Effects of aromatic compounds on the production of bacterial nanocellulose by Gluconacetobacter xylinus

Abstract

Background: Bacterial cellulose (BC) is a polymeric nanostructured fibrillar network produced by certain microorganisms, principally Gluconacetobacter xylinus. BC has a great potential of application in many fields. Lignocellulosic biomass has been investigated as a cost-effective feedstock for BC production through pretreatment and hydrolysis. It is well known that detoxification of lignocellulosic hydrolysates may be required to achieve efficient production of BC. Recent results suggest that phenolic compounds contribute to the inhibition of G. xylinus. However, very little is known about the effect on G. xylinus of specific lignocellulose-derived inhibitors. In this study, the inhibitory effects of four phenolic model compounds (coniferyl aldehyde, ferulic acid, vanillin and 4-hydroxybenzoic acid) on the growth of G. xylinus, the pH of the culture medium, and the production of BC were investigated in detail. The stability of the phenolics in the bacterial cultures was investigated and the main bioconversion products were identified and quantified.

Results: Coniferyl aldehyde was the most potent inhibitor, followed by vanillin, ferulic acid, and 4-hydroxybenzoic acid. There was no BC produced even with coniferyl aldehyde concentrations as low as 2 mM. Vanillin displayed a negative effect on the bacteria and when the vanillin concentration was raised to 2.5 mM the volumetric yield of BC decreased to ~40% of that obtained in control medium without inhibitors. The phenolic acids, ferulic acid and 4-hydroxybenzoic acid, showed almost no toxic effects when less than 2.5 mM. The bacterial cultures oxidized coniferyl aldehyde to ferulic acid with a yield of up to 81%. Vanillin was reduced to vanillyl alcohol with a yield of up to 80%.

Conclusions: This is the first investigation of the effect of specific phenolics on the production of BC by G. xylinus, and is also the first demonstration of the ability of G. xylinus to convert phenolic compounds. This study gives a better understanding of how phenolic compounds and G. xylinus cultures are affected by each other. Investigations in this area are useful for elucidating the mechanism behind inhibition of G. xylinus in lignocellulosic hydrolysates and for understanding how production of BC using lignocellulosic feedstocks can be performed in an efficient way.

Figure 1

The structure of model inhibitors and related compounds. (A) coniferyl aldehyde, (B) ferulic acid, (C) vanillin, (D) 4-hydroxybenzoic acid, (E) coniferyl alcohol, (F) vanillyl alcohol, and (G) vanillic acid.

Figure 2

Cultivation of G. xylinus in medium containing coniferyl aldehyde. The figure shows changes in (A) the glucose concentration in the culture medium, (B) the pH value of the culture medium, (C) the concentration of living cells, and (D) the concentration of coniferyl aldehyde. Coniferyl aldehyde was added on day one. Error bars show standard errors of means of three replicates.

Figure 3

Cultivation of G. xylinus in medium containing ferulic acid. The figure shows changes in (A) the glucose concentration in the culture medium, (B) the pH value of the culture medium, (C) the concentration of living cells, and (D) the concentration of ferulic acid. Ferulic acid was added on day one. Error bars show standard errors of means of three replicates.

Figure 4

Cultivation of G. xylinus in medium containing vanillin. The figure shows changes in (A) the glucose concentration of the culture medium, (B) the pH value of the culture medium, (C) the concentration of living cells, and (D) the concentration of vanillin. Vanillin was added on day one. Error bars show standard errors of means of three replicates.

Figure 5

Cultivation of G. xylinus in medium containing 4-hydroxybenzoic acid. The figure shows changes in (A) the glucose concentration of the culture medium, (B) the pH value of the culture medium, (C) the concentration of living cells, and (D) the concentration of 4-hydroxybenzoic acid. 4-Hydroxybenzoic acid was added on day one. Error bars show standard errors of means of three replicates.