Rickettsiae and Spirochetes
Contents
Ricketssiae
• Morphology
• Classification
• Diseases
Spirochetes
• Morphology
• Classification
• Diseases
Intended
learning objectives
At the end of this lecture, the student will be able to
• Explain the characteristic features of ricketssiae
• List certain ricketssial diseases and their symptoms
• Explain the special features in the morphology of
spirochetes
• Identify clinically important spirochetes
Study of
Rickettsiae
• They are named after Howard Taylor Ricketts
• Non-motile
• Gram-negative
• Non-sporeforming
• Highly pleomorphic bacteria
• Can present as cocci(0.1 μm in diameter)
• Rods (1–4 μm long)
• Thread-like (10 μm long).
• Obligate intracellular parasites
• Rickettsia survival depends on entry, growth, and
replication within the cytoplasm
• Cannot live in artificial nutrient environments
• Grown either in tissue or embryo cultures (typically,
chicken embryos are used) of eukaryotic host cells (typically endothelial
cells)
• In the past it was positioned somewhere between viruses
and true bacteria
Rickettsial
diseases
• Rickettsia species are carried by many chiggers, ticks,
fleas, and lice,
• Typical rickettsia is very similar to that of
Gram-negative bacteria.
• Outer structures – innermost cytoplasmic membrane, a thin
electron dense rigid cell wall and an outer layer.
• The outer layer resembles typical membranes in its
chemical composition and its trilaminar appearance.
Structure
• The cell wall is chemically similar to that of
Gram-negative bacteria but it contains diaminopimelic acid and lacks teichoic
acid.
• Intracytoplasmic invaginations of the plasma membrane
(mesosomes) and ribosomes are also seen.
• There are no discrete nuclear structures
• Cause diseases in humans such as
– Typhus
– Rickettsial pox
– Boutonneuse fever
– African tick bite fever
– Rocky Mountain spotted fever
– Flinders Island spotted fever
– Queensland tick typhus
Transmission
• In their arthropod vectors, the rickettsia multiply in the
epithelium of the intestinal tract
• They are excreted in the feces, but occasionally gain
access to the arthropods salivary glands.
• They are transmitted to man, via the arthropod saliva,
through a bite.
• In their mammalian host, they are found principally in the
endothelium of the small blood vessels, particularly in those of the brain,
skin and heart.
• Hyperplasia of endothelial cells and localized thrombus
formation lead to obstruction of blood flow, with escape of RBC’s into the
surrounding tissue.
Pathogenicity
• Inflammatory cells also accumulate around affected
segments of blood vessels.
• This angiitis appears to account for some of the more
prominent clinical manifestations, such as petechial rash, stupor and terminal
shock.
• Death is ascribed to damage of endothelial cells,
resulting in leakage of plasma, decrease in blood volume, and shock.
Symptoms of
rickettsioses
• Rickettsioses are difficult to diagnose
• Common symptoms that typically develop within 1–2 weeks of
infection
• Fever, headache, malaise, rash, nausea, and vomiting
• Rickettsia infections damage the permeability of blood
capillaries, which results in a characteristic spotted rash
Diagnosis
and treatment
• Diagnosis is usually based on clinical recognition and
serology
• Treatment of patients with possible rickettsioses should
be started early and should never await confirmatory testing
• Immediate empiric treatment with a tetracycline is
recommended, most commonly doxycycline
• Broad-spectrum antibiotics are not usually helpful
• Chloramphenicol may be an alternative
• No vaccine is available for preventing rickettsial
infections
Study of
spirochaetes
• The phylum Spirochaetes contains gram-negative,
chemoheterotrophic bacteria
• Spirochetes can be anaerobic, facultatively anaerobic, or
aerobic.
• Distinguished by their structure and mechanism of motility
• They are slender, long bacteria (0.1 to 3.0 m by 5 to 250
m) with a flexible, helical shape
Distinguishing
spirochetes from other bacteria
• Spirochetes differ greatly from other bacteria with
respect to motility
• Can move through very viscous solutions though they lack
external rotating flagella
• When in contact with a solid surface, they exhibit
creeping
• Or crawling movements
• Their unique pattern of motility is due to an unusual
morphological structure called the axial filament
Spirochaete
morphology
• The central protoplasmic cylinder contains cytoplasm and
the nucleoid
• Bounded by a plasma membrane and gram-negative type cell wall
• Two to more than a hundred procaryotic flagella,
called axial fibrils, periplasmic
flagella or endoflagella, extend from both ends of the cylinder and often
overlap one another in the center third of the cell
• The whole complex of periplasmic flagella, the axial
filament, lies inside a flexible outer sheath or outer membrane.
• The outer sheath contains lipid, protein, and carbohydrate
and varies in structure between different genera
• The outer sheath of Treponema pallidum has few proteins exposed
on its surface.
• This allows the syphilis spirochete to avoid attack by
host antibodies
Spirochete
morphology
• One end of each axial filament is attached near a pole of
the cell
• By rotating its axial filament, the cell rotates in the
opposite direction, like a corkscrew
• The movement is very efficient in moving the organism
through liquids
• Many spirochetes are found in the human oral cavity
Spirochete
– classes
Treponema
• The spirochetes include a number of important pathogenic bacteria
• The best known is the genus Treponema, which includes Treponema
pallidum, the cause of syphilis
Borrelia
• Members of the genus Borrelia cause relapsing fever and
Lyme disease, serious diseases that are usually transmitted by ticks or lice
Leptospira
• Leptospirosis is a disease usually spread to humans by
water contaminated by Leptospira species.
• The bacteria are excreted in the urine of such animals as
dogs, rats and swine. so domestic dogs and cats are routinely immunized against
leptospirosis.
Symptoms
• High fever
• Headache
• Chills
• Muscle aches
• Vomiting
• Yellow skin and eyes
• Red eyes
• Abdominal Pain
• Diarrhoea
• Rash
Leptospira
interrogans, the cause of leptospirosis
Summery
• Ricketssia are non-motile, gram negative pleomorphic
bacteria
• They are obligate intracellular parasites and are
transmitted through arthropod vectors
• Certain ricketssial diseases are Typhus, Rickettsial pox
and Rocky Mountain spotted fever
• Spirochetes are gram negative motile bacteria
• Special morphological feature – axial filaments
• Exhibit corkscrew type motility
• Can move even through viscous media
• Clinically significant spirochetes include treponema
pallidum (causing syphillis) and Borrelia species (causing lyme disease)
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