Presentación de resultados

Bergey’s Manual* describes the genus Rickettsia, which is duly placed in the order Rickettsiales and family Rickettsiaceae of the ααααα-proteobacteria ; whereas, Coxiella is shown in the order

Legionellales and family Coxiellaceae of the γγγγγ-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

Examples: (i) Rickettsias are observed to make use of both NAD* and uridine phosphate glu- cose.

(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. 3.5.4. Spirochaetes

The phylum Spirochaetes [Greek: spira = a coil ; and chaete = hair] essentially and distinguish- ably comprises of Gram-negative, chemoheterotrophic bacteria characterized by their specific structure and mechanism of motility.

Salient Features : The various vital and important salient features of the spirochaetes are as

enumerated below :

(1) They are slender long bacteria having diameter 0.1 to 3.0 μm, and length 5 to 250 μm ; and predominantly with a flexible and helical shape that may sometimes also occur in the form of chains.

(2) Multiplication of the spirochaetes invariably takes place by transverse fission.

(3) The bacterial cells consist of protoplasmic cylinder interwined with either one or more

axial fibrils, that originate in nearly equal number from the subterminal attachment disc

strategically located at either ends of the aforesaid proto-plasmic cylinder. Importantly, both the protoplasmic cylinder as well as the axial fibrils are duly enclosed in the outer envelope meticulously. However, the unattached ends of the axial fibrils may invariably get extended beyond the terminals of the protoplasmic cylinder that finally be observed as ‘po-

lar flagella’.

(4) The motility existing in the spirochaetes are usually found to be of three types, namely : (i) Obtained by the rapid rotation about the long axis of the helix

(ii) Derived by the flexion of the bacterial cells, and

(iii) Brought about by the locomotion invariably observed along a helical or a serpentine

path

(5) It has been observed that many species of spirochaetes are so slim that they may exclusively and vividly visible in a light-microscope either by the help of a phase-contrast microscope or a dark-field optics.

(6) The spectacular and distinctive features of the spirochaete morphology are quite evident by means of an ‘electron micrograph’ which explicitely reveals the following characteristic features, such as :

* Nicotinamide adenine dinucleotide.

** The prevailing metabolic dependence vividly clarifies and explains why many of these organisms should be specifically cultivated in the yolk-sacs of chick embryos or in tissue culture cells.

• Central protoplasmic cylinder contains cytoplasm and nucleoid, which is subsequently bounded by a plasma membrane together with a Gram-negative type cell wall. • Central protoplasmic cylinder actually corresponds to the body of other accessible Gram-

negative bacteria.

• Evidently two or more than a hundred prokaryotic flagella, known as axial fibrils,

periplasmic flagella (or endoflagella), extend from either ends of the cylinder and

invariably overlap one another in the centre segment of the cell as depicted in Fig. 3.11(a), (b) and (c). Spirochaetes ( )a AF PC OS IP AF = Axial fibril ; PC = Protoplasmic cylinder ; OS = Outer sheath ; IP = Insertion pore ; ( )b N R AF PM OS M CW PC N = Nucleoid ; R = Ribosome ; AF = Axial fibril ; PM = Plasma membrane ; PC = Protoplasmic cylinder ; CW = Cell wall ; M = Microtubule ; OS = Outer sheath ; ( )c

Fig. 3.11. Spirochaete Morphology ; (a) Spirochaetes ; (b) A surface view of spirochaete structure

as interpreted from electron micrographs ; (c) A cross-section of a typical spirochaete displaying morphological details.

(7) Interestingly, the spirochaetes may be anaerobic, facultatively anaerobic or even-aerobic in nature.

(8) Carbohydrates, amino acids, long-chain fatty acids (e.g., palmitic acid, stearic acid, oleic acid etc.), and long-chain fatty alcohols may cater for carbon as well as energy sources. (9) Certain spirochaetes may have inclusions but no evidence of any ‘endospore formation’ has

been reported.

(10) Important genera essentially include: Borrelia, Cristispira, Leptospira, Spirochaeta, and

Treponema.

The characteristic features of the ‘Spirochaete Genera’ viz., dimensions (μm) and flagella, G + C content (mol %), oxygen relationship, carbon + energy source, and habitats are summarized in Table 3.10.

Table 3.10. Characteristic Features of Spirochaete Genera

S.No. Genus Dimensions G + C Oxygen Carbon + Habitats

(μμμμμm)/ Content Relation- Energy

Flagella (Mol %) ship Source

1 Borrelia 0.2 – 0.5 × 3 – 20 ; 27 – 32 Anaerobic Carbohy- Mammals and arthopods ;

14 – 60 or Micro- drates Pathogens (relapsing Periplasmic aerophilic fever, Lyme disease). flagella

2 Cristispira 0.5 – 3.0 × — Facultati- — Mollusk digestive

30 – 180 ≥ 100 vely track.

Periplasmic anaerobic ?

flagella

3 Leptospira 0.1 × 6 – 24 ; 2 35 – 49 Aerobic Fatty acids Free-living or pathogens

Periplasmic and of mammals, usually

flagella alcohols located in the kidney

(leptospirosis).

4 Spirochaeta 0.2 – 0.75 × Facultatively Carbohy- Aquatic and free-living.

5 – 250 ; 2 – 40 51 – 65 anaerobic drates Periplasmic or anaerobic flagella

(almost = 2)

5 Treponema 0.1 – 0.4 × 25 – 53 Anaerobic Carbohy- Mouth, intestinal tract,

5 – 20 ; 2 – 16 or Micro- drates or and genital areas of Periplasmic aerophilic amino acids animals.

flagella

Importantly, the 2nd edition of Bergey’s Manual divides the phylum spirochaetes into one

class, one order (Spirochaetales), and three families, namely : Spirochaetaceae, Serpulinaceae, and Leptospiraceae.

FURTHER READING REFERENCES