Chapter 20 Chlamydiae, rickettsiae and mycoplasmas
Chlamydiae, rickettsiae and mycoplasmas are a miscellaneous group of organisms with properties common to both bacteria and viruses. Although they are categorized together in this chapter for the sake of convenience, they differ markedly from each other and cause divergent human diseases. A comparison of bacteria, chlamydiae, rickettsiae, mycoplasmas and viruses is given in Chapter 2, Table 2.1.
The chlamydiae are a group of microorganisms related to Gram-negative bacteria. However, unlike bacteria, they are unable to grow on inanimate culture media. They are therefore obligatory intracellular parasites. Their main characteristics include the following:
Rickettsiae are pleomorphic organisms, smaller than bacteria but resembling them structurally and metabolically, including cell wall formation. They, like Chlamydia and viruses, are obligate intracellular parasites. The best-known human rickettsial disease is typhus, which spreads wildly in conditions of malnutrition and poverty. Rickettsiae are: