Isolation and characterization of new poly(3HB)-accumulating star-shaped cell-aggregates-forming thermophilic bacteria

Abstract

Aims: This study aimed at isolating thermophilic bacteria that utilize cheap carbon substrates for the economically feasible production of poly(3-hydroxybutyrate), poly(3HB), at elevated temperatures. Methods and Results: Thermophilic bacteria were enriched from an aerobic organic waste treatment plant in Germany, and from hot springs in Egypt. Using the viable colony staining method for hydrophobic cellular inclusions with Nile red in mineral salts medium (MSM) containing different carbon sources, six Gram-negative bacteria were isolated. Under the cultivation conditions used in this study, strains MW9, MW11, MW12, MW13 and MW14 formed stable star-shaped cell-aggregates (SSCAs) during growth; only strain MW10 consisted of free-living rod-shaped cells. The phylogenetic relationships of the strains as derived from 16S rRNA gene sequence comparisons revealed them as members of the Alphaproteobacteria. The 16S rRNA gene sequences of the isolates were very similar (> 99% similarity) and exhibited similarities ranging from 93 to 99% with the most closely related species that were Chelatococcus daeguensis, Chelatococcus sambhunathii,Chelatococcus asaccharovorans, Bosea minatitlanensis, Bosea thiooxidans and Methylobacterium lusitanum. Strains MW9, MW10, MW13 and MW14 grew optimally in MSM with glucose, whereas strains MW11 and MW12 preferred glycerol as sole carbon source for growth and poly(3HB) accumulation. The highest cell density and highest poly(3HB) content attained were 4 center dot 8 g l-l (cell dry weight) and 73% (w/w), respectively. Cells of all strains grew at temperatures between 37 and 55 degrees C with the optimum growth at 50 degrees C. Conclusions: New PHA-accumulating thermophilic bacterial strains were isolated and characterized to produce poly(3HB) from glucose or glycerol in MSM at 50 degrees C. SSCAs formation was reported during growth. Significance and Impact of the Study: To the best of our knowledge, this is the first report on the formation of SSCAs by PHA-accumulating bacteria and also by thermophilic bacteria. PHA-producing thermophiles can significantly reduce the costs of fermentative PHA production

Production of Polyhydroxyalkanoate (PHA) Biopolyesters by Extremophiles?

“…Ibrahim et al [107] isolated and screened a range of thermophilic microbes able to convert inexpensive carbon feedstocks to PHA under elevated temperature conditions. The microbes were enriched from a German aerobic waste treatment plant, and from Egyptian hot springs.…”

High-Cell-Density Cyclic Fed-Batch Fermentation of a Poly(3-Hydroxybutyrate)-Accumulating Thermophile, Chelatococcus sp. Strain MW10

“…However, none of them was used for the fermentative production of PHAs. Very recently, new thermophilic strains were isolated which accumulate poly(3-hydroxybutyrate) [poly(3HB)] up to 73% (wt/wt) of CDW at 50°C in cheap medium and which utilize renewable resources such as glucose or glycerol as carbon sources for growth and poly(3HB) synthesis (19). From these strains, Chelatococcus sp.…”

“…In this context, all costs of bioreactor operation are important, especially the cost of cooling during the exothermic HCD fermentation process (28), where thermophilic fermentations are recommended (49). Since most PHA-producing strains are mesophiles growing optimally at temperatures between 30 and 37°C, alternative PHA-producing thermophilic strains should be used (19).…”

“…MW10 was cultivated in FBF at the 2-liter scale to provide excess carbon during growth, with the aims of achieving HCD growth and high poly(3HB) productivity by the strain and overcoming the problem of poly(3HB) degradation, which has been observed before in flask batch cultures (19). For this purpose, glucose was fed occasionally to keep its concentration above 20 g/liter.…”

“…MW10 was grown in mineral salts medium (MSM) as described previously (19). FBF was performed in a 2-liter glass bioreactor (Biostat B plus; Sartorius BBI Systems GmbH, Melsungen, Germany).…”