2013; Kügler et al. 2015). The yeast Candida sp. has been reported to produce
another group of promising biosurfactants which are sophorolipids (Elshafie et al.
2015).
As apparent from the above, biosurfactants produced by microbes are chemically
variable compounds; however, all these emulsifying agents are naturally amphipathic molecules, having both a hydrophobic moiety and a hydrophilic moiety. The
hydrophobic component may comprise fatty acid chains of 10–18 carbons or proteins or peptides with hydrophobic side chains. The hydrophilic components are
often esters, hydroxyl, phosphate, carboxylate, or carbohydrate groups. These have
the ability to emulsify the petroleum oil, producing micro-droplets.
The rhamnolipids produced by Pseudomonas sp. are composed of rhamnose
sugars attached to one or two beta hydroxy fatty acids (Abdel-Mawgoud et al.
2009). Surfactin is an anionic, cyclic lipopeptide-type biosurfactant, comprising a
heptapeptide chain (LLDLLDL), linked to a hydroxyl fatty acid (Peypoux et al.
1999). A Rodococcus sp. of marine origin has been reported to produce an extracellular trehalolipid biosurfactant in the presence of a hydrophobic substrate (White
et al. 2013).
A recently reported thermophillic strain of Aeribacillus pallidus (strain SL-1) was
shown to produce a bio-emulsifier which was composed of a mixture of polysaccharides and proteins, with the latter providing the major emulsifying function.
Being thermophillic, this strain has applications in bioremediation at temperatures
around its optimal temperature of 60
C (Tao et al. 2019).
It has also been suggested that bacteria may show adaptation to the low bioavailability of hydrophobic carbon sources by changing the hydrophobicity of their cell
surface. Mycobacterium sp. LB50IT was shown to grow to confluency as a biofilm
on solid anthracene, a poorly water-soluble carbon source, when provided as the sole
carbon source. However, a similar biofilm/confluent growth was not observed when
glucose was provided as an additional carbon source. The anthracene-grown cells
were found to be more hydrophobic and more negatively charged compared to
glucose grown cells. The authors concluded that biofilm formation and attachment
may be an adaptation to optimize substrate bioavailability (Wick et al. 2002).
11.5.3 Transmembrane Transport
After adsorption of the hydrocarbon to the cell surface, its uptake into the cell may be
by passive or active methods. Both simple and facilitated diffusion as well as energy
utilizing active transport mechanisms have been reported (Hua and Wang 2014).
In Gram-negative bacteria, several outer membrane (OM) proteins have been
shown to transport petroleum hydrocarbons into the cell interior. The E. coli FadL
(fatty acid degradation protein L) (Van Den Berg 2005) and OmpW (Outer membrane protein W) (Hong et al. 2006), as well as the FadL subfamily proteins TodX
(Toluene dioxygenation X) from Pseudomonas putida and TbuX (Toluene
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S. Jayasena and M. Perera
another group of promising biosurfactants which are sophorolipids (Elshafie et al.
2015).
As apparent from the above, biosurfactants produced by microbes are chemically
variable compounds; however, all these emulsifying agents are naturally amphipathic molecules, having both a hydrophobic moiety and a hydrophilic moiety. The
hydrophobic component may comprise fatty acid chains of 10–18 carbons or proteins or peptides with hydrophobic side chains. The hydrophilic components are
often esters, hydroxyl, phosphate, carboxylate, or carbohydrate groups. These have
the ability to emulsify the petroleum oil, producing micro-droplets.
The rhamnolipids produced by Pseudomonas sp. are composed of rhamnose
sugars attached to one or two beta hydroxy fatty acids (Abdel-Mawgoud et al.
2009). Surfactin is an anionic, cyclic lipopeptide-type biosurfactant, comprising a
heptapeptide chain (LLDLLDL), linked to a hydroxyl fatty acid (Peypoux et al.
1999). A Rodococcus sp. of marine origin has been reported to produce an extracellular trehalolipid biosurfactant in the presence of a hydrophobic substrate (White
et al. 2013).
A recently reported thermophillic strain of Aeribacillus pallidus (strain SL-1) was
shown to produce a bio-emulsifier which was composed of a mixture of polysaccharides and proteins, with the latter providing the major emulsifying function.
Being thermophillic, this strain has applications in bioremediation at temperatures
around its optimal temperature of 60
C (Tao et al. 2019).
It has also been suggested that bacteria may show adaptation to the low bioavailability of hydrophobic carbon sources by changing the hydrophobicity of their cell
surface. Mycobacterium sp. LB50IT was shown to grow to confluency as a biofilm
on solid anthracene, a poorly water-soluble carbon source, when provided as the sole
carbon source. However, a similar biofilm/confluent growth was not observed when
glucose was provided as an additional carbon source. The anthracene-grown cells
were found to be more hydrophobic and more negatively charged compared to
glucose grown cells. The authors concluded that biofilm formation and attachment
may be an adaptation to optimize substrate bioavailability (Wick et al. 2002).
11.5.3 Transmembrane Transport
After adsorption of the hydrocarbon to the cell surface, its uptake into the cell may be
by passive or active methods. Both simple and facilitated diffusion as well as energy
utilizing active transport mechanisms have been reported (Hua and Wang 2014).
In Gram-negative bacteria, several outer membrane (OM) proteins have been
shown to transport petroleum hydrocarbons into the cell interior. The E. coli FadL
(fatty acid degradation protein L) (Van Den Berg 2005) and OmpW (Outer membrane protein W) (Hong et al. 2006), as well as the FadL subfamily proteins TodX
(Toluene dioxygenation X) from Pseudomonas putida and TbuX (Toluene
276
S. Jayasena and M. Perera