Biomedical Engineering Reference
In-Depth Information
1999, all of the group species were assigned to Genus Chlamydia. Molecular
studies indicated that these two genera can be cleanly distinguished from
one another, based on genome size, DNA re-association, and sequence
dissimilarities.
Most obligate intracellular infectious agents require a vector to transfer
themselves from one host to another. For example, Rickettsia (Chapter 5),
another obligate intracellular bacteria, are transmitted via fleas, ticks, or lice.
Malaria and babesiosis, caused by obligate intracellular members of Class
Apicomplexa (Chapter 19), are transmitted by arthropods. Surprisingly, the
human pathogens in Class Chlamydiaceae manage to move from one host
to another, without the aid of a vector. How do they do it? These tiny organ-
isms create even smaller, infective forms, known as elementary bodies.
Elementary bodies have a rigid outer membrane and are resistant to environ-
mental conditions outside their hosts. They travel in expelled droplets, in the
case of a pneumonic infection, or in secretions, in the case of a venereal
infection or an eye infection. The elementary bodies attach to host cell mem-
branes, and are internalized within host cell endosomes. Elementary bodies
transform into reticulate bodies, the metabolically active form of the organ-
ism. Chlamydia and Chlamydophila organisms inhibit the fusion of host
endosomes with host lysosomes, and thus escape the normal cellular mecha-
nism by which phagocytosed bacteria are killed by eukaryotes. This bacterial
survival trick is similar to that employed by Coxiella burnetii (Chapter 7),
another obligate intracellular bacteria that is resistant to degradation by host
cells. When the endosome is filled with organisms of Class Chlamydiaceae,
the enlarged endosome becomes a cytoplasmic inclusion body, visible under
the light microscope. Active infections are characterized by eukaryotic cell
lysis. Most cases of infection with Chlamydia or Chlamydophila are asymp-
tomatic, indicating that cytopathic effects are often minimal. Immune defi-
ciency exacerbates the clinical virulence of infections caused by pathogenic
members of Class Chlamydiae.
Chlamydiae
Chlamydiae
Chlamydiales
Chlamydiaceae
*Chlamydophila
*Chlamydia
In terms of documented infections and disease, the most clinically important
species in Class Chlamydiaceae is Chlamydia trachomatis. This species can be
divided into several biological types, and the biological types can be subdiv-
ided into distinct serologic variants (serovars). The different diseases caused
by Chlamydia trachomatis are each associated with their own variants of
the species. Estimates would suggest that worldwide, more than half a billion
people are infected with one or another subtype of Chlamydia trachomatis.
