BIOSURFACTANTS AND BIOPOLYMERS 459 The chemical bases of RL production are deoxythymidine diphosphate-L-rhamnose sugar— derived from D-glucose and 3-(3-hydroxy alkanoyl oxy) alkanoic acid—or HAA—derived by the fatty acid amalgamation of two-carbon units. The derivation of HAA is catalyzed by one of the key RL enzymes called RhlA this is not found in the bacteria itself and makes up the hydrophobic end of RLs. There also exist other two enzymes which are key in the production, namely, RhlB and RhlC responsible for producing mono and di RLs, respectively (40). Mo norhamnolipid or Rha-C10-C10 and di-rhamnolipid or Rha-Rha-C10-C10 in chemical nomenclature are known as l-rhamnosyl-3-hydroxydecanoyl-3-hydroxydecanoate and l- rhamnosyl-l-rhamnosyl-3-hydroxydecanoyl-3-hydroxydecanoate, respectively (Figure 2) (41). The previously mentioned RhlA and RhlB are controlled by bacterial quorum sens- ing (QS). This is a method of signal transmittance by bacteria where the microbial cells produce an auto-inducer which is bonded to an environmental sensor that propagates the production of the functional QS system and begins a regulatory chain effect (42). The majority of the RL molecules are produced 20–36 h post-initiation or during the station- ary phase where the rate of generation of new cells is equal to the rate of dissipation of the old cells. This is a yield-controlling factor that can be taken into consideration (43). The source of carbon for RL production can be obtained from more sustainable substrates such as xylose (food industry by-product) and glycerol (biodiesel) (44). Th e second classifi cation of glycolipids known as SLs is primarily biosynthesized from nonpathogenic yeast strains such as Candida apicola and Starmerella bombicola (45). SL con- sists of a sophorose- or glucose-derived disaccharide head and a fatty acid tail which can either be lactonic or acidic owing to the presence or absence of the ester linkage between the fatty acid tail and sophorose head, respectively (Figure 2) (46). SL s can be biosynthesized from various glucose and fatty acid sources. The hydrophilic sugar sources undergo reduction, followed by glycosylation. The resultant compound acts as a catalyst for microbial growth. A part of glucose also reacts to become a glucose ester of F igure 2. RL chemical structures. (A) Monorhamnolip i d and (B) dirhamnolipid.

JOURNAL OF COSMETIC SCIENCE 460 pyrophosphoric acid along with a nucleoside uridine and forms the predominant structure of SLs. The 16–18 carbon atom fatty acid is introduced into the microbial host through an aldehyde dehydrogenase enzyme, and consequently nonacetylated acidic SL is formed (47). Chemical alterations of this can produce acetylated SL and further be catalyzed to lactonic forms by lactone esterase (48). The sources of the sugars and fatty acids can be obtained from several renewable raw materials or as industrial by-products. Different combinations of the sugar and fatty acid sources vary the SL yield. For example, glucose with canola oil yielded a greater amount of SL (80%) than lactose with canola oil (only 45%). Several studies showed that glucose was the optimal sugar for highest SL conversion (49). Other industry by-products can also add to the long list of sugar alternatives such as sugarcane molasses medium. Renewable fatty acids such as plant and animal esters can be used for good SL production. One interesting proposition being studied is the production of SL using the waste oil of food industries however, the methods have to undergo several modifi cations before being fully used, such as fed-batch culture and SSR or solid-state fermentation (50). Fi nally, the third classifi cation of glycolipids known as trehalose lipids or trehalolipids (TLs) meant for biosurfactant production is most commonly obtained from the species Mycobacterium, Nocardia, Corynebacterium, Rhodococcus erythropolis, and Arthrobacter. TL com- prises a trehalose head group bound to an esterifi ed fatty acid tail where the trehalose head group is two α glucose units linked by 1,1-glycosidic bonds (Figure 4) (51). The long- chain fatty acid tails are known as mycolic acids which have hydroxy functional groups in the α and β positions (52). Similar to all amphiphilic biosurfactant biosynthesis, the mycolic tail is produced as a result of the hydrocarbon metabolic pathways, whereas the carbohydrate metabolic path- ways lead to the formation of the trehalose head. The induction mechanism to synthesize TL involves the introduction of hydrocarbons to the bacterial propagation medium. In the past years, trehalose lipids have been found to be important for bioremedial applica- tions, for example, solubilization and biodegradation of nonpolar molecules (53). LIPOPEP TIDES Cyclic lipopeptides, in particular surfactins, produced by the species Bacillus subtilis, are known for their powerful surfactancy (54). Surfactins are peptides bonded with a 14-carbon fatty acid chain, where the peptide comprises seven amino acids—GluOMe-Leu-Leu-Asp-Val-Leu-Leu (Figure 5). Similar to glycolipids, surfactin or subtilisin strains can also be produced by being grown on polyol compounds obtained from biodiesel manufacturing units. The initial strain Figure 3. SL chemical structures. (A) Acidic f o rm and (B) lactonic form.