According to the difference of morphology and function, the mycelia can be divided into substrate mycelium and aerial mycelium ( Figure 1). Classic actinomycetes have well-developed radial mycelium. The whole structure of a hyphae cell corresponds to bacterial organization: the cytoplasm contains genomic DNA regions, ribosomes, and various inclusions, presumably reserve substances such as polyphosphates, lipids, or polysaccharides. The basic morphological characteristics of actinobacteriaĪctinobacteria display the greatest morphological differentiation among gram-positive bacteria however, the cell structure of actinobacteria are typical prokaryotes and totally different with fungi. The morphological characteristics are still one of the most basic indexes which provide in-depth information on a taxon.Ģ. Most actinobacteria are characterized and classified on the basis of their morphology in the first place. The key elements that should be acquired and analyzed in characterization studies of prokaryotes were outlined : the phenotypic features are the foundation for description of taxa. Īctinobacteria are currently characterized using the polyphasic approach that brings together a variety of phenotypic, chemotaxonomic, and genotypic data that comprise the formal description of a novel taxon. But the characterization of a strain is a key element in actinobacteria systematics in any period and prokaryotic morphologies are consistent with their phylogenetic reconstructions. On one hand, the quantitative determination results are more objective on the other hand, the research results of polyphasic taxonomy not only enrich the taxonomic content greatly, but also enrich the essence of life phenomenon. The modern classification method is an important means to understand the biological origin and species diversity. This approach combines as many different data as possible, for instance, phenotypic, chemotaxonomic, genotypic, and phylogenetic information. Recently, the most widely accepted system is the polyphasic approach. In recent years, more and more genotypic approaches were applied on the classification of actinobacteria, such as multilocus sequence analyses (MLSA), average nucleotide identity (ANI), and whole genome analysis. The development of prokaryotic classification has experienced different stages: (i) the classical or traditional classification mainly based on microbial morphological traits, growth requirements, physiological and biochemical features (ii) numerical taxonomy analyzing huge volumes of phenotypic data to derive meaningful relationships amongst a large number of microorganisms can be carried out using computer programs (iii) chemotaxonomic methods studied the chemical variation in actinobacteria and used chemical characters in classification and identification, and it dealt with the discontinuous distribution of specific chemicals, especially amino acids, lipids, sugars, proteins, and other substances in whole cells, parts of cells or fermentation products, and with enzymes (iv) genotypic classification based on genetic relatedness, inferred mainly from DNA-DNA hybridization (DDH) and comparative sequence analyses of homologous macromolecules, especially, rRNA.
The history of the classification of prokaryote clearly demonstrates that changes were caused by the availability of new techniques. Both morphogenesis and antibiotic production in the streptomycetes are initiated in response to starvation, and these events are coupled. The morphological differentiation of actinobacteria, especially streptomycetes, is controlled by relevant genes. Actinobacteria have different cultural characteristics in various kinds of culture media, which are important in the classification identification, general with spores, aerial hyphae, with or without color and the soluble pigment, different growth condition on various media as the main characteristics. The structure of hyphae and ultrastructure of spore or sporangia can be observed with microscopy. Some actinobacteria can form complicated structures, such as spore, spore chain, sporangia, and sporangiospore. Classic actinomycetes have well-developed radial mycelium, which can be divided into substrate mycelium and aerial mycelium according to morphology and function. The actinobacteria morphological characteristics are basic foundation and information of phylogenetic systematics. Among gram-positive bacteria, actinobacteria exhibit the richest morphological differentiation, which is based on a filamentous degree of organization like filamentous fungi. Actinobacteria is a phylum of gram-positive bacteria with high G+C content.
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