Polyhydroxyalkanoates (PHA) are a type of biopolymer that can be biosynthesised and accumulated by several specialised bacterial species. Their favourable mechanical properties coupled with their ability to naturally biodegrade within the environment make PHAs a potential candidate to replace tradition plastics, which cause harm to our environment, our wildlife and ourselves. One of the main drawbacks associated with PHAs, is the difficulty of producing them in large quantities, which hinders their potential usage. However, the manipulation of quorum sensing circuits has yet to be explored in the context of increasing the amount of PHA produced by bacterial species. Quorum sensing is a process of cell-to-cell communication, that allows for bacteria to share information about cell densities and coordinate gene expression, such as altering the production of secondary metabolites like PHAs.
During this study two Gram-negative bacterial species (Cupriavidus necator H16 and Pseudomonas putida KT2440) were grown in specific production media and supplemented with three different exogenous quorum molecules sensing (C4-HSL, C6-HSL and 3-oxo-C12-HSL), to study if there was a change in the amount of PHA produced. It was observed that C. necator H16 increased production of PHAs, specifically small chain length PHA (SCL PHA) poly(3-hydroxybutyrate) (P(3HB)), by 13.03% and 17.85% when grown in shake flasks and bioreactors, when supplemented with the quorum sensing molecule 3-oxo-C12-HSL. Whilst PHA production increased in culture of P. putida KT2440, when supplemented with C4-HSL. PHA production of the medium chain length polymer (MCL PHA) of poly(3-hydroxyoctanoate-co-3-hydroxydecanoate-co-3-hydroxydodecanoate) (P(3HO-co-3HD-co-3HDD)) increased by 7.24% and 10.89% when grown in shake flasks and bioreactors, respectively.
To understand as to why there was a change in production of PHAs produced by the two species of bacteria, qPCR was used to measure the expression of two key Pha genes (PhaC1 and PhaZ) when exposed to exogenous quorum sensing molecules in bioreactors. C. necator H16 displayed an increase in regulation of the PhaC1 gene, within 24 hours of fermentation by 2.53-fold when supplemented with 3-oxo-C12-HSL. Alternatively, PhaZ displayed an increase in transcription at the same time point, but by 1.48-fold. P. putida KT2440 cultures displayed a significant increase in regulation of PhaC1 at 48 hours of fermentation when supplemented with both C4-HSL and 3-oxo-C12-HSL. Although an increase of PhaZ was also observed, it was less than PhaC1.