However, the comparison to fungi is only superficial: like all bacteria, actinomycetes' cells are thin with a chromosome that is organized in a prokaryotic nucleoid and a peptidoglycan cell wall furthermore, the cells are susceptible to antibacterial agents ( Fig. Indeed, like filamentous fungi, many Actinobacteria produce a mycelium, and many of these mycelial actinomycetes reproduce by sporulation. Traditionally, actinomycetes were considered transitional forms between fungi and bacteria. This is what gave them their name, which derives from the Greek words for ray (aktis or aktin) and fungi (mukēs). They grow by a combination of tip extension and branching of the hyphae. Actinobacteria are Gram-positive filamentous bacteria with a high guanine-plus-cytosine (G+C) content in their genomes.
The majority of the Actinobacteria are free-living organisms that are widely distributed in both terrestrial and aquatic (including marine) ecosystems ( 3). The actinobacterial genomes sequenced to date belong to organisms relevant to human and veterinary medicine, biotechnology, and ecology, and their observed genomic heterogeneity is assumed to reflect their biodiversity ( 2). The phylum Actinobacteria is one of the largest taxonomic units among the major lineages currently recognized within the Bacteria domain ( 1).
This review presents an update on the biology of this important bacterial phylum. Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities. Actinobacteria play diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma), soil inhabitants (e.g., Micromonospora and Streptomyces species), plant commensals (e.g., Frankia spp.), and gastrointestinal commensals ( Bifidobacterium spp.). Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture.
They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds.
Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation. Actinobacteria are Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems.
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