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Index >> Soil Microorganisms >> Actinomycetes

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Soil Microorganisms Development of Soil Microbiology Significant Developments in Soil Microbiology Biological Nitrogen Fixation of Microorganisms Bacteria Cell Wall Actinomycetes Fungi Fungal Cell Structure Myxomycetes

Actinomycetes

	Actinomycetes These are soil organisms which have characteristics common to bacteria and fungi and yet possess sufficient distinctive features to delimit them into a distinct category. In the strict taxonomic sense, actinomycetes are clubbed with bacteria in the same class of Schizomycetes but confined to the order Actinomycetales. On agar plates, they can easily be distinguished from true bacteria. Unlike slimy distinct colonies of true bacteria which grow quickly, actinomycete colonies appear slowly, show powdery consistency and stick firmly to agar surface. A closer look at a colony under the compound microscope reveals slender unicellular branched mycelium (diameter of the hypha rarely exceeding one micron) forming asexual spores for propagation.
They bear certain similarities to Fungi Imperfecti in the branching of the aerial mycelium which profusely sporulate and in the formation of distinct clumps or pellets in liquid cultures. Certain actinomycetes, on the other hand, resemble Mycobacterium and Corynebacterium in all respects both morphologically and physiologically including susceptibility to the attack by viruses. Viruses are known to attack bacteria and actinomycetes but not fungi. Actinomycetes differ from fungi in the composition of their cell wall. They do not have chitin and cellulose which are commonly found in the cell walls of fungi.
The number of actinomycetes increases in the presence of decomposing organic matter. As a rule, they are intolerant to acidity and their numbers decline at pH 5.0. The most conducive range of pH is between 6.5 and 8.0. Waterlogging of soil is unfavourable for the growth of actinomycetes whereas desertic soils of arid and semi-arid zones sustain sizeable population, probably due to the resistance of spores to desiccation. The percentage of actinomycetes in the total microbial population increases with the depth of soil and actinomycetes can be isolated in sufficient number even from soil samples obtained from the C horizon of a soil profile.
Delineation of species within different genera of actinomycetales has always been a difficult problem owing to many characteristics which are common with bacteria. For instance, Mycobacterium and Mycococcus of the family Mycobacteriaceae have many characteristics common to bacteria and in fact, are spoken of as bacteria in common parlance. Nevertheless, the order Actinomycetales has been classified into four families―Mycobacteriaceae, Actinomycetaceae, Streptomycetaceae and Actinoplanaceae. In the order of abundance in soils, the commonest genera of actinomycetes are Streptomyces (nearly 70%), Nocardia and Micromonospora although Actinomyces, Actinoplanes and Streptosporangium have also been encountered occasionally.
	Temperatures between 25 and 30.C are conducive for the growth of actinomycetes although thermophilic cultures growing at 55 and 65°C are common in compost heaps where they are numerically extensive and mostly belong to the genera Thermoactinomyces and Streptomyces.

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