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There are three species: Chlamydia trachomatis, which infects the
eye and the genital tract; and two respiratory pathogens, C. pneumonia
and a related species Chlamydophila psittaci. They are obligate,
intracellular bacteria that exist in two forms: the reticulate
body (a non-infective, intracellular, vegetative form) and the elementary
body (an extracellular form that permits the organism to
survive and be transmitted), which is derived from the reticulate
body by binary fission.
The major outer membrane protein may participate in attachment
to mucosal cells. A 60-kDa cysteine-rich protein may also be associated
Chlamydia pneumoniae is transmitted from person to person by the
respiratory route. It produces pneumonia or bronchitis, which is
usually clinically mild, but may be associated with pharyngitis,
sinusitis and laryngitis.
There are multiple serotypes of C. trachomatis: A-C are associated
with trachoma (see Ocular infections
) and neonatal conjunctivitis
(see Congenital and perinatal infections
); D-K are associated with acute urethritis
and pelvic inflammatory disease (see Urinary and genital infections
L1-L3 are associated with lymphogranuloma venereum (see
Urinary and genital infections
Chlamydophila psittaci, a pathogen of birds and mammals, causes
psittacosis in humans. After an incubation period of 10-14 days,
fever, a dry unproductive cough, dyspnoea, headache and myalgia
develop. Clinical examination reveals few signs of consolidation,
but chest X-ray may show patchy consolidation.
Mycoplasma and Ureaplasma
- For C. trachomatis, usually by nucleic acid amplification test
(NAAT) although enzyme immunoassay (EIA) and culture are
- For C. pneumoniae, usually by NAAT or serology.
- Psittacosis is usually diagnosed serologically.
Mycoplasma and Ureaplasma are small bacteria that lack a cell
- Mycoplasma pneumoniae, a human respiratory pathogen, is the
second most common cause of respiratory infection after Streptococcus pneumoniae.
- Mycoplasma hominis and Ureaplasma urealyticum have uncertain
roles in relation to genital infection.
- Mycoplasma genitalium may be associated with urethritis.
Mycoplasma pneumoniae adheres to host cells by the P1 protein,
localizing to the base of the cilia where it induces ciliostasis.
Secreted hydrogen peroxide damages host membranes and interferes
with superoxide dismutase and catalase. Opsonized M. pneumoniae
is readily killed by macrophages and by the activity of the
Patients present with fever, myalgia, pleuritic chest pain and a
non-productive cough; headache is a prominent symptom. Antibodies
that agglutinate the host's red blood cells at low temperature
(cold agglutinins) cause peripheral and central cyanosis.
Infection is associated with reactive (postinfective) arthritis, and
NAAT is the method of choice for all Mycoplasma spp.
serology is an alternative.
- They are sensitive to erythromycin, tetracycline, aminoglycosides,
rifampicin, chloramphenicol and quinolones.
- They are resistant to β-lactams.
These organisms are obligate intracellular bacteria with biochemical
similarities to Gram-negative bacteria. Clinically, they are
divided into three groups:
1 spotted fever;
2 scrub typhus;
Spotted fever is generally transmitted by ticks. Scrub typhus is
caused by a single species, Rickettsia tsutsugamushi
. The typhus
group includes R. prowazekii
and R. typhi
, which cause epidemic
and murine typhus respectively. Typical features include:
- a 14-day incubation period;
- initial non-specific symptoms, which are followed by possible
development of fever, arthralgia and malaise, then a rash, conjunctivitis
- confusion, which occurs only in a proportion of Rocky Mountain
Spotted Fever cases;
- relapse of R. prowazekii infection months or years later, which
is known as Brill-Zinsser disease and is usually milder than the
Diagnosis is usually by NAAT or IgM-specific EIA. Tetracyclines
and chloramphenicol are the treatments of choice, but must
be initiated early to influence the outcome.
Coxiella burnetii is a pathogen of cattle, sheep and goats, which
localizes in the placenta. It survives desiccation in the environment
and is transmitted by contact with infected animals or their products
via the aerosol route. Typical features include:
- an acute febrile illness with fever, myalgia and cough;
- a chronic infection, which occurs in approximately 5% of cases;
- Q fever, which may present as atypical pneumonia, pyrexia of
uncertain origin and hepatitis.
- various complications including relapses, which can take
the form of culture-negative endocarditis or granulomatous
Diagnosis is made by NAAT or EIA and treatment is usually