Bio-plastic (P-3HB-co-3HV) from Bacillus circulans (MTCC 8167) and its biodegradation

Abstract

Polyhydroxyalkanoates (PHAs) are naturally occurring polyesters synthesized by bacteria for carbon and energy storage and it has commercial potential as bioplastic. The bacterial species Bacillus circulans MTCC 8167, isolated from crude oil contaminated soil, can efficiently produce medium chain length polyhydroxyalkanoates (P-3HB-co-3HV) from cheap carbon sources like dextrose. The molecular mass of P-3HB-co-3HV was reported as 5.1 × 10⁴ Da with polydispersity index of 1.21 by gel permeation chromatography. In the present investigation different bacteria and fungi species were used for testing the biodegradability of the extracted polymer. The FTIR spectra of the biodegraded PHBV film showed a decrease in the peak from 1735 cm⁻¹ (untreated film) to 1675 cm⁻¹, and disappearance of a peak present in the control at 2922 cm⁻¹ indicating the breakdown of ester (>CO) or O–R group and –CH bond, respectively. From biodegradability testing, the tested microorganisms were found to have decisive contribution to the biodegradation of P-3HB-co-3HV polymer.

Introduction

Plastic has become prevalent as it is inexpensive and can be engineered for wide range of applications and its special property like liquid phase. Total global plastic production dramatically increased from 1.5 million tons in 1950 to 245 million tons in 2008 with an annual growth rate of 9% [1]. Such an increase demands raw materials like crude oil which is in short supply, for the synthesis of plastics and the recycling of waste plastics becoming more important for our own environmental safety. Lack of degradability property and the closing of landfill sites as well as growing water and land pollution problems by waste plastic materials have led to concern about plastics. The use of plastics has enabled the development of innumerable disposable products, which has increased the amount of trash that is disposed. It can also be said that cheap cost of the material plays a major role in its pollution because these plastic products are not biodegradable; they are persistent and accumulated in the ecosystem, resulting in a significant burden on waste management. Natural polymers tend to be readily biodegradable, although the rate of degradation is generally inversely proportional to the extent of chemical modification.

Polyhydroxyalkanoates (PHAs) are completely renewable, biodegradable thermoplastics with similar material properties to petroleum-based plastics [2]. The microbial polyesters known as PHAs positively impact global climate change by reducing the amount of non-degradable plastic used [1]. PHAs belong to the family of microbial aliphatic polyesters that accumulate as storage material in microbial cells under stress conditions [3], [4]. Polymers of the PHA family are constantly increasing in number due to continuous discovery of new homopolymers and copolymers having different properties [5].

The stereochemistry of PHA goes well with its biodegradability. PHAs are readily degraded in different environments by a variety of bacteria and fungi. These degradations enable carbon dioxide and organic compound recycling in the ecosystem which provides a buffer to climate change [6]. A number of microorganisms excrete PHA depolymerases to hydrolyze the ester bonds of a polymer into water-soluble monomers and oligomers small enough to be transported into a microbial cell and metabolized to carbon dioxide and water [7], [8]. Degradation of PHA polymer by different Gram positive, Gram negative bacteria and some methanogenic co cultures were reported by different researchers [9], [10], [11]. Mabrouk and Sabry [12] reported that a marine Streptomyces sp. SNG9 utilize poly (3-hydroxybutyrate) (PHB) and its copolymer poly (3-hydroxybutyrate-co-3-hydroxyvalerate P (3HB-co-HV) as the sole carbon source and degraded the polymer particles in 4 days. Many microorganisms like Pseudomonas lemoignei, P. pseudomallei, Acidovorax facilis, A. delafieldii, Comamonas testosteroni, Variovorax paradoxus, Zoogloea ramigera, and Bacillus sp., as well as Streptomycetes are able to degrade P (3HB) extracellularly [13]. Colak and Guner [14] reported that three Pseudomonas sp. namely P. fluorescens, P. aeruginosa and P. putida were isolated from fuel oil contaminated soil to investigate the biodegradation of PHAs where morphological changes in the polymer were observed by the help of scanning electron microscope (SEM).

In our previous paper it was mentioned that Bacillus circulans (MTCC 8167) was isolated from the crude oil contaminated soil of Assam Asset Basin, ONGC, India which has biosurfactant producing capability and found to degrade different components of crude oil [15]. In the present investigation, B. circulans was used for the production of PHA in the detection medium. Polymer characterization and a study on their degradation by different bacteria as well as different fungi are reported in the present research. SEM and Fourier transform infrared spectroscopic (FTIR) analysis was done to characterize degradation of isolated PHA polymer. The results from the analysis of SEM suggest that the degradation of PHA occurred from surface and the leaching of degraded material as compared to the control can be visualized. FTIR spectroscopic analysis revealed the ester bonds of the PHA polymer is the main site of attack by microorganisms.