8.1 Introduction
Actinobacteria constitutes a major phylum within the domain Bacteria and includes
six classes: Actinobacteria, Acidimicrobiia, Coriobacteriia, Nitriliruptoria,
Rubrobacteria, and Thermoleophilia, with the class Actinobacteria comprising 43 of
the 53 families integrating the phylum (Barka et al. 2016 ; Gao and Gupta 2012 ;
Goodfellow et al. 2012 ). Actinobacteria are Gram-positive mostly aerobic bacteria
exhibiting diverse morphologies that range from unicellular organisms tofila-
mentous forms. Due to their growth style often involving the formation of
branching hyphae that can generate dense mycelia and produce spores, these
microorganisms were misclassified for a long time as fungi. However, they are
indeed prokaryotes having no nuclear membrane. Actinobacteria typically have a
high G+C content in their genomes (>50%), and are commonly known for their
remarkable capacity to produce bioactive compounds. More than half of the
bioactive substances described in the literature is attributed to microorganisms
belonging to this phylum (Barka et al. 2016 ; Berdy 2005 ), including antibiotics,
anticancer agents, immunosuppressive agents, antiviral agents, antioxidants,
enzymes, plant growth hormones, etc., that are highly important for applications in
medicine, industry and agriculture (Castillo et al. 2002 ; Fiedler et al. 2008 ; Igarashi
et al. 2007 ; Strobel and Daisy 2003 ). Within Actinobacteria, the genus
Streptomycesis particularly prolific in the production of a wide range of bioactive
compounds, being responsible for the production of ca. 80% of all natural products
produced by actinobacteria, including agriculturally relevant compounds like
insecticides and herbicides, holding a biosynthetic capacity that remains without
parallel in the microbial world (Berdy 2005 ; Jizba et al. 1991 ; Tanaka and Omura
1993 ). These microorganisms are widely distributed, being found in both terrestrial
and aquatic ecosystems, including marine environments. They are common
inhabitants of soils where they spend a significant part of their life cycles as
semi-dormant spores, and constitute ca. 20–30% of the rhizospheric microbial
community (Bouizgarne and Ben Aouamar 2014 ; Coombs and Franco 2003 ).
Actinobacteria are mostly saprophytic microorganisms that play important eco-
logical roles in the recycling of nutrients, in the decomposition of organic matter,
especially complex polymers derived from dead plants and animals, like lignin,
starch, and chitin (Coombs and Franco 2003 ; Minotto et al. 2014 ; Sharma 2014 ), in
the degradation of agricultural and urban wastes as well as in the removal of several
environmental pollutants, such as petroleum, dyes, and other recalcitrant com-
pounds (Amorim et al. 2014 ; Bagewadi et al. 2011 ; Kekuda 2016 ; Khedkar and
Shanker 2015 ). Most actinobacteria are mesophilic, having optimal growth tem-
peratures between 25 and 30 °C, and grow in soils with a neutral pH.
164 M.F. Carvalho et al.