Monday, September 19, 2011

Streptomyces and antibiotics

Streptomyces are Gram-positive, aerobic, high-GC, filamentous soil bacteria known for the production of secondary metabolites and biologically active materials (Stackebrandt et al., 1991).
Streptomycetes are members of the order Actinomycetales within the classes Actinobacteria (Stackebrandt et al., 1997) and have a DNA G+C content of 69±78 mol% (Korn-Wendisch and Kutzner, 1992).
Streptomycetes produce an extensive branching substrate and aerial mycelium. The substrate hyphae are approximately 0.5–2.0 μm in diameter and often lack cross-walls during the vegetative phase. Growth occurs at the hyphal apices and is accompanied by branching, thus producing a complex tightly woven matrix of hyphae during the vegetative growth phase. As the colony ages, aerial mycelia (sporophores) are produced which develop into chains of spores (conidia) by the formation of crosswalls in the multinucleate aerial filaments. This is followed by separation of individual cells directly into spores (Wildermuth and Hopwood, 1970).
The surface of the conidial wall often has convoluted projections which, together with the shape and the arrangement of the spore-bearing structures, are characteristic of each species and were often used for the separation of Streptomyces species (Pridham et al., 1958; Korn-Wendisch and Kutzner, 1992). They can be distinguished from other actinomycetes by their cell wall type which is characterized as Type I sensu (Lechevalier and Lechevalier, 1970b).
Besides antibiotics, which present the largest group of bioactive secondary metabolites, the streptomycete compounds show several other biological activities . The secondary metabolites from streptomycetes can be broadly separated into four classes according to their biological activity: (1) antagonistic agents, including antibacterials, antifungals, antiprotozoans as well as antivirals, (2) pharmacological agents, including antitumorals, immunomodulators, neurological agents and enzyme inhibitors, (3) agrobiologicals, including insecticides, pesticides and herbicides, and (4) compounds with regulatory activities, such as growth factors, siderophores or morphogenic agents. To detect simultaneous bioactivities for a given compound, pharmacological and agricultural screens are increasingly being used in combination with antimicrobial tests. This has revealed several novel therapeutic and agrobiological agents and previously unknown biological activities for antibiotics (Sanglier et al., 1996; Bérdy, 2005). Many reports have shown that since streptomycetes are frequently screened for antimicrobial activity, the existence of secondary metabolites with other activities may have been missed (Garcia et al., 2000; Nunes et al., 2005).


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