Rhamnolipids produced by Pseudomonads have been extensively covered and not surprisingly thanks to their potential for application in a huge number of industries. This has been reflected in the blogs with numerous reviews of P. aeruginosa and its potentials and downfalls.
This review takes into account bacteria other than P. aeruginosa that produce rhamnolipids. This includes bacteria that are non- pathogenic and could therefore be produced on a large scale without the health hazards the P. aeruginosa brings about. Also, it is suggested likely that these bacteria have acquired the rhl genes involved in rhamnolipid production through lateral or horizontal gene flow.
P. aeruginosa is an opportunistic pathogen and can effect humans. It is resistant to many antibiotics and is more likely to effect those that are immunocompromised. This is an important and unfortunate limitation for the mass production of the sought after rhamnolipids it produces. Therefore the investigation into other rhamnolipid producing bacteria is useful and could potentially lead to the reduction of a health hazard. Firstly, production occurs in three sequential reactions. The first reaction is catalysed by RhlA gene which produces 3 - hydroxyfatty acid dimers. These are coupled with dtDP - L - rhamnase in the second reaction, catalysed by RhlB. This coupling yields mono - rhamnolipids of which are also a substrate of RhlC, dirhamnolipids can therefore be produced. This production takes place in an heterlogous host, one of a different species and also relies on quorum sensing. This important process depends on bacterial autoinducers such as N - acyl homoserine lactones. These activate gene expression (such as the rhl genes responsible for rhamnolipid production) at high bacterial density.
Rhamnolipids produced with genetic engineering
Attempts to produce rhamnolipid from E. Coli that expresses the rhl AB has only resulted in a very limited supply, not enough for mass production. Other pseudomonads species have even been used to produce rhamnolipid even though it is not a normal trait. P. fluorescens and P. putida have been shown to produce a significant amount but this is still only 10 - 25% of that produced by aeruginosa. However introduction of a plasmid containing P. aeruginosa's rhlAB, rhlR and rhll genes to P. putida greatly increased rhamnolipid production to an even greater extent than the original strain. Caution however has to be taken when applying these techniques to heterologous hosts; the metabolites that are rhamnolipid precursors are part of central metabolic pathways and overexpression of rhl genes could lead to cell death or decreased viability.
Non - P. aeruginosa rhamnolipid producing bacterial isolates
Several pseudonomad strains, including P. putida and P. fluorescens mentioned above, can produce their own rhamnolipids but it rarely occurs. P. chlororaphis’ rhamnolipid production has been addressed but it is very unclear and its rhlA, B and C genes have not been reported, nor has the regulation of their expression been identified.
Non- Pseudomonad rhamnolipid producing sp, such as Enterobacter hormaechei, Pantoea stewartii and E. aburiae, have also been identified and isolated. However in none of these cases have the genes responsible for rhamnolipid production been identified and the possibility of lateral of horizontal gene transfer has not been investigated either. Although, later on in the paper, it is mentioned that the origin of rhl synthesis is likely due to this (lateral) form of transfer as, the newly named, Burkhalderia sp (another rhamnolipid producing strain) has very mobile genetic elements making lateral transfer plausible.
Some Burkhalderia sp are also pathonogeni but B. thalandensis is not, and this is the only case of non – pathogenic rhamnolipid producing bacteria where the sequence of rhl genes has been reported. More investigation into the potential of this species for mass production of rhamnolipid could therefore reveal promising results.
Jeiry Toribio, Ana E. Escalante, Gloria Soberón-Chávez (2010) Rhamnolipids: Production in bacteria other than Pseudomonas aeruginos European Journal of Lipid Science and Technology Vol112 pgs 1082 - 1087
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