Bergey’s Manual describes the genus Rickettsia, which is duly placed in the order Rickettsiales and family Rickettsiaceae of the α-proteobacteria ; whereas, Coxiella is shown in the order Legionellales and family Coxiellaceae of the γ-proteobacteria.
Rickettsia
and Coxiella
Bergey’s
Manual describes the genus Rickettsia, which is duly placed in the order Rickettsiales and family
Rickettsiaceae of the α-proteobacteria ; whereas, Coxiella is shown in the order
Legionellales and 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 discussed together.
Salient Features : The salient-features of Rickettsia and Coxiella are as enumerated under:
(1) The
bacteria belonging to these two
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.
Examples:
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 either parasitic 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 in blood-sucking
arthropods viz., ticks, lice, mites, fleas, tse-tse
flies that essentially serve either as vectors or as primary hosts.
(3) By
virtue of the fact that these two
genera predominantly comprise of vital and important ‘human-active pathogens’, both their metabolism as well as reproduction have been inves-tigated 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 cytoplasm.
Coxiella: In contract, it remains within
the phagosome after it has undergone fusion strategi-cally 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
trans-port systems; and thereupon, the
host cell nutrients as well as the ensuing coenzymes get 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 fever
Rickettsia rickettsii — associated with Rocky Mountain spotted fever.
Related Topics
TH 2019 - 2023 pharmacy180.com; Developed by Therithal info.