**3. Habitats, recovery, and growth requirements of MACA**

MACA are widely distributed in the environment including natural habitats such as mangroves, soil, freshwater, and deep ocean sediments as well as man-made sites such as activated sludge foams, biofilters, industrial wastewater and indoor building materials. Although predominantly saprophytic, many species are opportunistic pathogens forming parasitic associations with plants and animals, including humans, notably immunocompromised individuals. Several members of the genus *Mycobacterium* cause a plethora of diseases most notably tuberculosis caused by *Mycobacterium bovis* and *Mycobacterium tuberculosis*.

#### **Figure 4.**

*Types and key structural features of various biosurfactants produced by MACA. (Adapted from [13]).*

MACA capable of producing various biosurfactants have been isolated from environments (**Table 3**) including oil-contaminated soils [24, 25], water from oil wells [26], wastewater from the rubber industry [21], activated sludge, and effluent and sediment from pesticide manufacturing facilities [23]. The ability of MACA to produce biosurfactants in these habitats appears to be driven by the environmental conditions to which they are exposed whereby the biosurfactants act as mediators for the biodegradation of hydrophobic carbon substrates. Genes involved in biosynthesis of rhamnolipids by *Dietzia maris* for example have been shown to be upregulated in the presence of hydrophobic substrates including *n*-hexadecane, *n*-tetradecane and pristane [15]. However, the true distribution of biosurfactantproducing MACA in the environment may not solely depend on the presence of hydrophobic substrates.

*Biosurfactant Production by Mycolic Acid-Containing Actinobacteria DOI: http://dx.doi.org/10.5772/intechopen.104576*


#### **Table 3.**

*Various environmental sources of biosurfactant-producing MACA.*

Isolation of biosurfactant producers largely relies on selective isolation strategies, utilising hydrophobic compounds as sole carbon sources for energy and growth. Typically, strains are isolated and cultivated using mineral salt medium containing essential trace elements supplemented with a hydrocarbon substrate such as crude oil, diesel, n-alkanes, n-hexadecane, paraffin, polyaromatic hydrocarbons (PAHs), or vegetable oils such as olive oil and rapeseed oil, as the sole carbon source. These may be incorporated into the liquid or solid medium, spread across the agar surface or soaked onto a filter in the lid of petri dishes. Besides the selectivity of the culture medium, pre-enrichment techniques utilising hydrophobic compounds as the sole carbon source, can be used [27]. The principle of enrichment is to provide growth conditions that are favourable for the organisms of interest but not for competing organisms. This selective advantage allows target populations to expand through a series of passages, maximising the chances of successful recovery at the isolation stage. Incorporating antibiotics into the isolation media may provide a useful additional selective pressure to eliminate or reduce unwanted fungi and bacteria.

The ability of an organism to grow on hydrophobic compounds is a good indicator of biosurfactant production but is not a guarantee. It is therefore important that isolates of interest are tested in pure culture for biosurfactant production using further screening assays. It is also possible that biosurfactant-producing organisms may be present in an environment but not enriched by in the conditions provided or indeed producers may be recovered from the environment but not synthesize biosurfactants under the culture conditions imposed. Mining genomes for cryptic biosurfactant biosynthesis pathways, and metagenomic screening of DNA from environmental samples promise an alternative approach to biosurfactant discovery that may circumvent some of the issues associated with culture-dependent strategies [28].
