Bergey’s Manual* describes thegenus
andfamily Rickettsiaceae of the α-proteobacteria ; whereas, Coxiella is shown in the order
Legionellalesand family Coxiellaceae of the γ-proteobacteria. Based upon their close and intimate
similarity in the‘life-style’, despite their apparent ‘phylogenetic distance’, these two genera shall be
Salient Features :The salient-features of Rickettsia and Coxiella are as enumerated under:
(1) The bacteria belonging to thesetwo genera are found to be rod-shaped, coccoid, or pleomorphic
having typical Gram-negative walls and devoid of any flagella; however, their actual size
usually varies but they tend to be relatively very small.
Rickettsia— 0.3 to 0.5 μm (diameter); and 0.8 to 2.0 μm (length);
Coxiella— 0.2 to 0.4 μm (diameter); and 0.4 to 1.0 μm (length);
(2) It has been duly observed that all species happen to be eitherparasitic or mutualistic in
nature. Interestingly, the former species (i.e., parasitic ones) invariably grow in vertebrate
erythrocytes, macrophages, and vascular endothelial cells; and they usually reside inbloodsucking
arthropodsviz., ticks, lice, mites, fleas, tse-tse flies that essentially serve either as
vectorsor as primary hosts.
(3) By virtue of the fact that thesetwo genera predominantly comprise of vital and important
‘human-active pathogens’,both their metabolism as well as reproduction have been investigated
intensively and extensively.
Rickettsias:are found to gain entry into the host-cell by the induction of the phenomenon of
‘phagocytosis’.Thus the bonafide members belonging to the genus Rickettsia immediately
get free from the ensuing‘phagosome’ and get reproduced due to the ‘binary fission’ in the
Coxiella:In contract, it remains within the phagosome after it has undergone fusion strategically
with a‘lysosome’, and virtually undergo reproduction very much within the
‘phagolysosome’.Thus, the host-cell ultimately bursts, thereby providing the release of an
abundant quantum of newer organisms specifically.
(4)Physiology and Metabolism: Importantly, the rickettsias are prominently quite different in
comparison to most other bacteria with respect to physiology and metabolism. Some of the
highlights observed are as stated below:
(a) Rickettsias: normally lack the glycolytic path way and do not make use of ‘glucose’ as a
source of energy, but categorically oxidize both‘glutamate’ and ‘tricarboxylic acid
cycle (TCA-Cycle) intermediates,e.g., succinic acid.
(b) Rickettsial plasma membrane critically possesses the specific carrier-mediated transport
systems;and thereupon, the host cell nutrients as well as the ensuing coenzymesget absorbed and consumed almost directly.
Examples:(i) Rickettsias are observed to make use of both NAD* and uridine phosphate glucose.
(ii) The membrane of rickettsias also possesses particularly an adenylate exchange carrier which
meticulously exchanges ADP for the corresponding external ATP, whereby the latter (i.e., the host ATP)
may be able to cater for a good deal of‘energy’ essentially required for the ultimate growth.**
Rickettsial Pathogenic Organisms— are duly identified and recognized as given below :
Rickettsia prowazekii— associated with typhus fever
Rickettsia typhi— associated with typhus feverRickettsia rickettsii — associated with Rocky Mountain spotted fever.