Actinomycetes

Actinomycetes

The Actinomycetes [s., actinomycete], according to the latest edition of Bergey’s Manual (Volume 4), represent an aerobic, Gram-positive bacteria which predominantly and essentially give rise to specific branching filaments or asexual spores or hyphae. It has been duly observed that the elaborated morphology, arrangement of spores, explicit cell-wall chemistry, and above all the various kinds of carbohydrates critically present in the cell extracts are specifically vital and equally important requirement for the exhaustive taxonomy of the actinomycetes. Consequently, these informations are utilized meticulously to carry out the articulated division of these bacteria into different well-defined categories with great ease and fervour. It is quite pertinent to state at this juncture, that the actinomycetes do possess and exert an appreciable practical impact by virtue of the fact that they invariably play an apparent major role in the following two highly specialized and particular aspects, namely:

(a) Mineralization of organic matter in the soil, and

(b) Primary source of most naturally synthesized antibiotics.

General Characteristics

The general characteristics of the actinomycetes are as stated under :

(a) The branching network of hyphae usually developed by the actinomycetes, grows critically both on the surface of the solid substratum (e.g., agar) as well as into it to give rise to the formation of substrate mycelium. However, the septate**** mostly divide the hyphae into specific elongated cells (viz., 20 μm and even longer) essentially consisting of a plethora of nucleoids*****.

(b) Invariably, the actinomycetes afford the development of thallus. Noticeably, a large cross-section of the actinomycetes do possess an aerial mycelium that extends above the  solid subtratum, and produces articulately asexual, thin-walled spores known as conidia [s., conidium] or conidiospores at the terminal ends of filaments. In an event, when the spores are located strategically in a sporangium, they are termed as sporangiospores.

(c) The spores present in the actinomycetes not only vary widely in terms of shape and size, but also develop them (spores) by the help of septal formation at the tips of the filaments, invariably in response to nutrient deprivation. Besides, a larger segment of these spores are specifically devoid of any thermal resistance; however, they do withstand dessication quite satisfactorily, and thus exhibit considerable adaptive value.

(d) Generally, most actinomycetes are not found to be motile,* and the motility is particularly confined to the flagellated spores exclusively.

In the recent past, several taxonomically characteristic features and useful techniques are of immense value and worth, such as:

·        Morphological features and the colour of mycelia and sporangia

·        Surface properties and arrangement of conidiospores

·        % (G + C) in DNA

·        Phospholipid content and composition of cell membranes

·        Thermal resistance encountered in spores

·        Comparison of 16S rRNA sequences and their values

·        Production of relatively larger DNA fragments by means of restriction enzyme digestion, and

·        Ultimate separation and comparison of ‘larger DNA fragments’ by the aid of Pulsed Field Electrophoresis.

Significance of Actinomycetes

There are, in actual practice, three most important practical significances of the actinomycetes, as mentioned below:

(1) Actinomycetes are predominantly the inhabitants of soil and are distributed widely.

(2) They are able to degrade a large variety and an enormous quantum of organic chemical entities. However, these are of immense significance in the mineralization of organic matter.

(3) They invariably and critically give rise to a large excess of extremely vital ‘natural antibi-otics’ that are used extensively in the therapeutic armamentarium e.g., actinomycetin. Im-portantly, a plethora of actinomycetes represent free-living microbes, whereas a few are pathogens to human beings, animals, and even certain plants.

Fig. 3.5. illustrates the cross-section of an actinomycete colony with living and dead hyphae.

The substrate and aerial mycelium having chains of conidiospores have been depicted evidently.

Classification

The actinomycetes have been duly classified into three major divisions based upon the follow-ing characteristic features:

(a) Whole cell carbohydrate patterns of aerobic actinomycetes

(b) Major constituents of cell wall types of actinomycetes, and

(c) Groups of actinomycetes based on whole cell carbohydrate pattern and cell wall type.

The aforesaid three major divisions shall now be dealt with separately in the sections that follows.

(a) Whole Cell Carbohydrate Patterns of Aerobic Actinomycetes

The aerobic actinomycetes do have four distinct whole cell carbohydrate patterns as given in the following Table 3.5.

Table 3.5. Whole Cell Carbohydrate Patterns of Aerobic Actinomycetes

The above contents of Table: 3.5 vividly shows that none of the four carbohydrates are present in the Pattern ‘C’, whereas Pattern ‘B’ contains only madurose, and Pattern ‘A’ and Pattern ‘D’ con-tains each two carbohydrates out of the four cited above.

(b) Major Constituents of Cell Wall Types of Actinomycetes

The actinomycetes that possess major constituents of cell wall types also exhibit four different varieties as provided in Table 3.6.

(c) Groups of Actinomycetes Based on Whole Cell Carbohydrate Pattern and Cell Wall Type

There are in all five different varieties of cell wall types having carbohydrate and genera vari-ants in groups of actinomycetes, as given in Table 3.7 under :

Table 3.7. Actinomycetes Based on Whole Cell Carbohydrate Pattern and Cell Wall Type

One may observe from Table 3.7 that the cell wall type I is devoid of the characteristic feature pertaining to the specific carbohydrate pattern.

Actinomycetes with Multiocular** Sporangia***

The latest version of Bergey’s Manual has explicitly described the actinomycetes occurring as the ‘clusters of spores’ in a specific situation when a hypha undergoes division both transversely and logitudinally. In reality, all the three genera critically present in this section essentially possess chemotype III cell walls, whereas the cell extract carbohydrate patterns differ prominently.

Salient Features: The salient features of the actinomycetes with multiocular sporangia are as follows :

(1) The mole % (G + C) values varies from 57 to 75.

(2) Chemotype III C Cell Walls****: Geodermatophillus belonging to this category has motile spores and is specifically an aerobic soil organism.

(3) Chemotype III B Cell Walls : Dermatophillus invariably gives rise to pockets of motile spores having tufts of flagella. It is a facultative anaerobe and also a parasite of mammals actually responsible for the skin infection streptothricosis.

(4) Chemotype III D Cell Walls: Frankia usually produces non-motile sporangiospores evi-dently located in a sporogenous body. It is found to extend its normal growth in a symbiotic association particularly with the roots of eight distinct families of higher non-leguminous plant sources viz., alder trees. These organisms are observed to be extremely efficient microaerophilic nitrogen-fixers which frequently take place very much within the root nodules of the plants. Furthermore, the roots of the infected plants usually develop nodules that would eventually cause fixation of nitrogen so efficiently that a plant, for instance : an alder tree, may grow quite effectively even in the absence of combined N₂, when nodulated respectively. It has been duly observed that very much inside the nodule cells, Frankia in-variably gives rise to branching hyphae having globular vesicles strategically located at their ends. Consequently, these vesicles could be the most preferred sites of the N₂ fixation ultimately. However, the entire phenomenon of N₂ fixation is quite similar to that of Rhizobium wherein it is both O₂ sensitive and essentially and predominantly needs two elements, namely : molybdenum (Mo), and cobalt (Co).