Rickettsiae and Spirochetes

Rickettsiae and Spirochetes



• Morphology

• Classification

• Diseases


• Morphology

• Classification

• Diseases

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

• Identify clinically important spirochetes

Study of

• 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

• In the past it was positioned somewhere between viruses
and true bacteria


• 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.


• The cell wall is chemically similar to that of
Gram-negative bacteria but it contains diaminopimelic acid and lacks teichoic

• 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


• 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.


• 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

• Death is ascribed to damage of endothelial cells,
resulting in leakage of plasma, decrease in blood volume, and shock.

Symptoms of

• Rickettsioses are difficult to diagnose

• Common symptoms that typically develop within 1–2 weeks of

• Fever, headache, malaise, rash, nausea, and vomiting

• Rickettsia infections damage the permeability of blood
capillaries, which results in a characteristic spotted rash

and treatment

• Diagnosis is usually based on clinical recognition and

• 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

Study of

• The phylum Spirochaetes contains gram-negative,
chemoheterotrophic bacteria

• Spirochetes can be anaerobic, facultatively anaerobic, or

• 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

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

• Or crawling movements

• Their unique pattern of motility is due to an unusual
morphological structure called the axial filament


• 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


• 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

– classes


• The spirochetes include a number of important pathogenic bacteria

• The best known is the genus Treponema, which includes Treponema
pallidum, the cause of syphilis


• Members of the genus Borrelia cause relapsing fever and
Lyme disease, serious diseases that are usually transmitted      by ticks or lice


• 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


• High fever

• Headache

• Chills

• Muscle aches

• Vomiting

• Yellow skin and eyes

• Red eyes

• Abdominal Pain

• Diarrhoea

• Rash

interrogans, the cause of leptospirosis


• Ricketssia are non-motile, gram negative pleomorphic

• 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)

For Complete PDF Notes Click on Download Button

Leave a Comment