission and grows into small groups called morula
C. trachomatis are bacteria in the genus Chlamydia, a group of obligate intracellular parasites of eukaryotic cells. [3] Chlamydial cells cannot carry out energy metabolism and they lack biosynthetic pathways. [19] C. trachomatis strains are generally divided into three biovars based on the type of disease they cause.