Optimisation of Culture Conditions for PLA-food-packaging Degradation by Bacillus sp. SNRUSA4

Abstract

Polylactic acid (PLA) is increasingly used in food-packaging production. The screening of PLA-food-packaging-degrading bacteria and optimisation of culture conditions for the PLA-food-packaging degradation by PLA-food-packaging-degrading bacteria were investigated for bioplastic waste management purposes. Only bacterial strain SNRUSA4 exhibited an increase in optical density (OD) in Basal Medium (BM) supplemented with 1.0 g/L of PLA-food-packaging as sole carbon source after 4 weeks of incubation. A weight loss of 7.3% and the rough and porous surface of PLA-food-packaging indicated that SNRUSA4 was a PLA-food-packaging-degrading bacterium. SNRUSA4 was able to degrade pure PLA which was confirmed from the clear zone formation around its colony on emulsified pure PLA agar plate. The 16S rRNA gene sequence of SNRUSA4 showed the similarity with thirteen Bacillus species. Hence, the strain SNRUSA4 was assigned as Bacillus sp. SNRUSA4. Response surface methodology with Box-Behnken Design was used to optimise the culture conditions including yeast extract concentration, initial pH value, temperature and agitation speed for growth and PLA-food-packaging degradation of Bacillus sp. SNRUSA4. The optimal conditions of Bacillus sp. SNRUSA4 was discovered in BM at initial pH value 7.02 with yeast extract concentration of 2.56% and agitated at 205.28 rpm at 31.68°C. Under optimal conditions, the OD of Bacillus sp. SNRUSA4 was up to 1.955, and the different OD between before and after optimisation was up to 1.752. Furthermore, the PLA-food-packaging weight loss also increased from 7.30% to 87.10% indicating that the PLA-food-packaging degradation under optimal conditions was higher than the unoptimised conditions. Therefore, Bacillus sp. SNRUSA4 is an efficient strain for degradation of PLA and PLA-food-packaging.