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Rickettsias
Morphology and Biological properties
Most of these tiny non-motile Gram –negative Bacteria are obligate parasites able to grow only within
host cells as parasites. In morphological appearance they are bacilli or cocci .Rickettsias are cultivated on
yolk sac of chicken embryo are either rod like or rounded ranging from 0.5 m to 2 m.
The cell wall is the outer most and double or multiple layer . This surrounds a hyaline
envelope enclosing a dense cell wall . The cell wall is about 7-10 nm thick and has typical unit membrane
which is 6-8 nm thick and has typical unit membrane structure as seen in bacteria . Chemically the cell
wall of Rickettsia consists of glucosamine and muramic acid . In addition diamino pimellic acid is also
found in the wall . This points a close relationship between Ricketssia and Gram-negative Bacteria.
Internal to cell membrane , there are ribosomes ( 7- 20nm) which are presumably the same type
found in bacteria . The nuclear material may be organized in a central body with radiating fibres or
dispersed as a reticulum of fibrillar structure. Double stranded DNA is found in Coxiella burnetti.
Rickettsia except Coxiella burnetti are unstable outside the host cells and their cytoplasmic membrane
readily leak macromolecules such as RNA and intracellular ions into outside aqueous medium.
Suspensions of Ricketsia outside out side the host cells have a variety of enzymes , involved in metabolic
pathways of respiration , synthesis of lipid and proteins.
Staining Character : Giemsa or Machivelli stains used for blood smear of protozoans can be used .
They are classified as bacteria on the basis of the following characters .
They are pleomorphic rods 300x 800 nm or cocci in chain or in filaments.
Cultivation: In the nature of growth Rickettsias resemble virus and Chlamydias ; i.e, they arenot
cultivable on non living materials. However Rickettsiaa like bacteria are easily cultivated in live chick
embryo. They grow best in the live cell lining the egg yolk sac.
Metabolism: Rickettsia have autonomous though incomplete metabolic activity . They are able to
oxidize glucose and organic acids . Rickettsias posses a respiratory chain complete with cytochromes and
are able to carry out electron transport phosphorylation . They are able to synthesize at least some small
molecules needed for macromolecular synthesis and growth and abtain rest of their nutrients from the
host cell.
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Differences between Viruses and Rickettsia
Property
Nucleic acids
Rickettsia
Both are present
DNA /RNA
Multiplication by binary fission
Present
Cell wall containing muramic acid
Present
Ribosomes
Present
Metabolically active enzymes
Absent
Inhibition by antibacterial drugs
Inhibited
Synthesis of ATP as source of energy
Absent
Virus
Either of them
present
Absent
Absent
Absent
Absent
Not inhibited
Absent
In the 8th edition of Bergey’s Manual Rickettsia are grouped in part XVIII which consists of two orders.
Order : 1)Rickettsiales and 2) Order : Chlamydiales
The order Rickettsiales contain 3 famlies.
Family : Rickettsiaceae
Family: Bartonellaceae
Family : Anaplasmataceae
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The family Ricketsiaceae is further divided into 3 tribes. This family consists of rod shaped or Rickettsias
which are pleomorphic.
Tribe 1. Rickettsieae – pathogens to humans
Tribe 2. Ehrlichieae – pathogen to vertebrates other than humans
Tribe 3. Wolbachieae – found in arthopods only.
Oeder : Rickettsiales : This order contains the Rickettaiales which differ from the Chlamydiales by having
a more complex metabolism that allows them synthesise ATP and lacking a complete developmental
cycle. The tribe Rickettsieae contains 3 genera.
1.Genus : Rickettsia . The important features are :
1. trasmision to humans occurs through an arthopod vector like flies , ticks , mites depending on
the species,
2. the organisms multiply within the cytoplasm and sometimes within the nucleus of the host cells,
3. in the lab they cultivated in a host animal such as guinea pigs or mice,
4. embryonated chicken eggs particularly within the cells of yolk sac membrabne,
5. tissue cell cultures usually cells from 10 day old chicken embryo.
Disesase caused by Rickettsia species and arthpod vectors which transmit them include
a) Rocky mountain spotted fever ( Ticks ) R. ricketsii
b) Classical typhus fever ( Fleas) R.mooseri
c) Rickettsial pox ( Mites ) R.akari
d) Scrub typhus ( Mites) R.tsutsugamushi
e) Epidemic typhus fever ( Lice ) R.prowazekii
2.Genus : Rochelimaea
This genus is similar too Rickettsia except for the following features
a) although organisms are mainly parasitic to humans they can be cultivated in vitro or on lab
medium ( blood based agar)
b) they grow particularly on the sueface of the host cells rather than in cytoplasm or nucleus.
R . quintana causes a louse borne disease known as trench fever in humans.
3.Genus: Coxiella
a) growth occurs preferentially within the membrane bound vacuoles of host cells rather in
cytoplasm or nucleus.
b) The organisms have unusually high resistance to heat due to occurance of endospore like
structure in the cells.
c) Although transmission to vertebrates can occur via arthopod vectors it occurs mainly by
inhalation of air borne infectious dust and also by drinking contaminated unpasteurizes milk.
Coxiella burnetii causes Q –fever .
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Family: Bartonellaceae
1. It consists of parasites of RBCs
2. can be cultivated on lab media.
Genus Bartonella causes Oroya fever in humans and is transmitted by biting fleas.
Family : Anaplasmataceae
1. These organisms grow within or on erythrocyte or occur in plasma of various wild and domestic
animals.
2. none have cultivated on non living media
Property
1.Site & Multiplication
2.Obligate intracellular parasite
Rickettsia
Cytoplasm
Coxiella
Phagolysosome
+
+
-
-
-
+
Typhus
Q-fever
Rochalimeae
Exterior surface of
host cell
-
3.Can be cultivated axenically
+
4.Spore formation
5.Human disease
Scrub typhus
-
Trench fever