Organism:
Genus: Chlamydia
Species: trachomatis, psittaci
General Characteristics of Chlamydia :
The chlamydia, which are incorrectly called the PLT viruses or Bedsonia or basophilic viruses, are bacteria which are obligate intracellular parasites of higher animals (mammals and birds).
The members of this group share a unique development cycle, a common morphology and a common family antigen.
They are not transmitted by arthropods.
These organisms are termed basophilic because they take up the Giemsa stain (i.e., they stain blue).
They are gram-negative, non-motile and multiply in the cytoplasm of the host cell.
These organisms generally parasitize epithelial cells.
The methods used to study them are, in the main, those of the virologist rather than the bacteriologist.
Furthermore, the clinical features, pathogenesis, pathology and epidemiology of chlamydial infections are similar to those of viral infections.
Distinctive properties:
The Chlamydia have an unusual developmental cycle that involves two distinct forms: infectious elementary bodies & intracellular reticulate bodies
Elementary bodies attach and are internalized by susceptible host cells. Once inside, they reorganize into a replicative form (the reticulate body).
Over a 24 hour period, these reticulate bodies divide and begin to reorganize back into elementary bodies.
About 48-72 hours after infection, the cell is lysed and numerous infectious elementary bodies are released.
The genome of Chlamydia is only 25% the size of E. coli, making it one of the smallest prokaryotes.
The pathogenic mechanisms employed by Chlamydia are not well understood.
Taxonomy of Chlamydia :
The chlamydia fall into two main ecological groups:
Group A: are the agents causing trachoma, inclusion conjunctivitis, and lymphogranuloma venereum, which seem to infect man only.
Group B: are those agents transmitted to man as zoonotic infections (mainly birds)
About 100 species of birds are naturally infected with chlamydia. This includes 71 species of parrots as well as finches, pigeons, chickens, ducks, turkeys and seabirds.
Morphology and Structure:
· The chlamydial cell is roughly spherical and measures between 0.3 and 1.0 u in diameter, according to the stage of development.
· Both the small and the large cell types contain complete cell walls which are similar to the cell walls of gram-negative bacteria.
· Under the cell wall lies a separate cytoplasmic membrane made up of large amounts of lipid. .The DNA occurs as an irregular mass in the cytoplasm.
· There is no nuclear membrane.
· Ribosomes can be seen throughout the cytoplasm.
· The cells contain no capsule or flagella.
Metabolism :
· There are no detectable flavoproteins or cytochromes.
· It appears that the basis of the obligate intracellular parasitism is due to a lack of ATP-generating ability and the need to obtain ATP from the host cell.
· The cells can synthesize DNA, RNA and protein.
Growth and multiplication :
Chlamydia passes through a series of developmental forms while multiplying by binary fission.
This is termed the "developmental cycle." Two morphologically different developmental forms with a continuous gradation of intermediates between them can be recognized.
One is a small cell about 0.3 u in diameter, with an electron-dense nucleoid.
The other is a large cell, 0.5 to 1.0 u in diameter without a dense center.
There appears to be no significant difference in morphology or developmental cycle among the various chlamydia, and a single generalized description applies to all.
The development cycle may be regarded as an orderly alternation of the small and large cell type.
It is initiated by the highly infectious small cell which is taken into the host cell by phagocytosis.
The engulfed small cell retains its morphological integrity in vacuoles bound by membrane derived from the surface of the host cell, and there is no eclipse (period in which the parasite loses the infectious ability).
Without loss of individuality, the small cell is reorganized into a large cell which is the vegetative multiplying form of these organisms.
Then, still in the membrane-bound vacuole, the large cell grows in size and multiplies by repeated binary fission.
The developmental cycle is completed by the reorganization of most of the large cells into small ones which are then available for infection of new host cells.
The time required for completion of a cycle varies from 24-48 hours, depending on the particular host/parasite system involved.
ELEMENTARY BODY (EB) .
RETICULATE BODY (RB) .
Cell size 0.3 um in diameter.
Size 0.5 - 1.0 um in diameter.
RNA : DNA content = 1.1
RNA : DNA content = 3.1
· Under the cell wall lies a separate cytoplasmic membrane made up of large amounts of lipid. .The DNA occurs as an irregular mass in the cytoplasm.
· There is no nuclear membrane.
· Ribosomes can be seen throughout the cytoplasm.
· The cells contain no capsule or flagella.
Metabolism :
· There are no detectable flavoproteins or cytochromes.
· It appears that the basis of the obligate intracellular parasitism is due to a lack of ATP-generating ability and the need to obtain ATP from the host cell.
· The cells can synthesize DNA, RNA and protein.
Growth and multiplication :
Chlamydia passes through a series of developmental forms while multiplying by binary fission.
This is termed the "developmental cycle." Two morphologically different developmental forms with a continuous gradation of intermediates between them can be recognized.
One is a small cell about 0.3 u in diameter, with an electron-dense nucleoid.
The other is a large cell, 0.5 to 1.0 u in diameter without a dense center.
There appears to be no significant difference in morphology or developmental cycle among the various chlamydia, and a single generalized description applies to all.
The development cycle may be regarded as an orderly alternation of the small and large cell type.
It is initiated by the highly infectious small cell which is taken into the host cell by phagocytosis.
The engulfed small cell retains its morphological integrity in vacuoles bound by membrane derived from the surface of the host cell, and there is no eclipse (period in which the parasite loses the infectious ability).
Without loss of individuality, the small cell is reorganized into a large cell which is the vegetative multiplying form of these organisms.
Then, still in the membrane-bound vacuole, the large cell grows in size and multiplies by repeated binary fission.
The developmental cycle is completed by the reorganization of most of the large cells into small ones which are then available for infection of new host cells.
The time required for completion of a cycle varies from 24-48 hours, depending on the particular host/parasite system involved.
ELEMENTARY BODY (EB) .
RETICULATE BODY (RB) .
Cell size 0.3 um in diameter.
Size 0.5 - 1.0 um in diameter.
RNA : DNA content = 1.1
RNA : DNA content = 3.1
Infectious
Not infectious
Adapted for extracellular survival
Adapted for intracellular growth
Hemagglutinin present
Hemagglutinin absent
Induces endocytosis
Does not induce endocytosis
Metabolically inactive
Metabolically active
Characteristics of the elementary and reticulate bodies of Chlamydia can be found in the table below.
Not infectious
Adapted for extracellular survival
Adapted for intracellular growth
Hemagglutinin present
Hemagglutinin absent
Induces endocytosis
Does not induce endocytosis
Metabolically inactive
Metabolically active
Characteristics of the elementary and reticulate bodies of Chlamydia can be found in the table below.
Pathogenicity :
Sub group A organisms, primarily infect the mucous membranes of the eye or the genitourinary tract of humans.
Sub group B organisms, although primarily parasites of birds can be transmitted to man where they cause a lung infection.
The mechanism by which chlamydia cause disease or injure cells is unknown.
Chlamydial infections of mucous membranes cause damage to tissues deep in the epithelial layer; for example, in trachoma, scarring of the tarsal plate occurs frequently.
There is some evidence that a toxin is produced.
Epidemiology:
Trachoma is prevalent in Africa and Asia, generally in hot and dry areas.
The organisms are very persistent. Their habitat is similar to that of Neisseria and Haemophilus.
Infection can occur via swimming in unchlorinated pools, sharing towels or by passage through the birth canal.
Sub group A organisms, primarily infect the mucous membranes of the eye or the genitourinary tract of humans.
Sub group B organisms, although primarily parasites of birds can be transmitted to man where they cause a lung infection.
The mechanism by which chlamydia cause disease or injure cells is unknown.
Chlamydial infections of mucous membranes cause damage to tissues deep in the epithelial layer; for example, in trachoma, scarring of the tarsal plate occurs frequently.
There is some evidence that a toxin is produced.
Epidemiology:
Trachoma is prevalent in Africa and Asia, generally in hot and dry areas.
The organisms are very persistent. Their habitat is similar to that of Neisseria and Haemophilus.
Infection can occur via swimming in unchlorinated pools, sharing towels or by passage through the birth canal.
Laboratory Diagnosis :
Laboratory diagnosis is made by one or more of the following:
1. Isolation of the organism from infected tissue:
The tissue is inoculated into the yolk sac of seven-day chick embryos or in McCoy human cells.
2. Characteristic cytoplasmic inclusion bodies infected cells.
3. Serological diagnosis:
a. Microimmunofluorescent tests in tears of patients with eye infections for the presence of anti-chlamydia antibody. In neonatal conjunctivitis and early trachoma, direct immunofluorescence of conjunctive cells with fluorescein - conjugated monoclonal antibody is sensitive and specific.
b. Delayed-type skin reaction (type IV hypersensitivity) to killed organisms in genitourinary infections (Frei test).
c. Rising titer of antibody against the chlamydial family antigen in lung infecitons. This accomplished with the complement fixation test or the fluorescent antibody test.
Treatment :
Chlamydia exhibit low pathogenicity except in a compromised host. The chlamydial diseases are relatively easy to treat, but present two problems.
1. Latency of infection:infections may remain latent or sub-clinical for years.
2. Susceptibility of compromised host to reinfection:
the compromised host usually remains compromised because of genetic and/or environmental factors and becomes reinfected.
3. Minimal symptomology :
Chlamydia trachomatis - doxycycline or azithromycinChlamydia pneumonia - doxycycline or azithromycin or erythromycinChlamydia psittaci - doxycycline or erythromycin
b. Delayed-type skin reaction (type IV hypersensitivity) to killed organisms in genitourinary infections (Frei test).
c. Rising titer of antibody against the chlamydial family antigen in lung infecitons. This accomplished with the complement fixation test or the fluorescent antibody test.
Treatment :
Chlamydia exhibit low pathogenicity except in a compromised host. The chlamydial diseases are relatively easy to treat, but present two problems.
1. Latency of infection:infections may remain latent or sub-clinical for years.
2. Susceptibility of compromised host to reinfection:
the compromised host usually remains compromised because of genetic and/or environmental factors and becomes reinfected.
3. Minimal symptomology :
Chlamydia trachomatis - doxycycline or azithromycinChlamydia pneumonia - doxycycline or azithromycin or erythromycinChlamydia psittaci - doxycycline or erythromycin
Diseases:
The chlamydial diseases include: Chlamydia has on its surface, a peptide that resembles one in heart myosin.
The peptide, when displayed by antigen-presenting cells, can trigger T-cells that attack both Chlamydia and heart cells, thus causing heart muscle inflammation (myocarditis).
This autoimmune reaction also plays a role in the formation of the artery-clogging plaques of atherosclerosis.
Chlamydia induce interferon and are sensitive to it.
Subgroup A Diseases (Persn to person transmition ) :
The chlamydial diseases include: Chlamydia has on its surface, a peptide that resembles one in heart myosin.
The peptide, when displayed by antigen-presenting cells, can trigger T-cells that attack both Chlamydia and heart cells, thus causing heart muscle inflammation (myocarditis).
This autoimmune reaction also plays a role in the formation of the artery-clogging plaques of atherosclerosis.
Chlamydia induce interferon and are sensitive to it.
Subgroup A Diseases (Persn to person transmition ) :
Trachoma :
Causual agent : Chlamydia trachomitis
host : Man
Subgroup A (person-to-person transmition)
Inclusion conjunctivitis :
Causual agent : Chlamydia trachomitis
host : Man
Subgroup A (person-to-person transmition)
Inclusion conjunctivitis :
Causative agent: Chlamydia trachomitis
Host : Fowl, Man
Urethritis :
Host :Man
Host : Fowl, Man
Urethritis :
Host :Man
Causative agent: Chlamydia trachomatis
Cervicitis :
Causative agent :Chlamydia trachomatis
Host :Man
Ophthalmia neonatorum :
Causativ agent :Chlamydia trachomatis
Host :Man
Myocarditis :
Causative agent :Chlamydia trachomatis
Host : Man
Lymphogranuloma venereum :
Causative agent : Chlamydia trachomatis
Host : Man
Atherosclerosis :
Causative agent : Chlamydia trachomatis
Host :Man
Neonatal Pneumonia :
Causative agent :Chlamydia trachomatis
Host : Man
Cervicitis :
Causative agent :Chlamydia trachomatis
Host :Man
Ophthalmia neonatorum :
Causativ agent :Chlamydia trachomatis
Host :Man
Myocarditis :
Causative agent :Chlamydia trachomatis
Host : Man
Lymphogranuloma venereum :
Causative agent : Chlamydia trachomatis
Host : Man
Atherosclerosis :
Causative agent : Chlamydia trachomatis
Host :Man
Neonatal Pneumonia :
Causative agent :Chlamydia trachomatis
Host : Man
Subgroup B (mostly bird-to-human (transmition) :
Bronchitis/pneumonia/sinusitis :
Causative agent : Chlamydia pneumoniae
Host : Man
Atherosclerosis :
Causative agent : Chlamydia pneumoniae
Host : Man
Meningopneumonitis :
Causative agent : Chlamydia psittaci
Birds --> Man
Hepatic and renal dysfunction :
Chlamydia psittaci
Birds --> Man
Conjunctivitis :
Chlamydia psittaci
Birds --> Man
Abortion :
Chlamydia psittaci
Birds --> Man
Endocarditis :
Chlamydia psittaci
Birds --> Man
During infection, antibodies are synthesized but recovery is not generally protective.
CONTROL:
Sanitary: Good hygiene, treatment of sexual partners and the quarantine of birds all reduce the incidence.
Immunological: No vaccine is available or likely since specific antibodies fail to neutralize elementary bodies in vivo.
Chemotherapeutic: Tetracycline or erythromycin are drugs of choice.
Summary :
1. The chlamydia are extremely small, gram-negative, coccal-shaped, obligate intracellular parasites with limited metabolic capability which are classified as bacteria.
2. The chlamydia lack flavoproteins and cytochromes.
3. Chlamydia undergo a unique developmental cycle which is an alternation in size between the small elementary body and the relatively large reticulate body.
4. The elementary body is relatively metabolically inactive, adapted for extracellular survival and is the infectious unit.
5. The reticulate body is metabolically active, adapted for intracellular growth and is not infectious.
6. The major target tissue of the chlamydia is mucous membranes.
7. Serological diagnosis of chlamydial diseases is via a fluorescent antibody test, complement fixation test or a delayed type hypersensitivity test (Frei test) for lymphogranuloma venereum.
Sanitary: Good hygiene, treatment of sexual partners and the quarantine of birds all reduce the incidence.
Immunological: No vaccine is available or likely since specific antibodies fail to neutralize elementary bodies in vivo.
Chemotherapeutic: Tetracycline or erythromycin are drugs of choice.
Summary :
1. The chlamydia are extremely small, gram-negative, coccal-shaped, obligate intracellular parasites with limited metabolic capability which are classified as bacteria.
2. The chlamydia lack flavoproteins and cytochromes.
3. Chlamydia undergo a unique developmental cycle which is an alternation in size between the small elementary body and the relatively large reticulate body.
4. The elementary body is relatively metabolically inactive, adapted for extracellular survival and is the infectious unit.
5. The reticulate body is metabolically active, adapted for intracellular growth and is not infectious.
6. The major target tissue of the chlamydia is mucous membranes.
7. Serological diagnosis of chlamydial diseases is via a fluorescent antibody test, complement fixation test or a delayed type hypersensitivity test (Frei test) for lymphogranuloma venereum.
Best wishes
Prof.Dr. Fathi Awwad Mansour
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