**4. Antioxidants**

Antioxidants are molecules capable of counteracting at low concentrations the damage of mainly reactive oxygen and nitrogen species (ROS and RNS), which are generated from metabolic pathways such as mitochondrial respiratory chain and lipid β-oxidation among others [37, 38]; depending on the ROS/RNS, they can attack different targets [39, 40] whether biomolecules such as proteins, lipids and nucleic acids or cell organelles [22, 41]. Usually ROS and RNS at moderate concentration are useful for defense, signaling mechanisms and cellular maturation [42–45]; however when ROS and RNS concentration are in excess, different pathologies can be caused due to oxidative stress by causing tissue damage [41, 43, 45, 46].

In this regard actinobacteria have played their role as potential sources of antioxidants where [47] isolated *Streptomyces* strains in the Oman sea presented an inhibitory concentration 50 (IC50) that ranges from 356.8 to 566.4 μg/mL against 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical inhibition.

Growth media is important for the production of antioxidants such as the case reported by [48] on *Streptomyces variabilis* (isolated from the Gulf of Khambhat) using six different media: starch casein agar, yeast malt extract agar (ISP2), glycerol asparagine agar (ISP5), inorganic salt agar (ISP4), tyrosine agar (ISP7) and gause' synthetic agar (GSA), and incubated at 30°C for 7–9 days. GSA medium was selected because there was a larger quantity of cell mass compared to other media;

its metabolites were extracted with ethyl acetate and antioxidant activity was tested against DPPH, metal and hydrogen peroxide (H2O2) radical in a concentration range from 0.5 to 2.0 mg/mL. The best radical scavenging activity was against H2O2 radical (64% of antioxidant activity) at a concentration of 0.5 mg/mL.

Specific radical scavengers can be obtained depending on the microorganism such as the strain of *Streptomyces antioxidans* (isolated in the forest of Tanjung Lumpur), in a research reported by [49], which exhibited 79.84% of antioxidant activity against superoxide radical at an extract concentration of 1.5 mg/mL; most compounds present in the extract were pyrazines, fatty acids and a phenolic compound. Similar compounds have been found by [50] in a strain of *Streptomyces monachensis* isolated from a mangrove in Malaysia with an antioxidant activity against superoxide radical as well as metal chelating activity of 83.80 and 75.50% respectively.

Among other antioxidants found on microorganisms extracted due to their possible coloring properties are carotenoid pigments mainly used as vitamins in the case of carotenes and xanthophylls, which can be found on bacteria (*Gordonia rubropertincta*), yeast (*Blakeslea trispora)* and microalgae (*Haematococcus pluvialis*) [51].

In this regard, 50 carbon atom carotenoids identified as bacterioruberin derivatives have been detected as main pigments of *Haloterrigena turkmenica* grown in halobacterium medium, which were tested with DPPH and ferric reducing antioxidant power (FRAP) assays [52].

As mentioned earlier, growth media can influence in the production of antioxidants. Three yeasts isolated from Brazil were tested in different media. The highest carotenoid producer was *Rhodotorula mucilaginosa* in malt and yeast extract medium (MYM) followed by glycerol and corn steep liquor (GCSLM) with a biomass production of 13.5 and 7.9 g/L and a carotenoid content of 1068.5 and 224.8 μg/L respectively.

The authors noticed changes in the carotenoid profile with a higher content of β-carotene followed by astaxanthin and lutein in MYM (91.8, 6.9 and 1.3% respectively). With GCSLM, astaxanthin and lutein content increased (23.3 and 71.2% respectively) and β-carotene content decreased (71.2%).

This change in the carotenoid profile influenced greatly in the antioxidant activity where the pigments presented antioxidant activity against DPPH, 2.7 and 14.7% for MYM and GCSLM respectively. A similar, yet higher behavior was observed with 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and FRAP [53]. The increase in antioxidant activity could be due to the increase of xanthophyll content since the presence of oxygenated moieties in the carotenoid structures increases the antioxidant activity [54].

Similar experiments have been carried out by adding bivalent ions such as ferrous, calcium, copper and zinc among others as initiators of the Fenton reaction or as cofactors for carotenoid biosynthesis [55–57]. However, in a research reported by [58], such behavior was not observed on carotenoid pigments from marine strains of *Rhodococcus* and *Gordonia* genera (isolated from the Gulf of Mexico). However, a change in the carotenoid profile was observed on *Rhodococcus* sp., which may improve the antioxidant activity for two reasons:


*Microorganisms as Alternative Sources of New Natural Products DOI: http://dx.doi.org/10.5772/intechopen.92697*

These carotenoids were identified as glycosidic carotenoids; such carotenoid extracts demonstrated a better antioxidant activity against DPPH radical (IC50 of 1.07 and 0.09 μg/mL for *Rhodococcus* sp. and *Gordonia* sp. respectively) than β-carotene (IC50 of 19.59 μg/mL) [60]. Furthermore, these extracts were compared against those reported by [61] where the authors calculated an IC50 of 11.6 and 9.1 μg/mL for carotenoid extracts from two varieties of *Bactris gasipae*, presenting a better antioxidant activity than the bacterial extracts.

Some of these microbial carotenoid pigments are already commercially available for their use as supplement like Lycogen™, which is a carotenoid pigment from a mutant strain of *Rhodobacter sphaeroides* [62], which contains spheroidenone, bixin (a carotenoid found on *Bixa orellana* L.) and hydroxyspheroidenone [63, 64]. Another pigment that is already available is astaxanthin from microalgae *Haematococcus pluvialis*, whose production cost is estimated at \$552/Kg, being competitive with synthetic carotenoids (\$1000/Kg) [65].
