**2. Defining rare Actinobacteria**

Rare Actinobacteria are defined as certain types of Actinobacteria which are abundant in various habitats but are difficult to isolate [7]. These include all non-Streptomyces actinobacterial genera like Actinomadura, Actinoplanes, Amycolatopsis, Dactylosporangium, Planomonospora, Planobispora, Salinispora, Streptosporangium, Verrucosispora and Microbiospora [8]. Even though a major percentage of antibiotics are secreted by bacteria from the genus Streptomyces [9] rare actinobacteria account for 25–30% of these. The importance of rare actinobacteria is further demonstrated by the fact that many of the successful bioactive products in the market like rifamycin, erythromycin, teicoplanin, vancomycin, and gentamycin are produced by such rare actinobacteria [10]. Consequently, rare actinobacteria are being unveiled as highly prospective sources of bioactive compounds.

Members of the genus Actinomadura have been reported to secrete about 350 different types of bioactive compounds exhibiting a wide range of mechanisms of action [11, 12]. Some strains of the genus Actinoplanes are known for the production of acarbose, a secondary metabolite that is an α-glucosidase enzyme inhibitor [13] and is currently being used in treating type 2 diabetes [14]. While numerous bioactive compounds are isolated from the strains of Amycolatopsis, vancomycin and rifamycin are the most familiar ones [15]. Research also led to the discovery of the first naturally occurring tetracycline C2 amides called dactylocyclines which are produced by members of the genus Dactylosporangium [16]. Metabolomic studies conducted on the genus Planomonospora led to the discovery of multiple

#### *Anti-Quorum Sensing Compounds from Rare Actinobacteria DOI: http://dx.doi.org/10.5772/intechopen.106526*

compounds with biosynthetic activities such as Ureylene-containing oligopeptide antipain, the thiopeptide siomycin and sphaericin and lantibiotic 97,518 [17]. Planobispora rosea is found to secrete a novel antibiotic called GE 2770A and thiazolyl peptides which are protein synthesis inhibitors [18]. Arenimycin is another antibiotic produced by the marine actinobacterium Salinispora arenicola and is found to be effective against drug resistant S. aureus and a few other Gram-positive bacteria [19]. Molecular studies done on compounds isolated from Streptosporangium oxazolinicum, resulted in the discovery of three novel alkaloids having anti-trypanosomal activity [20]. Studies conducted using liquid chromatography mass spectrometry (LC–MS) and one strain-many compounds strategy led to the detection of another bioactive compound abyssomicin from the marine actinobacterium Verrucosispora [21]. Members of the genus Micromonospora have been a major source of Gentamicin, apart from which about 740 different antibiotics are found from other strains [22]. All the above mentioned actinobacteria and few more are a subject of intense investigations and prove to be a rich source of many novel antibiotics when modern approaches such as transcriptomics and metabolomics are employed.


#### **Table 1.**

*Habitats of rare actinobacteria.*
