Rickettsia and Coxiella

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.